Understanding Species: Definitions, Concepts, and Naming Rules
School
University of Alberta**We aren't endorsed by this school
Course
PALEO 200
Subject
Biology
Date
Dec 12, 2024
Pages
56
Uploaded by HighnessWombat3664
Lesson 7: What is a Species? Written by W. Scott Persons, Victoria Arbour, Matthew Vavrek, Philip Currie, and Eva Koppelhus Learning objective for lesson 7: Students will learn the different ways of defining what a species is and will be able to compare the strengths and weaknesses of different species concepts. Learning objective 7.1: Describe the requirements for erecting a new species. In the late 18thcentury, the Swedish naturalist Carl Linnaeusdid a great and enduring service to all biologists by introducing a new system for scientifically naming organisms. Linnaeus is considered to be the founding father of modern taxonomy. Taxonomyis the science of naming and organizing organisms into related groups. Prior to Linnaeus, there was not an agreed-upon system for assigning names to organisms, and this had led to considerable confusion. Under Linnaeus’ system, which we still use today, every unique species of organism is given a binomial name. The binomial nameof a species consists of two parts: the genus name, or generic name, and the specific epithet. Here are two examples, the dinosaur Tyrannosaurus rexand our own binomial name Homo sapiens. Tyrannosaurus and Homoare the genus names, and rexand sapiensare the specific epithets. Note that the genus name is always capitalized and that the specific epithet is not. Also note that a binomial name is always italicized. Organisms that are different species but that belong to the same genus (meaning that they are very similar in overall form and are more closely related to each other than to members of any other genus) have the same genus name. For instance, our close relatives Homo erectusand Homo habilisshare our genus name. Specific epithets may be shared by many organisms, regardless of how closely related they are. For instance, Tyrannosaurus rexshares its specific epithet with Othnielia rex(an ornithopod dinosaur), Nuralagus rex(a giant extinct rabbit), Comitas rex(a sea snail), and Cattleya rex(a flower). However, the specific combination of genus name and specific epithet are not permitted to be shared by any two species. There are a few other rules governing how a species gets its binomial name. The rule of prioritystates that, once a species has officially been given a binomial name, the name cannot be changed (unless it turns out that the organism is not really a new species, in which case, the binomial name is abandoned). To officially give a new species a binomial name, a biologist must publish a description of the species in a widely distributed and peer-reviewed scientific publication and must designate a holotype specimen. The published description must include a list of characteristics or combination of characteristics that makes the new species unique. A peer-reviewedscientific publication is one that is not published until it has been reviewed by other scientists to
University of Alberta - PALEO 200/201 2 verify that the contents of the publication are legitimate and scientifically reasonable. A holotype specimen is a physical example of the new species, and it must be kept in a research institution, such as a university or a museum, so that other scientists may study it and be able to both verify that it is a distinct species and compare it to other potentially new species that are later discovered. A holotype specimen does not necessarily need to be a complete specimen (a broken or partial specimen will do, as long as it shows the unique characters that make it a distinct species). Holotype specimens of dinosaur species are hardly ever complete. As an example, the University of Alberta Laboratory for Vertebrate Paleontology houses the holotype specimen (UALVP 48778) of a small dinosaur called Hesperonychus elizabethae. The genus name is Hesperonychus, and the specific epithet is elizabethae. UALVP 48778 includes only a partial pelvis, but this pelvis provided enough information for palaeontologists to determine that it represented a new kind of dromaeosaurid theropod. The description of the new genus and species was published in the peer-reviewed journal Proceedings of the National Academy of Sciences. Learning objective 7.2: Describe sources of morphological variation You might think that identifying a new fossil species is as simple as pointing out the differences in the anatomy of two animals, and that's that. However, there are many reasons that one individual may look different from another individual, even if both belong to the same species! Individuals that differ in morphology because they belong to different species represent interspecific variation. Individuals that belong to the same species, but that have different morphologies, show intraspecific variation. There are several potential sources of intraspecific variation that we need to take into account when trying to identify a new species of dinosaur. First of all, males and females of the same species can look different, and this is called sexual dimorphism(or, sexual variation). Think about animals like deer, where the males have antlers but the females do not. There are many examples of sexual dimorphism in modern animals, and so dinosaurs may have exhibited sexual dimorphism as well. Ontogenetic variationis the variation that you can see between young individuals and old individuals of the same species. Besides size differences, animals can change shape as they progress through ontogeny, which is a fancy way of saying 'as they grow up'. Puppies often have shorter snouts relative to their overall head size, compared to fully grown dogs. Individual variationis the normal variation that exists among individuals of a given species. Take humans, for example: we come in a variety of sizes as adults, and things like eye and hair colour vary among individuals. The last source of variation that we need to consider when thinking about fossils is not biological in origin, but geological: taphonomic processes like plastic deformation can change the shape of a bone, resulting in taphonomic variation
University of Alberta - PALEO 200/201 3 Learning objective 7.3: Identify the necessary types of evidence required to delimit species based on the biological species concept versus the morphological species concept. Now that we have described how a species gets its name, we come to a serious question: what exactly is a species? There is no single definition or agreed-upon concept of what a species is. The most common species concept is the biological species concept, which defines a species as a group of organisms that can successfully interbreed. This species concept works well when applied to most modern animals and many plants. However, it cannot be applied to the majority of those modern organisms that reproduce asexually and which, therefore, cannot be said to interbreed at all. Nor can the biological species concept be applied to extinct organisms of any kind, since testing whether or not two fossils can mate is impossible. To make matters even more complicated, a single species can sometimes be divided into separate groups by geographic barriers. Each of these geographically separate groups of individuals is called a population. A populationis any grouping of organisms that live in the same geographic area and interbreed. One or more populations make up a single species. Instead, paleontologists rely on the morphological species concept. The morphological species concept defines a species as a group of organisms that share a certain degree of physical similarity. In dinosaur paleontology, the morphological species concept is often applied as it relates to the biological species concept. That is, fossil specimens are assumed to belong to the same species if their physical similarities are consistent with the similarities that would be expected (based on the general pattern of physical similarity observed in modern species) between members of a group that can successfully interbreed. Learning objective 7.4: Explain what types of evidence are available in the fossil record to define a species You might think that identifying a new fossil species is as simple as pointing out the differences in the anatomy of two animals. However, there are several factors that may cause one individual to look different from another individual, even if both belong to the same species. Individuals that differ in morphology but belong to the same species are said to exhibit intraspecific variation. Obviously, defining species using the morphological species concept is not an exact science, and trying to do so may be confounded by a variety of factors. Consider, for example, the taxonomic conundrum that a future paleontologist might face when discovering the skulls of an adult female moose, an adult male moose, and a juvenile male moose. Assuming that the futuristic scientist was not familiar with the sexual dimorphism and ontogenetic patterns of moose and their relatives, this paleontologist might be inclined to consider the more robust and antlered skull of the adult male moose to belong to a different species than the female and juvenile. The paleontologist might also be inclined to reason that the juvenile was just as likely to be a separate species of small deer or to be a juvenile form of either adult. Sexual dimorphism and ontogenetic change are both
University of Alberta - PALEO 200/201 4 Life restoration of Hesperonychusby Sydney Mohr, used with permission.factors that make the morphological species concept tricky to apply, especially given the incompleteness of the fossil record. Because of these and other confounding factors, there is considerable disagreement among paleontologists over how much of a morphological difference is needed to reasonably consider one species of dinosaur as distinct from another. Paleontologists who require more differences before they consider two species to be distinct are called lumpers. Paleontologists who require fewer differences before they consider two species to be distinct are called splitters. Whether you are a lumper or a splitter can have a big effect on the total number of dinosaur species you recognize and on your interpretations of dinosaur species diversity. Let's finish off with another look at Hesperonychus. Hesperonychusis one of the smallest known dinosaurs from North America. How did paleontologists know that it was a new species, rather than an individual of an already named species? Since it is small, could it be a juvenile of another species? The specimen had several unique features on the pelvis that were not seen in any other dromaeosaurid theropod, which suggests that it was a new species. Additionally, the bones of the pelvis were tightly fused together. In juveniles, the bones of the skull, vertebrae, and pelvis are not tightly fused together, and you can see the sutures between individual bones. The sutures were not visible in the Hesperonychuspelvis, suggesting that it was a fully-grown individual that had a small adult size. Sexual dimorphism is harder to test, but none of the differences in the pelvis seemed likely to relate to sex-specific functions. Finally, the pelvis was well preserved, and taphonomic deformation could not have produced the unique features. So, naming the specimen as the holotype of a new species seemed to be the right choice.
University of Alberta - PALEO 200/201 5 Supplementary Materials. Understanding variation in and between species:Laelaps: Out of many Psittacosaurus, one. [blog post] Dinosaur Tracking –Goodbye, Anatotitan?[blog post] Tetrapod Zoology - The explosion of Iguanodon, part 3: Hypselospinus, Wadhurstia, Dakotadon,Proplanicoxa...When will it all end?[blog post] Tetrapod Zoology –Stegosaur Wars[blog post] Optional: Royal Tyrrell Museum Speaker Series: Who-oplocephalus? Euoplocephalus![video lecture] New dinosaurs from Alberta!Laelaps –New horned dinosaur had funky frill. [Blog post] Dinosaur Tracking - Paleontologists welcome Xenoceratopsto the ceratopsian family tree. [blog post] Cleveland Museum of Natural History –Scientists name new species of dinosaur, "bone-headed" Acrotholusaudeti[video] Green Tea and Velociraptors –Theropod dinosaurs were waaay more diverse than previously thought.[blog post]
Lesson 8: Evolution Written by Victoria Arbour, W. Scott Persons, Philip Currie, and Eva Koppelhus Learning objective for lesson 8: Students will understand the basic theory of evolution and understand how to interpret a phylogenetic tree Learning objective 8.1 –Describe the principles underlying the theory of natural selection. Evolution is the great unifying theory of all modern biology. The theory of evolution was first conceived by the British naturalist Charles Darwin. Darwin’s theory explains how new species come into existence, how organisms become adapted to their environments, and why specific groups of organisms share specific traits. It also correctly postulated that all life on Earth is related and shares a single common origin. Despite its importance and tremendous explanatory power, evolution is a simple concept and is based on four basic principles of life. First, many of the traits of an organism (everything from how it looks to how it behaves) are heritable. Heritablemeans that the trait is part of an organism’s genetic code and, therefore, either will be, or (depending on the type of reproduction) has a chance to be, copied to the organism’s offspring. Heritability is the reason that sons and daughters tend to resemble their parents. A trait must be heritable in order for that trait to evolve. Second, sometimes organisms have heritable traits that are new, not copied from the organism's parent(s). One source for new heritable traits is random genetic mutation. In order for selection to occur on any given trait, there must be variationin that trait in a population. Third, an organism’s traits affect how successfully that organism is able to reproduce. Often, a trait’s effect is indirect –that is, it improves or hinders an organism’s ability to survive, which, in turn, affects how many reproductive opportunities the organism has. One variation of the trait must provide an advantage (or, differential success)over the other variations in order for evolution to occur. Fourth, natural environments have limited resources, and competitionfor these resources permits only some organisms to successfully reproduce before they die. Some of the versions of a given trait must be 'selected out' of a population in order for evolution to occur, and this must occur because of competition for limited resources. The theory of evolution combines these four basic principles of life together: the differential successof certain variationsof a heritabletrait, because of competitionfor limited resources, leads to the change over time (evolution) of that trait in a population. If an organism is born with a new trait (say, for example, a woodpecker has a genetic mutation that makes the keratin covering of its beak a little stronger)
University of Alberta - PALEO 200/201 2 and this new trait improves the organism’s ability to successfully survive and reproduce (the harder-beaked woodpecker is better able to obtain nutritious food, which improves the quality of the woodpecker’s mating plumage, allows it to lay healthier eggs, and makes the woodpecker better able to feed its young), then that organism is more likely to outcompete other similar organisms (the harder-beaked woodpecker consumes more food and attracts more mates while other woodpeckers starve and are mate-less). The beneficial trait will likely be inherited by the organism’s more abundant offspring (many clutches of harder-beaked woodpeckers hatch). Over time and generations, the beneficial trait is likely to become widespread (harder-beaked woodpeckers thrive, while softer-beaked woodpeckers may eventually become extinct). At the same time that one beneficial trait is becoming widespread, so too may many other beneficial traits become the norm (for example, sharper woodpecker claws that can better hold onto tree bark, more sensitive woodpecker ears that can better detect boring insects, broader wings that can better maneuver through forests, or an additional mutation that helps to make the beaks even harder). Eventually, when many new traits become widespread, the organisms that have these accumulated new traits (the super-hard-beaked, sharp-clawed, sensitive-eared, broad-winged woodpeckers)are so different from their ancestors (the soft-beaked, dull-clawed, insensitive-eared, narrow-winged woodpeckers)that they constitute a new species. The evolution of a new species does not necessarily require the extinction of its ancestor. For instance, a new species might simply branch off from an ancestral species if only a single population of the ancestral species was exposed to new environmental conditions that favored new traits. While the population in the new environment would acquire new traits better adapted to that environment and evolve into a different species, the ancestral species might continue to exist in its ancestral environment. Thus, evolution uses only basic principles of the natural world to explain how one species can give rise to others. It is often mistakenly said that the evolution of new species is a random processes. Just the opposite is true. While new traits may be introduced by random mutations, the determination of which traits are successfully passed on to later generations is not random. Instead, it is based on a specific criterion: how well each trait improves an organism’s reproductive success. Of course, most random mutations are more likely to diminish than they to improve an organism’s success. The competitive selective process by which detrimental traits are competitively discarded and advantageous traits are retained is called natural selection. Learning objective 8.2 –Show how shared derived characters can be used to establish relationships between dinosaur groups. Evolution provides a framework that modern taxonomy uses to categorize organisms. Organisms are grouped together based on their most recent shared common ancestors. For instance, all hadrosaurs, ceratopsians, ankylosaurs, and stegosaurs share a more recent common ancestor with each other than they do with sauropods, prosauropods, and theropods. Thus, hadrosaurs,
University of Alberta - PALEO 200/201 3 pachycephalosaurs, ceratopsians, ankylosaurs, and stegosaurs are grouped together, and we call this group the Ornithischia. These ornithischians and the saurischians share a more recent ancestor with each other than they do with all other amniotes, and we call this group Dinosauria. In other words, all dinosaurs are classified together in a group because all dinosaurs evolved from a single species of amniote tetrapod. Within the dinosaurs, all ornithischians are classified together because they evolved from a single particular species of dinosaur, while saurischians are classified together in a different group because they evolved from another particular species of dinosaur. How can we figure out which dinosaurs share a more recent common ancestor, and, therefore, should be grouped together? This is done by studying characters. A characteris any heritable trait that can be described and labeled. A shared derived characteris a character that is present in two or more groups and their common ancestor, but is not present in any more distantly related groups. A shared derived character is also called a synapomorphy. For instance, all species of ornithischians have a special bone in the lower jaw that forms a beak, called the predentary, and no other dinosaurs have this special beak bone. Thus, the character of the predentary was passed on to all ornithischians from their ancient shared ancestor and is a synapomorphy that testifies to this shared ancestry and can be used to define the group. Learning objective 8.3 –Distinguish between similarities resulting from convergent evolution or shared derived characters. Identifying shared derived characters and using them to identify evolutionary related groups sounds easy, but often it is not. One of the biggest challenges to determining evolutionary relationships is the common phenomenon of convergent evolution. Let's take a look at two dinosaurs, Spinosaurusand Ouranosaurus. Spinosaurusis a theropod with a long snout, sharp teeth, clawed fingers, and long processes on the vertebrae of the back that form a sail. Ouranosaurusis an iguanodont with a beak and grinding teeth, hoofed toes, and long processes on the vertebrae of the back that form a sail. Spinosaurushas the saurischian hip arrangement where the pubis points forwards, and Ouranosaurushas the ornithischian hip arrangement where the pubis points backwards. Spinosaurusand Ouranosaurusboth have a sail, but we do not consider the sail to be synapomorphy, or shared derived character, between Spinosaurusand Ouranosaurus. Spinosaurushas many of the synapomorphies of saurischians and theropods, and Ouranosaurushas many of the synapomorphies of ornithischians and ornithopods. Therefore, the most likely evolutionary scenario is that the sail in Spinosaurusevolved independently of the sail in Ouranosaurus. A sail evolved twice in Dinosauria: once in the lineage leading to Spinosaurus, and once in the lineage leading to Ouranosaurus. The evolution of similar traits in two different lineages is termed convergent evolution.
University of Alberta - PALEO 200/201 4 The dinosaur family tree, showing convergent evolution of the sail in the theropod Spinosaurusand the iguanodont ornithopod Ouranosaurus. Diagram by V. Arbour, with dinosaur illustrations by J. Ang.Usually, convergent evolution results when two lineages must adapt to similar environments and to similar modes of life. We aren't really sure what the function of the sail was in Spinosaurusand Ouranosaurus, but we have talked about many examples of convergent evolution in the lesson on feeding adaptations. For example, Spinosaurushas a long snout with conical teeth, similar to the jaw structure seen in some crocodilians and other piscivorous animals. The need to adapt to feeding on aquatic prey is probably responsible for the similarity in the tooth and jaw structures of the two groups. Another excellent example of convergent evolution is the evolution of wings in flying vertebrates. Birds, pterosaurs, and bats all evolved wings from the forelimbs, but did so in different ways. Pterosaurs have a wing made of a membrane supported by just one long finger. Bats (which are mammals) have a wing made of a membrane supported by several fingers. And birds have a wing made of feathers, and have fused the hand bones into a single unit. Learning objective 8.4 –Use a family tree to show the relationships between groups of dinosaurs. Suppose that in the previous example you had no particular reason to suspect that the similarities between Spinosaurusand Ouranosauruswere the result of convergent evolution. How could you figure out that Ouranosauruswas an ornithopod dinosaur and not a theropod? The answer is obvious. Although Ouranosaurushas a sail like Spinosaurus, it shares many more traits in
University of Alberta - PALEO 200/201 5 common with iguanodonts (such as grinding teeth and a beak), and ornithischians (a backwards-pointing pubis, a predentary bone, and more). It is simpler to assume that the one character in common between Ouranosaurusand Spinosaurus(the sail) is the result of convergent evolution, than it would be to assume that the huge number of similarities between Ouranosaurusand iguanodonts are all the result of convergence. The idea that "all other things being equal, the simplest answer is usually the right one" is called parsimony. Parsimony is also referred to as Occam’s razor. Figuring out the relationships between only a few kinds of distantly related organisms is usually not too hard, as we can rely on our own ability to judge which of a handful of alternative relationships is the most parsimonious. To determine the evolutionary relationships between large numbers of species, many of which may be closely related, and to take into consideration a large number of characters, paleontologists use computer programs. These programs analyze a list of characters that is first compiled by the researcher. This list is called a character matrix. Based on the character matrix, the computer program applies the principle of parsimony to arrange the organisms in a sequence of relationships that requires the fewest instances of convergent evolution. The resulting arrangements look like diagrams of a “family tree” and are called phylogenetic trees. Phylogenetic trees are composed of nodes and branches. A node is where two branches diverge, and shows the point at which two linages shared a common ancestor. After a node, the pattern of subsequent branches and nodes shows how the descendants of that common ancestor continued to diverge from each other. A group of species that share a common node is called a clade. Clades can be very small (even as small as two species), or very large –there are no size limits. A clade must contain the ancestor of a group and all of its descendants. A group that does not include all of the descendants of a common ancestor is therefore not a clade. Only one of these phylogenetic trees shows a clade highlighted in grey –the top tree. The middle and bottom trees do not include all of the branches from the earliest node. Diagrams by V. Arbour.
University of Alberta - PALEO 200/201 6 In previous biology courses you may have been taught a system of classification that we call the Linnean Hierarchy, which classifies organisms as belonging to a Kingdom, Phylum, Class, Order, Family, Genus, and Species. While the Linnean hierarchy works pretty well, especially considering it was formed long before our current ideas about natural selection and evolution had solidified, the original classifications do not always work well with our current understandings of the evolutionary relationships of organisms. For example, Mammalia (mammals), Aves (birds), Pisces (fish), and Amphibia (amphibians, lizards, snakes, crocodiles, and turtles, but also sharks, rays, and some fish!) were all classified at the equivalent rank of Class. But we can trace all four-limbed animals (the tetrapods, including mammals, reptiles, birds, and amphibians) back to a fish ancestor –Class Mammalia, Aves, and Amphibia are all a subset of Class Pisces (now called Osteichthyes). One of the best examples of this problem is what to do with the birds…Learning objective 8.5 –Evaluate the evidence for a dinosaurian origin of birds. When the theory of evolution was first proposed by Charles Darwin in the 1800’s, many scientists were skeptical and wanted to see more evidence supporting evolution in the fossil record. These skeptics reasoned that, if indeed new species evolved from ancient species and if all life shared a common evolutionary history, then fossils should exist of major “missing links” –that is, organisms that show an evolutionary connection between two major groups of organisms by displaying some traits that are characteristic of one group and some traits that are characteristic of the other group. Thomas Henry Huxleywas a close colleague of Charles Darwin and one of the earliest advocates for the theory of evolution. Huxley was also the first scientist to recognize that birds evolved from dinosaurs, and he cited the newly discovered specimens of Archaeopteryxas fossils of a “missing link” between dinosaurs and birds. Specimens of Archaeopteryx had been found exquisitely preserved in fossil lake deposits. These specimens clearly show that Archaeopteryxhas long wing-feathers and tail feathers just like a bird, but they also show that Archaeopteryxhad teeth, clawed fingers, and a long series of tail vertebrae just like a dinosaur. With the help of Archaeopteryx, Thomas Henry Huxley showed that transitional forms do exist in the fossil record, just as the theory of evolution predicted, and also showed that birds are a branch of the dinosaur family tree. Of course, not everyone was convinced by Huxley’s arguments. It took many years before the theory of evolution was fully accepted by the entire field of biological science and even longer for the theory of a dinosaur origin of birds to be fully accepted. Since Huxley’s time, paleontologists have come to recognize an increasing number of characters that birds and other theropod dinosaurs share. One of the most significant shared characters was discovered in a specimen of the little dinosaur Sinosauropteryx. Sinosauropteryxwas the first non-avian (non-bird) dinosaur to be discovered with feathers. The feathers of Sinosauropteryxhad a simple structure compared to the feathers of modern birds and were used for insulation, not for flight, but they were feathers just the same. Many other small theropod specimens have since been found with feathers, some with complex flight feathers. Recently, feathers have also been found on specimens of
University of Alberta - PALEO 200/201 7 Sinosauropteryxlife restoration by Matt Martyniuk, used under the CC-BY-SA license. the large tyrannosauroid Yutyrannus, showing that some large dinosaurs had them as well. Birds are the only clade of dinosaurs alive today. Birds can be classified as theropod dinosaurs, because they evolved from theropod dinosaurs. But where we draw the line between non-avian theropod dinosaurs and 'birds' is more complicated than you might think. So many fossils of bird-like dinosaurs and dinosaur-like birds have been found, that the line between dinosaur and bird is blurry. Let's finish this lesson by discussing some different definitions for 'bird'. What is a bird? Many palaeontologists use Aves to refer to the crown group of birds, which includes all living birds as well as extinct taxa like the dodo and moa. Another clade name, Avialae, is generally equivalent to 'flying dinosaurs', which includes extinct species that looked very similar to modern birds, including Archaeopteryx. However, 'birds' can be defined in several ways: "The Ascent of Bird" by Matt Martyniuk, used with permission. From left to right: the tyrannosaur Dilong, the possible ornithomimid Nqwebasaurus, the alvarezsaur Haplocheirus, the maniraptoran Yixianosaurus, the dromaeosaur Xiaotingia, Archaeopteryx, Confuciusornis, the enantiornthine Bohaiornis, the ornithurine Apsaravis, and Ichthyornis. Where would you draw the line between birds and non-avian dinosaurs?
University of Alberta - PALEO 200/201 8 Definition 1:Archaeopteryxand all of its descendants. Problem: New phylogenetic analyses sometimes show that Archaeopteryx is more closely related to the dromaeosaurid theropods than to modern birds. Diagrams by V. Arbour.Definition 2: Feathered dinosaurs. Problem: As more and more feathered dinosaur fossils, like Yutyrannus, are found, more dinosaur are included in this definition. For instance tyrannosaurs would now be considered birds!
University of Alberta - PALEO 200/201 9 Definition 3: Flying dinosaurs. Problem: It is difficult to determine exactly which dinosaurs were capable of flying (as opposed to simply gliding). Definition 4: Crown dinosaurs. This a somewhat technical term that means the last common ancestor of all extant birds and its descendants. Problem:This definition fails to recognise many feathered and flying dinosaurs that are more closely related to modern birds than to Archaeopteryxas birds. However, this is the definition favoured by many palaeontologists. We then use the name Avialae for the clade containing Archaeopteryxand all of its descendants.
University of Alberta - PALEO 200/201 10 Supplementary Materials. Evolution:How evolution works[video] University of California, Berkeley - Evolution 101[website; contains some material that we don't cover in Dino101, like genetic drift, but you should still have a look through the whole site.] Tetrapod Zoology –Species recognition vs. Sexual selection in dinosaurs. [Blog post] PhD TV –Dogs vs. Hyenas(another example of convergent evolution in the fossil record, but not dinosaurs) [video] OneZoom Tree of Life Explorer: Although it currently only includes living species, and therefore there are no dinosaurs on the tree, this is a really excellent tool for visualizing the branching nature of evolution! Try zooming in on the very tip of the bird branch! Birds are Dinosaurs:TED-Ed - How did feathers evolve?[video] Dinosaur Tracking - Did all dinosaurs have feathers?[Blog post] Dinosaur Tracking - Feathery ostrich mimics enfluffle the dinosaur family tree. [Blog post] Optional: PBS –The Four-Winged Dinosaur[video; long, but very good!]
Lesson 9: Stratigraphy and Geologic Time Written by Victoria Arbour,W. Scott Persons,Philip Currie, and Eva Koppelhus Learning objective for lesson 9: Students will understand basic stratigraphic concepts and the scale of earth history. Students will understand the evolution of dinosaurs through time, including which groups evolved when and where.Learning objective 9.1: Describe the geological Principle of Superposition Learning objective 9.2: Arrange geological formations in order by interpreting a simple geological cross-section As has been previously discussed, sedimentary rocks form from small organic or inorganic particles (called sediments) that accumulate and are cemented or compacted together. As sediments are deposited, they gradually build up on top of each other in layers. Over time, deep sequences of layered sedimentary rocks result. In such a sequence, the oldest rocks (which are formed from the oldest deposited sediments) are at the bottom, and the layers become increasingly younger towards the top. The tendency for rock layers to be chronologically stacked is called the Principle of Superposition. The Principle of Superposition is only an expression of how things normally work, and there can be exceptions. Igneous rocks that form from volcanic activity may vertically cut through horizontally arranged layers of rocks, and mountain building events may tilt, fold, and even flip rock layers. This diagram shows the complex stratigraphy that can happen when rocks are folded, broken, and eroded. A are ancient folded sedimentary layers that have been folded and broken (faulted). B is a large igneous intrusion that partially melted A. These rocks were then uplifted and eroded, creating a flat surface called an unconformity. New sedimentary layers were deposited on top of the old rocks. D is a new igneous intrusion that cut through A, B, and C, and then E is yet another horizontal layer of sedimentary rocks deposited over top of A-D. Image by Woudloper via Wikimedia Commons, used under the CC-BY-SA license.
University of Alberta - PALEO 200/201 2 Alternating light and dark sedimentary layers at Dinosaur Provincial Park. Photo by V. Arbour. Stratigraphyis the science of using the arrangement and composition of rock layers to interpret geological history. A large uninterrupted sequence of rock that is made of multiple layers that all share similar properties (such as mineral composition and average sediment grain size) and that all formed under similar conditions is termed a formation. When a sequence of rock changes from one formation to another, it indicates that a large scale change occurred in the environment where the rocks were being deposited. The Principle of Superposition allows a stratigrapher to infer the relative age of rock layers (that is, how old one layer is relative to another), but it does not determine the absolute age (that is, how old in years the layers are). To age rocks in absolute terms, a technique called radiometric datingis used. All matter (including rocks) is composed of chemical elements, which are atoms composed of a particular number of protons and electrons (positive and negatively charged particles). Some chemical elements are also composed of neutrons (neutrally charged particles), and some of these chemical elements may exist as isotopes. An isotopeis a variant of a chemical element that has an unusual number of neutrons. Some isotopes are unstable and will undergo radioactive decay, whereby energy is released and a new atom (or atoms) with a different composition of particles results. These resulting atoms with different particle compositions are called the decay products. At what time a single isotopic atom will undergo radioactive decay is unpredictable, but a large collection of isotopes will radioactively decay at a mathematically predictable rate. When a new rock forms, it has a ratio of isotopes and decay products that matches that of the environment. As the rock ages, the isotopes decay and the ratio of isotopes to decay products decreases. Using a special machine called a mass spectrometer, it is possible to measure the isotope ratio of a rock, and this ratio can tell you how long ago the rock formed. Unfortunately, sedimentary rocks are never really “new” -- that is, they are made of sediments that had already formed and that were already potentially undergoing radioactive decay. Thus, sedimentary rock cannot usually be radiometrically dated. On the other hand, igneous rocks areformed anew and can usually be radiometrically dated. But, if fossils are usually found in sedimentary rocks and not in igneous rocks, how can we ever tell how old a fossil and its rock layer is? By combining radiometric dating and the principle of superposition. If sedimentary rocks that contain fossils are found between two horizontally deposited layers of igneous rocks, then dating the igneous rocks above the sedimentary layer will tell us what age the fossils must be older than, and dating the igneous rocks below the sedimentary layer will tell us what age the fossils must be younger than. So, we can confidently bracket the age of the fossils.
University of Alberta - PALEO 200/201 3 In some instances, it is even easier to date fossils, because we can be certain that particular particles of igneous rocks that compose the sedimentary rocks that the fossils are buried in were incorporated into the sediment at nearly the same time that they were formed. For instance, fossils may be buried by, or be buried near, deposits of volcanic ash. Volcanic ash forms at the moment of an eruption, and the time between when an eruption occurs and when its ash falls to the surface is inconsequently short. Volcanic ash deposits are a key tool in fossil dating.Learning objective 9.3 –Describe important events in the geological history of Earth Stratigraphy and radiometric dating are combined to piece together the history of the earth and to create the geologic time scale. The geologic time scaleis a standardized series of chronological divisions that parses the Earth’s history into discrete named units. The largest units in the time scale are Eons, followed by Eras, Periods, and Epochs. We'll post a PDF of the most recent version of the official geologic time scale on eClass, or you can download it from the International Commission on Stratigraphy: http://www.stratigraphy.org/index.php/ics-chart-timescale. Here is a brief overview of the geologic time scale and of some of the important events that occurred during its different divisions. THE HADEAN EON –4.6 TO 4 BILLION YEARS AGOThe Hadean Eon is named for Hades (the Greek god of the underworld), and by the beginning of this eon, the rest of the universe was already over nine billion years old. The formation and early years of the earth were a tumultuous time, with the surface of the earth partially molten and with volcanic activity widespread. At roughly 4.5 billion years ago, the young earth collided with a smaller planetoid. This collision ejected a large mass of debris, which was held in orbit by the earth’s gravity and eventually formed the moon. By the end of the Hadean, the earth had cooled and large oceans covered much of its surface. Complex organic molecules are thought to have formed in these early oceans and possibly the earliest true life forms. The oldest rocks on earth have been dated at only about 4.4 billion years old, though rocks discovered on the moon are older. THE ARCHEAN EON –4 TO 2.5 BILLION YEARS AGOThe oldest known fossils come from the Archean Eon. These fossils are of simple single-celled organisms. More advanced forms later evolved in the Archean, including cyanobacteria. The cyanobacteria were photosynthetic and eventually produced large amounts of oxygen gas, which became concentrated in the earth’s atmosphere. Some cyanobacteria formed structures called stromatolites, which are some of the best records of early life. Stromatolites look like lumpy stones, but when you cut them in half you can see the layers that were created as the cyanobacteria secreted sticky films that trapped particles of sediment.
University of Alberta - PALEO 200/201 4 A stromatolite on display in the Paleontology Museum at the University of Alberta. Photo by V. Arbour. THE PROTEROZOIC EON –2.5 BILLION TO 541 MILLION YEARS AGOAt approximately 1.7 billion years ago, the first multicellular organisms evolved. Because single-celled and early multicellular life had no bones or other hard parts and was usually microscopic, the fossil record of this early life is poor. Within the Proterozoic, the time span from 630 to 542 million years ago is known as the Ediacaran Period. During the Ediacaran, large forms of life with some harder parts evolved, including the first animal life. THE PHANEROZOIC EON –541 TO 0 MILLION YEARS AGOThe Phanerozoic Eon is subdivided into three eras, which are themselves subdivided into numerous periods. It is during the Phanerozoic that animal life rapidly evolved into a multitude of diverse forms, including dinosaurs. We will consider the events of the each Phanerozoic era and period. The Paleozoic Era –541 to 252 million years agoAt the start of the Paleozoic, animal life was restricted to primitive invertebrates living in the oceans, but, by its close, great forests covered the land and teamed with reptiles, amphibians, and insects. The Cambrian Period –541 to 485 million years agoThe beginning of the Cambrian marks such a dramatic diversification of aquatic animal life that it is often referred to as The Cambrian Explosion. Sponges, molluscs, worms, and many kinds of arthropods (including trilobites) evolved. A close early relative of the vertebrates, called Pikaia, didn't have vertebrae, but had several other features found in vertebrates. The invertebrate Anomalocariswas once the top predator in the Cambrian oceans. Illustration by Rachelle Bugeaud.
University of Alberta - PALEO 200/201 5 With a bizarreness characteristic of Cambrian life, Opabiniaremains a mystery in teams of both its life habits and its evolutionary relationships to modern animals. Illustration by Rachelle Bugeaud. Though it lacked a hard skeleton, Pikaiawas one of the earliest ancestors of the vertebrate animals. Illustration by Rachelle Bugeaud. The Ordovician Period –485 to 443 million years agoGlobal sea levels were high. Life in the oceans continued to diversify, with fish increasingly becoming the dominant large aquatic animals. The Silurian Period –443 to 419 million years agoUntil this point, fish had not yet evolved jaws. With the evolution of jaws came the evolution of large predatory fish. Primitive plant life began to flourish on land. Osteostracans, a group of early jawless vertebrates, in the Paleontology Museum at the University of Alberta. Photo by V. Arbour. The Devonian Period –419 to 359 million years agoThe first forests appeared on land. Huge jawed fishes, like Dunkleosteus, evolved in the seas, and the first true sharks appeared. Lobe-finned 'fishapods', like Tiktaalik, ventured onto land, and give rise to the tetrapods. The skull of Dunkleosteusin the Paleontology Museum at the University of Alberta. Photo by V. Arbour.
University of Alberta - PALEO 200/201 6 The Carboniferous Period –359 to 299 million years agoAmphibians were widespread in the abundant swamps, and reptiles, the first amniotes, evolved. Much of the coal that is mined today formed from the rotting plants of Carboniferous swamps. The Permian Period –299 to 252The continents collided together and formed a single super continent called Pangaea. Reptiles evolved into three main lineages: the anapsids (which would go on to evolve into turtles), the synapsids (which go on to evolve into mammals), and the diapsids (which would go on to evolve into lizards, snakes, crocodilians, and dinosaurs). Many of the terrestrial rocks from this period of time represent dry, desert environments. The single greatest mass extinction in our planet’s history occurred at end of the Permian, with one of the most notable losses being the trilobites. The Mesozoic Era –252 to 66 million years agoThe Mesozoic is often referred to as the Age of Dinosaurs. It is during this time that dinosaurs evolved and became the dominant form of large terrestrial animal life. Many kinds of marine reptiles evolve, including the ichthyosaurs, plesiosaurs, and mosasaurs. The first true turtles, crocodilians, lizards, snakes, mammals, and birds evolved at this time as well. The first flowering plants evolved towards the end of the Mesozoic. The Mesozoic Era has been, and will continue to be, examined and discussed throughout this course. We'll go into more detail on the Mesozoic Era in the next section! The Cenozoic Era –66 to 0 million years agoThe Cenozoic is often referred to as the Age of Mammals. Although mammals had been around since the Triassic, the extinction of the dinosaurs (except for birds) allowed mammals to evolve larger forms and to fill many new ecological roles. Grasses only become abundant at this time! The Paleogene Period –66 to 23 million years agoGlobal temperatures began to cool. Mammals diversified into a variety of new forms, including primates, bats, and whales. Birds also diversified. The Neogene Period –23 to 2.6 million years agoGlobal temperatures continued to cool. The first hominids evolved in Africa. The Quaternary Period –2.6 to 0 million years agoThe earth experienced several large glaciation events, or “ice ages”. The first anatomically modern humans evolved. Human civilization spread. Learning objective 9.4 –Classify types of dinosaurs based on the geological age in which they were most common. Many dinosaur books and movies depict dinosaur species from vastly different periods existing at the same time and place. This can be a big mistake! The non-avian dinosaurs existed for 135 million years. There is less time separating the first humans from the last
University of Alberta - PALEO 200/201 7 dinosaurs than there is separating the last dinosaurs from the first dinosaurs, and not all clades of dinosaurs were present throughout the entire Mesozoic. The Triassic Period –252 to 201 million years agoDuring the first ten million years of the Triassic, life gradually recovered from the mass extinction that occurred at the end of the Permian. The first mammals and dinosaurs evolved during the later portion of the Triassic, and so did the first pterosaurs - the first vertebrates to fly. The supercontinent Pangaea began to break apart. Many of the dinosaurs from this period of time look fairly similar to each other. The first representatives of the ornithischians (like Pisanosaurus), theropods (like Eoraptorand Herrerasaurus), and sauropodomorphs (like Panphagia) were all mostly small and bipedal. "Prosauropods" like Plateosauruswere some of the first large herbivorous dinosaurs. We'll discuss life on land in the Triassic in more detail in lesson 11. As dinosaurs were evolving to fill large-bodied ecological roles on land, other amniote groups were evolving to fill them in the sea and air. It was in the Triassic that the first ichthyosaurs evolved. The name ‘ichthyosaur’ literally means “fish lizard,” but ichthyosaurs are not lizards, and they certainly aren’t fish. Even so, the name ‘ichthyosaur’ still seems fitting, because theyare a group of reptiles that took on a fish-like lifestyle and evolved a very fishy body form. The ancestors of ichthyosaurs were fully terrestrial reptiles, but, just like the ancestors of modern whales, dolphins, seals, and sea turtles, the group found success by making an evolutionary return to the water. To adapt to an aquatic life, ichthyosaurs evolved paddle-like front and hind limbs, a finned tail, and even a shark-like dorsal fin. The long snouts of most ichthyosaurs resemble those of dolphins, and are filled with conical teeth –good equipment for a piscivorous diet. Despite their many fish-like adaptations, ichthyosaurs never evolved gills and needed to come to the surface in order to breathe air. Late into the Triassic, ichthyosaurs were joined in the seas by another group of reptiles that also evolved a secondarily aquatic lifestyle: the plesiosaurs. Most plesiosaurs had large chests and torsos, broad paddle-shaped limbs, and relatively short tails. In front of their shoulders, plesiosaurs varied tremendously. Some had short necks and huge jaws, other had elongated serpentine necks with small heads. Elasmosaurusis an example of a plesiosaur, although it lived during the Cretaceous Period. Art by Rachelle Bugeaud. The airways of the Dinosaur Age were also busy places. Insects had already evolved flight at least as far back as the Carboniferous, and dinosaurs would take to the air and give rise to birds in the Jurassic. In addition, there was a third group fluttering over the heads of dinosaurs: the pterosaurs. Pterosaurs, or as they are commonly called, “pterodactyls”, are close relatives of dinosaurs who branched off from the reptilian family tree at roughly the same time in the Triassic that dinosaurs did. Pterosaurs were the first vertebrates to fly.
University of Alberta - PALEO 200/201 8 Unlike birds, which have arms that support wings made of feathers, and bats, which have wings made from skin stretched between multiple fingers, pterosaurs have membranous wings supported by a single extremely elongated finger. Early pterosaurs belong to a group called rhamphorhynchoids, which were common in the Late Triassic and throughout the Jurassic. The Triassic of New Mexico: the small, early theropod Coelophysischases an early mammal, while the synapsid Placeriaslooks on. Art by Jan Sovak.The Jurassic Period –201 to 145 million years agoDinosaurs diversified. This was the peak of sauropod diversity, and they were the dominant terrestrial herbivores. Small and medium sized ornithopods were common. Non-coelurosaurian theropods, like Allosaurus, were the dominant terrestrial carnivores. The stegosaurs are almost completely restricted to the Jurassic, and the first ankylosaurs, ornithopods, and ceratopsians appear at this time, although they are not particularly abundant or diverse. The first birds, including Archaeopteryx, evolved during the Jurassic. The Morrison Formation of the western USA, and the Solnhofen limestone of Germany, are some of the best and most famous records of dinosaurs from this time.
University of Alberta - PALEO 200/201 9 In the Jurassic, rhamphorhynchoid pterosaurs gave rise to a new pterosaur group: the pterodactyloids. Pterodactyloids differed from rhamphorhynchoids in the morphology of their tails, which were short, and the carpels in their wrists, which were elongated and made a greater contribution to the length of the wing. Unlike rhamphorhynchoids, many pterodactyloids had large head crests, which were presumably display structures. There were many species of small pterodactyloids, some smaller than a robin, but some species had wingspans of over ten meters, making them the largest animals to ever fly. Left: Pteranodon is an example of a Cretaceous pterodactyloid pterosaur. Illustration by Rachelle Bugeaud. Below: The Morrison Formation of the western USA: sauropods dominate the landscape, like the slender-necked Diplodocus(to the right), tall Brachiosaurus(center), and stubby-faced Camarasaurus(to the left). Art by Jan Sovak.
University of Alberta - PALEO 200/201 10 The Early Cretaceous Period –146 to 100million years agoDinosaurs continue to diversify and the first flowering plants evolved. In the Early Cretaceous new theropods, like spinosaurids and carcharodontosaurids evolve, coelurosaurian theropods become more diverse, and iguanodonts become larger and more abundant. The Yixian Formation of China, the Wealden Supergroup of England, and the Cedar Mountain Formation of Utah are important Early Cretaceous fossil-rich rock units. Sometime in the Cretaceous Period, a third major reptilian group began patrolling the Mesozoic waters. Mosasaurs were relatives of modern monitor lizards and snakes. Like ichthyosaurs and plesiosaurs, mosasaurs had tail fins and limbs modified into paddles, but the bodies and tails of mosasaurs were more elongate. Many mosasaurs are the right size to have preyed on small and medium sized fish, but some were true sea monsters with huge jaws and bodies over eighteen meters long. These aquatic giants seem adapted for deep-sea big-game hunting, and they likely ate large fish and other marine reptiles. A Wealden scene: the theropod Neovenatorattacks a pair of Iguanodon, while the small heterodontosaurid Echinodonflees; the early ankylosaur Polacanthusis well protected by its armour. Art by Jan Sovak.
University of Alberta - PALEO 200/201 11 Meanwhile in China, the coelurosaurian theropods show a wide variety of feathery integuments. A pair of the Sinosauropteryx(to the right; these are compsognathid theropods) chase a small mammal, a pair of Caudipteryx(center; these are oviraptorosaurian theropods) display their tail feathers, and a pair of Microraptor(to the left; these are dromaeosaurid theropods) glide using their four wings. Art by Jan SovakThe Late Cretaceous Period - 100 to 65 million years agoOften considered the apex of non-avian dinosaur diversity, many of the most famous dinosaurs come from this period of time. The coelurosaurian theropods are abundant and diverse in the northern hemisphere, and include the tyrannosaurs, ornithomimids, therizinosaurs, oviraptorosaurs, dromaeosaurids, troodontids, and many more interesting clades. The ankylosaurs have diverged into two groups, the tail-clubbed ankylosaurids and the clubless nodosaurids. Ceratopsians and hadrosaurs are the dominant large herbivores in the northern hemisphere. Only a single lineage of sauropods remains, but the titanosaurid sauropods are the dominant herbivores in the southern hemisphere. Pachycephalosaurs are only known from the Late Cretaceous. The first flowering plants evolved. At the end of the Cretaceous, a large meteor collided with the earth, and this event along with its catastrophic consequences is thought to have brought about a mass extinction, which killed all non-avian dinosaurs. We'll go into more detail about the end Cretaceous extinction in lesson 12.
University of Alberta - PALEO 200/201 12 The Horseshoe Canyon Formation of Alberta: the tyrannosaurid theropod Albertosauruschase the ceratopsian Arrhinosaurus; to the right, the ankylosaurid ankylosaur Anodontosaurusbattles another Albertosaurus. Art by Jan Sovak. Supplementary Materials. The Evolution of Life in 60 Seconds[video] Dinosaur Tracking –Dinosaur turnover.[blog post] Everything Dinosaur –Palaeocommunities in the Upper Campanian strata of Alberta. [blog post] Smithsonian.com –Where have all thesauropods gone?[blog post] British Museum of Natural History –Dino Timeline. [interactive timeline of dinosaurs –explore!] University of Alberta: UAlberta provides a copy of Pteranodonto American Museum of Natural History. [Video] Optional: Royal Tyrrell Museum Speaker Series –Revisiting the Horseshoe Canyon Formation: Why Drumheller Rocks. [Lecture]
Lesson 10: Palaeogeography and Plate Tectonics Written by W. Scott Persons, Victoria Arbour, Matthew Vavrek, Philip Currie, and Eva Koppelhus Learning objective for lesson 10: Students will understand basic concepts of plate tectonics and the evolution of the Earth's surface Learning objective 10.1: Summarize the evidence for plate tectonics. Today, the Earth has several continents: North America and South America, Europe and Asia, Africa, Australia, and Antarctica. In addition to these continents are large islands like Greenland, New Zealand, Madagascar, and the southeast Asian islands. However, if you were to travel back in time to the Permian period and observe the Earth from a distance, you would immediately notice a huge difference. Instead of multiple continents, only a single, enormous continent was present on Earth at that time. This was a supercontinent called Pangaea. How do we know that there was only one continent at that time? And how did the continents change positions through time? In 1912, a German researcher named Alfred Wegenerdrew the scientific community’s attention to several curious facts. Wegener had noticed that the eastern coastline of South America and the western coastline of Africa looked like two connectable puzzle pieces, that the fossils of many ancient animals (which, as far as anyone could tell, were not animals that would have been capable of swimming across the Atlantic Ocean) could be found in both South America and Africa, and that several geologic formations in South America had seemingly identical twins in Africa. Wegener suggested that Africa, South America, and possibly other continents had once been connected and had since drifted apart. Wegener’s reasoning was sound and his evidence was tantalizing, but his theory of continental drift had a huge hole in it: Wegener could not offer a convincing mechanism for how land masses as big and as seemingly immobile as continents could move. Many years later, Wegener’s idea of moving continents was vindicated, and an explanation for how such a massive phenomenon occurs was discovered.Below its surface, the earth is not a uniform mass of rock. The outermost layer of the earth consists of the continents and ocean basins and is called the crust. The thickness of the crust varies but is usually between 5 and 25 kilometers deep. By comparison to the other layers of the earth, the crust is thin. Below the crust is a layer called the mantle. The mantleis a layer over 2,500 kilometers deep. The uppermost portion of the mantle is solid. Along with the crust, this upper solid portion of the mantle is called the lithosphere. The
University of Alberta - PALEO 200/201 2 lithosphere is not one unbroken layer, but is actually composed of many discrete pieces, or plates, that fit together. Below the lithosphere is a portion of the mantle called the asthenosphere. While the lithosphere is rigid, the asthenosphere is viscous, slowly flowing, and its shape may be deformed under the uneven weight of the lithosphere. Although it flows, the mantle is not a liquid, but a viscous solid that flows. The intense heat and pressure at great depths causes the solid mantle to behave like a fluid –similar to plasticine or play-doh that is a solid at rest, but that squishes when you squeeze it. Below the mantle is the core. The core is primarily composed of iron and nickel and is subdivided into the outer core and the inner core. The outer coreis molten liquid, while the inner coreis a solid ball. The temperature of the inner core is estimated to be roughly 5,700° C (the same as the surface temperature of the sun). Stylized diagram showing the layers of the Earth –note that the layers are not to scale! By V. Arbour. The extreme heat of the inner layers of the earth creates convection currents in the viscous asthenosphere. Lower portions of the asthenosphere slowly heat, expand, rise upwards, and then slowly cool and sink. Plates, or pieces of the lithosphere, are affected by these currents. The currents pull along the undersurfaces of the lithosphere’s various pieces, causing them to slowly move. Additionally, the cool crust is more solid and dense than the layers below it. This causes lithosphere plates to slowly sink and to melt into the lower layers. This sinking does not happen all at once, but occurs gradually along one of the edges of a plate. As one edge sinks, a small gap is created along the opposite edge, and, through this gap, molten rock is free to escape. This rock then cools, solidifies, and adds its own mass to the edge of the plate. This cycle continues and, ever so slowly, the newly erupted rock will eventually progress to the sinking edged and be melted once more. The movement of the lithosphere is called plate tectonics, and it provides the explanation for the drifting continents that Alfred Wegner theorized. Plate tectonics has now been verified in a variety of ways. The discovery of mid-ocean ridges revealed plate edges where new crust was being formed. Studies of mid-ocean ridges show that the crustal rocks on either side of the ridges have indeed been slowly drifting apart. Advanced global positioning satellites tracking systems can detect the ongoing movements of the continents and even record their speeds. As plates move, they sometimes come into conflict and collide. The boundary where two plates collide can be a place where tremendous pressure builds. Such plate boundaries are often sites of sudden pressure releases, in the
University of Alberta - PALEO 200/201 3 form of volcanoes and earthquakes, and/or of gradual pressure releases, which can slowly build mountain ranges. Learning objective 10.5 –Identify palaeogeographic features. and Learning objective 10.6 –Classify types of dinosaurs based on the geographic area where they were most common. Due to the actions of plate tectonics, Earth during the Age of Dinosaurs was different from what it is today. By the end of the Permian period and the beginning of the Triassic period, all the world’s continents had collided together and formed the single supercontinent Pangaea. This meant that all the world’s oceans were also one. We call this single super-ocean Panthalassa. Because Pangaea was a single unbroken land mass, the first dinosaurs that appeared during the Triassic were able to spread across the entire planet, with no major sea barriers standing in their way. For this reason, during the late Triassic and early Jurassic, dinosaurs all across the world are fairly similar. Prosauropods and small theropods similar to Coelophysisare found worldwide. Massospondylus, a typical prosauropod, by Joy Ang.The Late Permian. All of the continents have collided to form the supercontinent Pangaea. Prosauropods are found worldwide. Paleomaps throughout this lesson are by R. Blakey and used with permission.
University of Alberta - PALEO 200/201 4 This similarity continued into the Jurassic. For example, the 'classic' dinosaurs from the Jurassic Morrison Formation of the western USA have very similar counterparts in the Tendaguru Formation of Tanzania and the Lourinha Formation of Portugal. The first true sauropods appeared very late in the Triassic, alongside their prosauropod relatives. During the Early Jurassic, while all the continents were still connected, sauropods rose to new heights, surpassing prosauropods in both abundance and body-size. Among the thriving Jurassic long-necks were the diplodocids. Even compared with other sauropods, most diplodocids have extremely long necks. They are also characterized by front legs that are much shorter than their hind legs, and by their unusual faces. The skull of a diplodocid is elongated and resembles the general shape of a horse’s or a deer’s. Diplodocid teeth are simple, peg-like, and are positioned only at the front of the mouth, not on the sides. They are nipping teeth -- good for cropping off leaves and other tender growth. Diplodocids shared their Jurassic world with another group of sauropods called the macronarians. Macronarians do not have the whip-tails of diplodocids. Their bodies are generally more robust, and their front legs are usually not noticeably shorter than their back legs. In fact, in macronarians like Brachiosaurus and Giraffatitan the front legs were much longer than the back legs. Most macronarians still have the long necks characteristic of sauropods, and they too filled the ecological niche of high browsers. A niche is an animal’s way of life. Think of it like the animal’s job in the ecosystem –it’s how a particular species makes its living, what it must do to survive. With so many kinds of sauropods living side-by-side in the Late Jurassic, it might seem like the niche of high browser would have been filled many times over and that sauropods would have faced excessive competition for their food resources. But that was not the case. Consider the macronarian Camarasaurus and the diplodocid Diplodocus (the namesake of the group). The bones of both these sauropods have been found side by side in many fossil quarries from the Morrison Formation, in the American West. In comparison, the snout of Camarasaurus is much shorter, and its teeth are not limited to the front. In fact, the teeth of Camarasaurus line the entire jaw, and the individual teeth are not simple pegs. They are broad, robust, and look like the heads of spoons. While Diplodocus has the mouth of a selective nipper, Camarasaurus has the mouth of a powerful muncher. Camarasaurus, at the Royal Tyrrell Museum. Diplodocids were adapted to reach high and prune off the most delectable Jurassic foliage, while macronarians were less picky eaters. They
University of Alberta - PALEO 200/201 5 could crunch much harder, even woody, vegetation, and they could eat what the diplodocids left behind. Thus, these two rather similar animals avoided direct competition for food resources. This is an example of a common ecological phenomenon called niche partitioning. Grazing and browsing in the shadows of sauropods were a variety of smaller Jurassic herbivores. Among them were the thyreophorans, a group that includes the ornithischians with body armor. By far the most well-know of the Jurassic thyreophorans was Stegosaurus. Stegosaurs were a widespread group of thyreophorans in the Jurassic, and their fossils have been found in Africa, Asia, Europe, and North America. Another group of common Jurassic ornithischians were the ornithopods. Small ornithopods had long legs and appear to have made up for their diminutive size with speed, earning themselves the nickname “Jurassic gazelle”. A few Late Jurassic ornithopods obtained greater size, like Camptosaurus -- an early iguanodont. The Jurassic niche of big predator was filled by an array of carnivorous dinosaurs. There were giant megalosaurids and ceratosaurids, both ancient lineages of theropods. But the Late Jurassic was a time of predatory change. A new group of big carnivorous dinosaurs had evolved, and they were mounting an ecological takeover. The allosauroids were different from the big predators that had come before them. Allosauroids have vertebrae that interlock more rigidly, so their spines were held stiffer. Their legs are also proportionately longer, suggesting that they were faster than either megalosaurids or ceratosaurids. The allosauroid Allosaurus is known from more fossil skeletons than any other big theropod dinosaur, and it was clearly among the most successful of the Late Jurassic’s predators. Allosaurus, by R. Bugeaud. Not all the Jurassic carnivores were big. The chicken-sized theropod Compsognathus is among the smallest of all known dinosaurs. Like Allosaurus, Compsognathus has a more rigid spine and long legs, but it belongs to another theropod group. It is a coelurosaur. Coelurosaurs are characterized by a long series of sacral vertebrae, narrow hands, and tails with back halves that are skinny, stiff, and lightweight. Allosauroids might have been the biggest predators around at the time, but in the Jurassic, it was the coelurosaurs that spawned the dinosaurs’ greatest success: birds. And, in the Cretaceous, some coelurosaurs would evolve their way to the very top of the food chain. Compsognathus, by R. Bugeaud.
University of Alberta - PALEO 200/201 6 During the Jurassic, Pangaea began to split into two massive continents. Laurasiawas the northern of the two and was composed of what we today call North America, Europe, and Asia (excluding India). Gondwana includes today's Gondwanawas the southern of the two and was composed of what we today call South America, Australia, Africa, Antarctica, Madagascar, and India. Later, Laurasia and Gondwana also split into smaller continents, but the continents did not assume their modern positions until long after the dinosaur extinction. As the continents drifted apart, so too did the populations of dinosaurs. Some groups went extinct in Laurasia or Gondwana, and some groups diversified. By the Early Cretaceous, there were significant regional differences among the world’s dinosaurs. Iguanodontians, ankylosaurs, and brachiosaurid sauropods were present in North America and Europe. In Africa, the dominant theropods were the spinosaurs and carcharodontosaurids. In Asia, coelurosaurian theropods became common, and the first ceratopsians evolved. The Early Cretaceous; spinosaurids in Gondwana, ankylosaurs and iguanodonts in Laurasia. (Map by R. Blakey, dinosaurs by J. Sovak).
University of Alberta - PALEO 200/201 7 By the Late Cretaceous, Gondwana had begun to break apart into its constituent continents, but Antarctica and Australia remained connected until close to the end of the Cretaceous. Sauropod dinosaurs went extinct in Laurasia, but thrived in Gondwana. However, they were not the same sauropods that populated the Jurassic. The diplodocids had gone extinct during the beginning of the Cretaceous, and although the macronarians survived, the brachiosaurid macronarians did not. Instead, a new type of macronarian dominated: the titanosaurs. Titanosaurs are the most robust of all sauropods. Their chests are broad and their hips are wide. Their hind limbs are spaced far apart –giving them a very stable base. Many titanosaurs had osteoderms and some even had large spiky armor. Titanosaurs ranged in size, but among their ranks were animals like Argentinosaurus –a sauropod that has been estimated to weigh over a hundred tons, making it the largest creature to ever walk the earth. With armor and sheer size to protect them, titanosaurs were not easy prey, but one group of Late Cretaceous theropods may have been specialized giant slayers. The carcharodontosaurs are named for the shape of their teeth, which resemble those of the Carcharodon –the great white shark. Carcharodontosaurs are a type of allosauroid, so they are descendants of the big theropods that first rose to prominence in the Late Jurassic. However, carcharodontosaurs differ from older allosauroids in a number of ways. Most noticeably, carcharodontosaurs have bigger heads, with longer jaws. As Late Cretaceous titanosaurs got bigger, so did carcharodontosaurs. The largest of all was the South American Giganotosaurus. At over thirteen meters in length, Giganotosaurus even out sized Tyrannosaurus rex. There was room for more than one kind of big carnivore in the Late Cretaceous of South America. Abelisaurs, like the famous horned species Carnotaurus, were the last survivors of the ceratosauroid lineage, and some grew to over eight meters in length. In the Cretaceous, the group was strictly limited to Gondwana, but they evidently thrived there, as abelisaur fossils have been found throughout the southern hemisphere.Giganotosaurus, by R. Bugeaud.
University of Alberta - PALEO 200/201 8 Living alongside carcharodontosaurs must have been tough, and the need for ecological niche partitioning drove abelisaurs to adapt a strikingly different morphology. While carcharodontosaurs have long jaws with big teeth, abelisaurs have short muzzles and proportionately tiny teeth. While carcharodontosaurs tended to have powerful forearms with large hooked claws, abelisaurs had ridiculously short and stubby arms, with small claws. And, while carcharodontosaurs likely preyed on huge titanosaurs, abelisaurs are thought by many paleontologist to have hunted the smaller species of titanosaurs and other less daunting herbivores. Carnotaurus, by R. Bugeaud. Hadrosaurs and ankylosaurs were rare in South America, but a few species found in the latest Cretaceous suggest that there was a land bridge late in the evolution of the dinosaurs between North and South America. While titanosaurs were abundant and diverse in Gondwana, they were far less common in Late Cretaceous Laurasia. In what is now Asia, titanosaurs were present, but comparatively rare components of the ecosystem. In North America, only a handful of titanosaur species are known. This relative under abundance of sauropods in the north is one of the biggest differences between the Late Cretaceous fauna of Laurasia and Gondwana, and meant that northern herbivorous niches were filled by other kinds of plant-eaters. Although stegosaurs never made it to the Late Cretaceous, another group of thyreophrans did: the ankylosaurs. In Laurasia, ankylosaurs split into two major groups. The ankylosaurids are the ankylosaurs with the famous tail clubs. Ankylosaurids also typically have large backwards-pointing horns at the rear of their skulls and a short rounded snout at the front. Nodosaurids are the second major group of ankylosaurs. They lacked tail clubs, but some have offensive weapons at the other end, in the form of large osteoderm spikes that project outwards from over their shoulders. Nodosaurids do not generally have the big skull horns of anklosaurids, and their snouts are significantly narrower and more elongated. After their start in the Jurassic, iguanodont ornithopods thrived and became common across the globe, during the Early Cretaceous. In Laurasia, a new kind of iguanodont evolved: the hadrosaurs. Hadrosaurs flourished in the Late Cretaceous and quickly became the northern hemisphere’s most successful herbivorous dinosaurs. We have more fossils of hadrosaurs and know more about hadrosaur biology than any other major dinosaur group. Among advanced hadrosaurs, there are two major groups. The lambeosaurine hadrosaurs had big crest on their heads, which can be thought of as a kind of musical instrument. Inside a lambeosaurine crest is a complex and hollow nasal passageway. Blowing air through this passage and then out the nostrils would have amplified the dinosaur’s calls. The hollow crests of lambeosaurines come in a variety of sizes and shapes.
University of Alberta - PALEO 200/201 9 The second major group of advanced hadrosaurs is the hadrosaurines (sometimes called the saurolophines). From the skulls of hadrosaurines, it is clear that they do not have the complex sound amplifying crests of the lambeosaurines. However, some hadrosaurines do still have crests. For instance, the hadrosaurine Saurolophus had a prominent, but solid, bony crest. Recently, a fossil mummy specimen of the hadrosaurine Edmontosaurus was discovered in Alberta, Canada, with a big fleshy crest, like the comb of a rooster, preserved on the top its head. This specimen revealed that at least some hadrosaurines had large crests, even though their skulls provide no record of them. Running a close second to hadrosaurs, in terms of Laurasian diversity and success, was another group of herbivorous dinosaurs: the marginocephalians. That name literally means “fringe heads” and refers to an overhanging lip of bone at the back margin of the skull. Pachycephalosaurs are one of the two major groups within the marginocephalians. The other is the ceratopsians. The first ceratopsians are a far cry from the later and famous forms, like Triceratops. Primitive ceratopsians, like Psittacosaurus, are small and bipedal dinosaurs, but they still show a few family resemblances. Like all ceratopsians, they have large beaks and small, jugal cheek-horns projecting from the sides of their face. Psittacosaurus and other primitive ceratopsians are well known in Asia, but the larger, more derived ceratopsians, like Triceratops, are known almost exclusively from North America. For much of the Cretaceous, the allosauroids were the top predators throughout Laurasia, just as they were in Gondwana. But they were rivaled by another theropod group. Coelurosaurs blossomed into the most diverse of all theropod groups, and they gave rise to the most infamous of all predatory dinosaurs: the tyrannosaurs. By the end of the Cretaceous, tyrannosaurs came to dominate the niche of alpha predators throughout Laurasia. To some extent, tyrannosaurs achieved their success by taking to a further extreme the same adaptations that had once set the allosauroids apart from their competitors; that is, tyrannosaurs evolved even longer legs and a much stiffer vertebral column. However, tyrannosaurs also evolved massive skulls with tremendous jaw muscles. The earliest tyrannosauroids, like the Asian species Dilong and Guanlong, or the European species Eotyrannus, have normal head and body proportions, and they look similar to most other small coelurosaurs. But as tyrannosaurs evolved, they grew in absolute size and in the relative size of their heads. These big heads added weight to the front half of their bodies. To compensate, tyrannosaurs reduced weight by shrinking the size of their arms and hands –culminating in the last and largest of all tyrannosaurs Tyrannosaurus rex. Eotyrannus, by R. Bugeaud. Not all Cretaceous coelurosaurs were huge and predatory. Ornithomimids are a kind of coelurosaur that evolved a body plan similar to
University of Alberta - PALEO 200/201 10 that of a modern ostrich or emu, but with long clawed forelimbs and a large tail. Another group of coelurosaurs developed a highly specialized wrist bone called a semilunate carpal. These crescent-shaped bones allowed the hand to be folded backwards at a sharp angle, and the dinosaurs that possess them are called the maniraptorans. Birds are one group of maniraptorans, and the semilunate carpals of birds allow them to delicately fold their wings when not flying. As close relatives of birds, the sickle-clawed dromaeosaurs are also maniraptorans, and so are the oviraptorosaurs. Like ornithomimids, oviraptorosaurs are a group of theropods that adapted to a mostly vegetarian life and lost their teeth in favor of large beaks. Many oviraptorosaurs had cranial crests and fans of feathers on the ends of their tails. Citipati, an oviraptorosaur, by R. Bugeaud. Perhaps the strangest of all Laurasian coelurosaurs are the therizinosaurs. Theriznosaurs are also probably the most confusing. The first theriznosaur fossil to be found was a huge claw, over 60 cm long. Paleontologists had no particular reason to think the claw belonged to a dinosaur. It certainly did not look like the claw of any dinosaur that had ever been found before. Instead, it was mistaken for the claw of a giant turtle. Even after more fossils were found, and it was clear that therizinosaurs were dinosaurs, not turtles, no one was quite sure what kind of dinosaurs they were. Therizinosaurs have small skulls on the end of long necks and hind feet with four forward pointing toes, so some paleontologists thought they might be prosauropods. Therizinosaurs also have a backwards-directed pubis and jaws with small herbivorous teeth in the back and a beak in the front, so some researchers classified them as ornithischians. Several fossil skeleton discoveries and a lot of research later, paleontologists are now in agreement that therizinosaurs are maniraptoran theropods (close relatives of oviraptorosaurs). Therizinosaurus, by R. Bugeaud. The movement of the continents does not always lead to geographic isolation –sometimes, plate tectonics brings continents
University of Alberta - PALEO 200/201 11 that were once separate back together. When this happens, dinosaurs from one region can move into another, leading to similar species in both regions. This phenomenon is called faunal interchange.The Late Cretaceous dinosaurs of Alberta are very similar to those found in Mongolia, which suggests that there was immigration between these two areas during the Late Cretaceous; Asia and North America were probably intermittently connected via Alaska at this time. Theropods are represented by tyrannosaurs, dromaeosaurs, therizinosaurs, ornithomimids, and oviraptorosaurs; ornithischians in both areas include hadrosaurs, ceratopsians, ankylosaurs, and pachycephalosaurs. Learning objective 10.6 –Summarize evidence for warmer climates during the Mesozoic. The average global climate was also different during the Age of Dinosaurs. Temperatures were, on average, much higher. This warmer global climate was largely caused by high volcanic activity, which released large quantities of carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas that holds in solar heat. The concentration of all Earth’s land masses in only one or two supercontinents may have also been a factor that contributed to the high average temperatures, because it affected the circulation of both air and water currents through the polar regions.Ocean currents are extremely important to distributing and moving heat from one part of the earth to another. Today, the Gulf Stream is an ocean current that flows from the Gulf of Mexico to the western coast of Europe. Gulf Stream water is warmed in the Gulf of Mexico and this heat is carried north as it flows. The Gulf Stream makes many European countries much warmer than places of similar latitudes on the other side of the Atlantic. This is why the maritime provinces of Canada are frigid places in winter, while Italy and Spain rarely see snow. Today a strong and cold ocean current encircles much of Antarctica, and this current helps to keep Antarctica cold. However, 70 million years ago, when Antarctica was still attached to Australia, this current had to go up and around Australia. As the current moved through more equatorial areas, it become warmer, and then, as it flowed back down, carried this heat to Antarctica. As a consequence of the high global temperatures, there were no polar icecaps or glaciers during the Mesozoic. Antarctica and Australia were located within the Antarctic Circle, and parts of North America were located above the Arctic Circle (North America was actually located further north than it is today). The discovery of lush plant fossils in polar regions indicates that the climate there must have been much warmer than today. Although it was once assumed that dinosaurs were limited to warm tropical climates, it is now known that many varieties of dinosaurs thrived in polar regions. The Early Jurassic theropod Cryolophosaurus, and the prosauropod Glacialisaurus, were discovered in Antarctica, along the Transantarctic Mountains. The small ornithopod Laeallynasaurainhabited what were then polar forests in the Early Cretaceous of Australia. The Late Cretaceous of Alaska was
University of Alberta - PALEO 200/201 12 home to a diverse assemblage of dinosaurs similar to what is found in Alberta, including the hadrosaur Edmontosaurus, the tyrannosaurid Albertosaurus, and the ceratopsian Pachyrhinosaurus. Although the climate was warmer and supported lush polar forests, the polar regions would still have experienced periods of reduced sunlight or total darkness, as they do today. Presumably photosynthesis would have been reduced during these darker periods. There is debate over whether during these periods polar dinosaurs overwintered at the poles or migrated to lower latitudes (south, for North America, or north, for Antarctica and Australia). J. Ang. The Jurassic-aged Hanson Formation of Antarctica. Cryolophosaurus(the large theropod in the foreground), small theropods, and the prosauropod Glacialisaurusall lived in the south polar region. J. Sovak.
University of Alberta - PALEO 200/201 13 Learning objective 10.7 –Summarize evidence for changing sea levels during the Mesozoic. Today, polar icecaps and glaciers hold large quantities of water, but, during the Mesozoic, this water was liquid and contributed significantly to high global sea levels. The warm climate also made the average global ocean temperature higher, which led to thermal expansion, causing the world’s oceans to further swell and rise. During the Mesozoic, sea levels were up to 250 m higher than they are today. This resulted in the flooding of vast regions of the earth, limiting the amount of exposed land and splitting areas that are now connected into isolated islands. For example, during the Late Cretaceous, much of the interior of North America was covered by a massive inland sea. This vast waterway, called the Western Interior Seaway, spread from the Arctic Ocean to the Gulf of Mexico. At various points during the Mesozoic, North America was subdivided into two separate island subcontinents –Laramidia in the west, from which the majority of North American dinosaur species are known, and Appalachia in the east, from which far fewer dinosaurs are currently known. Because of this ancient sea, Mesozoic marine fossils can be found in parts of Alberta and throughout the American Midwest, because these regions were underwater during much of the Mesozoic. In southern Alberta, marine sediments called the Bearpaw Formation have produced the remains of marine reptiles like mosasaurs and plesiosaurs, and invertebrates like ammonites (relatives of today's nautilus). Ammonite fossils by Ernst Haeckel, 1904. Supplementary Materials. Blakey Palaeomaps. The maps that you see during this week's lesson were created by Ron Blakey, a geologist in Colorado. Spend some time at his website checking out the palaeomaps for different time periods, including some maps not featured in this week's videos. Cretaceous North America:University of Colorado at Boulder - Geologic Evolution of North America: 600 Million Years to Present.[video] Whirlpool of Life - Dinosaurs of the Lost Continent[blog post]
University of Alberta - PALEO 200/201 14 Lalaeps - All Hail the Gore King[blog post] Smithsonian –Where have all the sauropods gone? [blog post] Polar Dinosaurs:Laelaps –L is forLeaellynasaura[blog post] The Field Museum - Polar Dinosaurs[video] The Field Museum –Antarctica Video Report 8, Sampling Ancient Plant Spores & Pollen[video; We haven't gotten to see much of Dr. Eva Koppelhus in this course, but she has been an important contributor to Dino101! This is one segment of a series on an expedition to Antarctica that Dr. Currie and Dr. Koppelhus participated in during the winter of 2010/2011. The entire series is really fun if you're interested in seeing what fieldwork in Antarctica looks like!]
Lesson 11: Dinosaur Origins Written by Victoria Arbour, W. Scott Persons, Philip Currie, and Eva Koppelhus Learning objective for lesson 11: Students will be able to describe the evolution of dinosaurs from non-dinosaurian archosaursLearning objective 11.1: Identify features that differentiate anapsids, synapsids, and diapsids. Recall that fenestrae are additional openings in the skull that do not house sensory organs. Usually, fenestrae provide an open area for large muscles to fill. The number and arrangement of fenestrae are key characters that are used to help classify amniotes into their major lineages. Amniotes that completely lack fenestrae are called anapsids. Modern turtles are one example, but anapsids are relatively rare today, and were more common earlier in the history of amniotes. Synapsidsare amniotes with one fenestra on each lateral side of their skull. All mammals are synapsids and so were our close reptilian ancestors, like the famous sail-backed synapsid Dimetrodon. Although it is commonly misidentified as dinosaur, Dimetrodonis more closely related to you and me than it is to any dinosaur. Dimetrodonlived during the Permian period, so it was millions of years older than the first dinosaurs. Top: the skull of a green sea turtle (Cheloniamydas), an anapsid. Middle: the skull of Dimetrodon, a synapsid. Bottom: the skull of Eoraptor, a diapsid. Photos by V. Arbour.
University of Alberta - PALEO 200/201 2 Amniotes with one set of fenestrae on the lateral sides of their skulls and one set on the top surfaces of their skulls are called diapsids. Learning objective 10.2 –Identify features that differentiate lepidosauromorph diapsids and archosauromorph diapsids . Diapsids are further subdivided into two groups, again based on fenestrae. Lepidosauromorphs (or lepidosaurs, as we'll also refer to them here) are diapsids with no additional fenestrae. Modern lepidosaurs include lizards, snakes, and tuataras. Archosauromorphs(or archosaurs, as we'll also refer to them here) are diapsids with an additional fenestra in front of each orbit (the antorbital fenestra) and an additional fenestra on the rear of the lower jaw (the mandibular fenestra). Crocodilians, birds, dinosaurs, and the extinct flying reptiles called pterosaurs are all archosaurs. Note that some lineages of archosaurs, such as modern crocodilians, have secondarily lost their antorbital fenestra and some, like the pterosaurs, secondarily lost their mandibular fenestra. This does not mean that crocodilians or pterosaurs lose their status as archosaurs, because “archosaur” is a name applied to the evolutionary lineage. As long as the ancestors of crocodilians and pterosaurs had the characters that define an archosaur (and they did), crocodilians, pterosaurs, and all other such descendants will be classified as part of this evolutionary group. Learning objective 11.3 –Identify features that differentiate pseudosuchian archosaurs from avemetatarsalian archosaurs. Top: Eoraptor, an archosauromorph. Bottom: Komodo dragon (Varanus komodoensis), a lepidosauromorph. Photos by V. Arbour. Dinosaurs, pterosaurs, and a few of their close relatives belong to a special group of archosaurs, and are known as avemetatarsalians. Avemetatarsalians are characterized by having ankles that flex like ahinge, while other archosaurs have ankles that rotate like a ball-and-socket. This adaptation gave avemetatarsalians stiffer ankles, which were better able to safely support their weight while running and were better suited to locomotion on upright (non-sprawling) limbs. The archosaurs are thus divided into two main lineages: the pseudosuchian archosaurs, which include today's living crocodiles, their ancestors, and many unusual extinct groups that we'll cover in greater detail shortly, and the avemetatarsalian archosaurs, which include dinosaurs and their immediate ancestors, pterosaurs, and birds.
University of Alberta - PALEO 200/201 3 Left: The pseudosuchian ankle condition, where the astragalus (pink) and calcaneum (blue) interlock like a peg-and-socket, rotating around each other during locomotion. Right: The avemetatarsalian condition results in a simple hinge joint. Illustrations released into the public domain by Philip Chalmers, via Wikimedia Commons. A simplified phylogenetic tree of the Amniota. Dinosaurs are dinosauromorphan, avemetatarsalian, archosaurian diapsids. As an aside: the phylogenetic placement of turtles and other anapsids, is still hotly debated; for simplicity, all anapsids have here been grouped as a single clade outside of Synapsida+Diapsida here. Remember that each node includes all of the clades above it on the tree (i.e. those that are more derived). Diagram by V. Arbour.
University of Alberta - PALEO 200/201 4 Learning objective 11.4 –Describe the characteristic vertebrates of the Permian Period and the changes in vertebrate community structure at the Permian-Triassic boundary. In the Permian (299 to 252 million year ago), all the world’s landmass was part of the supercontinent Pangaea. This single continent had an arid interior, with rapidly fluctuating temperatures and climates. The first group of amniotes to evolve large body size and to dominate the ecological roles of mega-herbivores and mega-carnivores across Pangaea was not the dinosaurs. It was the synapsids - our own lineage! Reptile-like synapsids, including Dimetrodon, became common and thrived for millions of years. Dimetrodon, one of the earliest synapsids, seems to have little in common with its living relatives the mammals at first glance. Image by Dmitry Bogdanov and used under the GNU Free Documentation License, via Wikimedia Commons. Gradually, these early synapsids become more mammal-like. Late in the Permian, large saber-toothed synapsids, called gorgonopsids,were the top predators, and synapsids, like the tusked dicynodonts,were the top herbivores. There was a diverse array of small and medium-sized synapsids, including the cynodonts. Cynodontswould go on to evolve into true mammals, and the early forms looked a little Permian synapsids. Gorgonops(top), a carnivorous gorgonopsid from South Africa, Estemmenosuchus(middle), one of the larger early synapsids at 3 m (10 feet) in length, a herbivore from Russia, and Dicynodontoides(bottom), a small herbivorous dicynodont from Africa and India. Images by Dmitry Bogdanov and used under the GNU Free Documentation License, via Wikimedia Commons. like short-legged dogs. Then, 252 million year ago, disaster struck the world of the synapsids. The end-Permian mass extinction was the most devastating extinction event in the history of life. The exact percentage of species that went extinct varies according to different researchers (think back to the problems palaeontologists can face when identifying fossil species), but palaeontologists agree that about 70% of all terrestrial vertebrate species, and 90-95% of all marine species, went extinct in a short span of time. This is a truly colossal loss of life and diversity.
University of Alberta - PALEO 200/201 5 The cause (or causes) of the end-Permian mass extinction remains uncertain. Huge lava deposits, known as the Siberian Traps, formed at this time. The volcanic eruptions that formed these deposits may have been ongoing for 200 000 years or more! These long-lasting eruptions must have released large quantities of volcanic gases into the atmosphere, leading to a greenhouse effect and increased global temperatures. Increased global temperatures may have also resulted in the melting of frozen chemicals called methane hydrates deep in the ocean, which in turn would have contributed to more global warming and even more melting of methane hydrates, and so on. Global temperatures may have increased by at least 6 degrees right at the end of the Permian. Some scientists have hypothesized that the extinction may have been brought about by a comet or meteorite impact, although a crater from such an impact has yet to be found. Whatever the cause, the end-Permian mass extinction was the single greatest extinction event ever, and it took millions of years for Earth’s ecosystems to recover. Learning objective 11.5 –Describe the characteristic vertebrates of the Triassic Period and the changes in vertebrate community structure at the Triassic-Jurassic boundary. and Learning objective 10.6 –Assess competing hypotheses for the rise of dinosaurs as the dominant terrestrial vertebrates. The synapsids had been cut down in their evolutionary prime, and this left vacant the ecological roles that they had previously filled. At first, the synapsid lineages that had managed to survive the extinction slowly rebounded and some groups re-evolved large body sizes and reassumed their roles as top predators and herbivores. Cynodonts and dicynodonts were among the synapsids that succeeded in rebounding, and it is during the Triassic that the first true mammals appeared. However, at the same time, a new group of diapsids, the archosaurs, also began to diversify and grow. Gradually, large archosaurs became more abundant, while large synapsids became less abundant. Kannemeyeria, a large Triassic dicynodont that had a nearly worldwide distribution. Image by Dmitry Bogdanov and used under the GNU Free Documentation License, via Wikimedia Commons. The first widely successful group of archosaurs was a lineage that would later go on to evolve into modern crocodilians. These crocodile-line archosaurs are called pseudosuchians. The pseudosuchians of the Triassic include the often huge and slender-snouted phytosaurs, which were semiaquatic predators like their distant crocodile relatives; the heavily armored and herbivorous aetosaurs; the rauisuchids and prestosuchians, which were terrestrial predators with upright limb posture; and the poposauroids, some of which were sail-backs
University of Alberta - PALEO 200/201 6 Non-dinosaurian archosaurs from the Triassic of Arizona: Smilosuchus(a phytosaur), Desmatosuchus(an aetosaur), and Poposaurus(a poposauroid) Images by Jeff Martz and used under the CC-BY 2.0 license, via PetrifiedForestNPS on Flickr. Silesauruswas a herbivorous, quadrupedal, non-dinosaurian dinosauromorph. It is known from the Triassic of Poland, but silesaurids are also known from many other locations worldwide. Image by Jeff Martz and used under the CC-BY 2.0 license, via PetrifiedForestNPS on Flickr. and demonstrate convergent evolution with the earlier Dimetrodon. Where were the dinosaurs? The oldest record of dinosaur-like archosaurs comes from footprints that have been dated at roughly 250 million years old. The earliest dinosaur-like archosaurs were small and bipedal and looked a lot like the true dinosaurs, but they lacked some of the specializations that characterize true Dinosauria, such as a hip socket with a hole through it –for this reason, we call these animals dinosauromorphs. Some early dinosauromorphs were quadrupedal, like the silesaurids. The best record of early dinosaur bones comes from 228 million year old fossil beds of Argentina. Eoraptor, Eodromaeus, Herrerasaurus, and Panphagiaare examples of early carnivorous saurischian dinosaurs, and Pisanosaurusis an early herbivorous ornithischian dinosaur. It seems clear that early dinosaurs were more successful and diverse as carnivores than as herbivores. Eoraptor, Eodromaeus, Panphagia,andPisanosaurus are all relatively small (under a meter in length), but Herrerasaurus was significantly larger (comparable in size to a modern tiger). Compared to the many other archosaurs, these early dinosaurs were rare components of their ecosystems. As the Triassic drew to a close, dinosaurs were gaining ground. Coelophysiswas a wolf-sized Triassic theropod that has been found in large bonebeds in New Mexico. Coelophysisappears to have been one of the most common predators of its time and place. Prosauropods evolved late in the Triassic and also were hugely successful. Plateosaurusis the best known of the prosauropods and would have weighed more than three tons. In the Triassic,
University of Alberta - PALEO 200/201 7 Eodromaeusand Eoraptorare both known from the Triassic of Argentina; Eodromaeusis thought to be an early member of the theropod lineage, and Eoraptoris thought to be an early member of the sauropod lineage. Early representatives of any given clade often look very 'primitive' and unspecialized. CC-BY 3.0 license by Conty via Wikimedia Commons. Coelophysis, a Triassic theropod dinosaur. Image by Jeff Martz and used under the CC-BY 2.0 license, via PetrifiedForestNPS on Flickr.prosauropods were record breakers, the largest herbivores that had ever evolved up to that time. Did dinosaurs suddenly appear and dominate their environment? Not at all. The story of the dinosaurs' rise to power was a slow, but steady one, pushed along by the misfortune of others. For example, after the end Permian extinction, pseudosuchian archosaurs diversified and became very common. The earliest dinosaurs coexisted alongside more primitive dinosauromorphs for some time, and many pseudosuchians had evolved dinosaur-like body plans. At the end of the Triassic another mass extinction event of unknown cause occurred. This extinction was not nearly as severe as the extinction at the end of the Permian. Still, it hit many of the thriving archosaur groups hard . . . but not dinosaurs. The story of the origin of dinosaurs is not so much one of dinosaurs conquering and defeating other groups, but rather one of chance and opportunity. Right now there isn't any evidence to suggest that dinosaurs outcompeted the pseudosuchians. Both pseudosuchians and dinosaurs were well adapted for their Triassic environments, and dinosaurs did not gradually replace pseudosuchians in their ecosystems. Instead, dinosaurs 'got lucky' –some aspect of their biology made them better able to survive the end-Triassic extinction than the pseudosuchians. The extinction left several ecological roles vacant, and dinosaurs quickly evolved to fill them. This success, at the time of Pangaea, allowed dinosaurs to spread to the far edges of every continent, without ever having to swim. As Pangaea broke apart, dinosaurs rode the plates, and different dinosaur groups had the opportunity to evolve and diversify in
University of Alberta - PALEO 200/201 8 geographic isolation. With the start of the Jurassic, the Age of Dinosaurs had truly begun. As we'll see in the next lesson, however, the odds caught up with dinosaurs in the end. Supplementary Materials. Laelaps –Lystrosaurus, the most humble badass of the Triassic[blog post] Laelaps –Poposaurus, Postosuchus, and the dinosaur mimic croc walk[blog post] Smithsonian –Why a pterosaur is not a dinosaur.[blog post] Pterosaur.net–Spend some time on this website reading about pterosaur origins, anatomy, and diversity. Smithsonian - Scientists Discover Oldest Known Dinosaur[blog post]. American Museum of Natural History –What is the earliest-known dinosaur?[video] Tetrapod Zoology –The surprising and hitherto undocumented late survival of non-dinosaurian dinosauromorphs[blog post] University of California –Rise of dinosaurs not so rapid at all.[video] The University of Chicago –Paul Sereno talks about Eodromaeusthe "Dawn Runner"[video]
Lesson 12: Dinosaur Extinction Written by Victoria Arbour,W. Scott Persons, Philip Currie, and Eva Koppelhus Learning objective for lesson 12: Students will be able to describe the end-Cretaceous extinction event, and provide examples of vertebrate groups that both persisted and died out during the event Learning objective 12.1: Compare background extinction to mass extinctions. Species that are still present today are called extant species. Species whose members have all died off are called extinctspecies. Naturally, the number of extant species is only a tiny fraction of the huge number of species that are now extinct. As environments gradually change and species evolve and compete, the extinction of some species is an inevitable result. At any time in the history of life, it is usual for some species to be going extinct. However, certain dramatic environmental changes can trigger the extinction of many species all at nearly the same time and across the entire planet. When such a sudden and global loss of species occurs, it is called a mass extinction event. Palaeontologists generally recognize five major mass extinctions. The End Ordovician mass extinction affected only marine organisms, but at that time terrestrial organisms had only just begun to evolve. The Late Devonian mass extinction was also largely limited to marine organisms, including some early vertebrate clades. As discussed in Lesson 11, the End Permian mass extinction saw the largest loss of diversity in all of Earth's history. Marine invertebrates were decimated, and this was the This figure is modified from a graph of "Family"-level diversity through time by Jack Sepkoski and David Raup published in 1982 –it is often referred to as the Sepkoski curve. Newer data has refined the graph somewhat, but the overall pattern still stands. Diagram by V. Arbour.
University of Alberta - PALEO 200 2 largest mass extinction of insects. On land, the synapsids were hard hit, as were the anapsids (which we have not discussed much in this course, but which included large and small herbivores and some aquatic species). The End Triassic mass extinction saw the extinction of most lineages of pseudosuchian archosaurs, as well as many of the synapsids that had survived the End Permian extinction, and also affected marine life. The last of the "Big 5" extinctions was the End Cretaceous. The End-Cretaceous Extinction event occurred roughly 66 million years ago and killed all non-avian dinosaurs. However, dinosaurs were not the only casualties of this extinction. In the oceans, large marine diapsids, called mosasaurs and plesiosaurs, died out, as did many varieties of corals, several forms of plankton, and ammonites (relatives of modern squids and octopi). Pterosaurs went extinct as well. Although birds ultimately survived, many types of Cretaceous birds (including hesperornithiform and enantiornithiform birds) perished. Land plants also lost many species in the extinction, and insect diversity fell. Mammals, turtles, crocodiles, amphibians, and fish all made it through the End-Cretaceous Extinction, although many of the larger species in all these groups did not. Generally, it seems that large animals and photosynthetic organisms were the most likely to die off. Small animals, and particularly those that were semiaquatic, had the best chance of surviving. However, not all groups of animals that survived the End Cretaceous extinction are still around today. Champsosaurs are a good example: these crocodile-like aquatic diapsids (completely unrelated to crocodilians, and another good example of convergent evolution at work) survived the End Cretaceous mass extinction, only to go extinct during the early Miocene (about 20 million years ago). Top, Mosasaurus, a giant marine lizard; middle, Quetzalcoatlus, a giant pterosaur; and bottom, Hesperornis, a flightless aquatic bird. Mosasaurusby D. Bogdanov and used with the GNU Free Documentation license. Quetzalcoatlusby D. Bogdanov and released into the public domain. Hesperornisby N. Tamura and used with the GNU Free Documentation license.Champsosaurus natatorin the University of Alberta Paleontology Museum. Photo by V. Arbour.
University of Alberta - PALEO 200 3 Extinct animals do not include only prehistoric species that died out millions of years ago –many species have also gone extinct more recently. In some of the most recent examples of extinction, species have been eliminated through the actions of humans. For example, the thylacine (also called the Tasmanian tiger or Tasmanian wolf) was a large carnivorous marsupial that went extinct sometime went the last individual died in a zoo in 1936. The Carolina parakeet, the only species of parrot native to the USA, went extinct in 1918. Perhaps one of the most famous examples, the passenger pigeon, went extinct in 1914 even though there were billions of passenger pigeons only a few decades prior. The extinction of these species was the result of intense hunting and habitat loss, both caused by humans. Many animals today are on the verge of extinction, with severely depleted populations. Although deliberate hunting does not play as much of a role in the depletion of modern animal populations, habitat loss and pollution are significant contributors to the extinction of different species.Based on the current rate of species extinction, many biologists have argued that the earth is presently in the middle of a sixth mass extinction event. This new mass extinction is being brought about by sudden global climactic change and large-scale ecosystem destruction and degradation (the results of human activities).Learning objective 12.5 –Describe geological features associated with meteorite impacts. In 1979, an Italian stratigrapher was studying rock layers at the boundary of the Cretaceous and Paleogene periods and noticed a strange thin layer of grey clay. Later, this same grey layer began to be discovered at the Cretaceous/Paleogene boundary all over the world and in very different formations. Close inspection of the grey clay layer revealed that it had high concentrations of iridium. Iridiumis a rare element on earth, but it is a common component of meteorites. That was not all: the layer was also rich in tektites and shocked quartz. Tektitesare tiny pieces of rock that have been melted and then cooled. Shocked quartzis a form of the mineral quartz with a unique internal structure that can only be created by exposure to a powerful shockwave, like those created by a nuclear explosion or a meteorite impact. Tektite specimens in the University of Alberta Geology Collection. By W Scott Persons.Both tektites and shocked quartz are telltale signs of a meteorite impact, but to spread iridium, tektites, and shocked quartz all across the globe would have required either an enormous shower of large meteorites or …a single tremendous meteorite impact. Could a meteorite impact have been responsible for the End Cretaceous mass extinction? The search was on for a giant crater.
University of Alberta - PALEO 200 4 For many years, no such crater was found. Then, geologists working near the town of Chicxulub in Mexico’s Yucatán Peninsula noticed a peculiar pattern of cenotes, or limestone sinkholes. The cenotes were arranged in a crescent shape many miles long. Each end of the crescent seemed to terminate at an edge of the peninsula. Investigation revealed that the cenotes were caused by a displaced portion of a limestone layer that had been pushed upwards, and that the structure did not actually end at the edges of the peninsula. Instead, it continued along the ocean floor and was actually a huge continuous ring over 180 km in diameter. Radiometric dating revealed that this massive ring of displaced rock was 66 million years old. The crater made by the meteorite responsible for showering the earth with debris at the end of the Cretaceous had been found. Based on the crater’s size, it has been calculated that the meteorite that made it must have been 10 kilometers in diameter, larger than Mount Everest. Learning objective 12.5 –Evaluate competing hypotheses for the cause of the end-Cretaceous extinction. There have been many ideas put forward to try to explain the cause of the End-Cretaceous extinction. Some of these ideas are more plausible than others. It has been suggested that dinosaurs went extinct because small mammals began eating all of the dinosaurs’ eggs. Not only does this idea not take into consideration the fact that mammals and dinosaurs evolved at roughly the same time (and, therefore, dinosaurs had been successfully coexisting with small mammals for over 160 million years and laying eggs all the The pattern formed by the limestone sinkholes called cenotes formed part of a ring. Geological imaging revealed a huge impact crater! Images by NASA, in the public domain.while), but it also fails to explain why so many other kinds of organisms died out at the same time. It is commonly and incorrectly thought that the Cretaceous period was immediately followed by an ice age and that widespread glaciations and freezing temperatures were responsible for the
University of Alberta - PALEO 200 5 dinosaurs’ demise. While average global temperatures did fall after the Cretaceous, this temperature fall was gradual, and it was millions of years before a true ice age resulted. Some hypotheses are very unlikely, and would be almost impossible to test scientifically. Sometimes you may see reports in the media about dinosaurs generating too much methane gas from their digestive systems, thereby causing climate change –these kinds of news items usually are based on mathematical estimates that make a lot of assumptions about dinosaur physiology and ecology. Other ideas, like that the dinosaur's demise was caused by a nearby supernova or a viral outbreak, leave little evidence in the fossil record. And remember, whatever caused the extinction needed to kill not just a few species of dinosaurs, but all of the dinosaurs, and all of the other disparate organisms that went extinct at the same time. A mass volcanic outgassing of carbon dioxide and ash plumes has also been suggested as a possible cause of the extinction. This scenario could potentially have affected the global climate enough to have caused the extinction of many kinds of organisms, and there is a record of high volcanic activity in the Deccan Traps of India at the end of the Cretaceous. The current prevailing theory for the cause of the End-Cretaceous extinction is more cosmic. Without a doubt, a very large meteorite struck the Earth in the Yucatan peninsula at about the same time we see a mass extinction in the fossil record. But how could a single meteorite impact, even a massive one, have killed off so many kinds of animal that lived all across the globe? The theory goes like this: The initial impact caused huge tsunamis and sent a great cloud of super-heated rock and dust high into the atmosphere. The rocks and larger pieces of debris quickly fell to earth and started wildfires. Smaller pieces of debris next began to fall and, as they fell, were heated by air friction. This rain of hot dust raised global temperatures for hours after the impact and cooked alive animals that were too large to seek shelter. Small animals that could shelter underground, underwater, or perhaps in caves or large tree trunks, may have been able to survive this initial heat blast. Finally, much debris would have remained in the atmosphere for perhaps months or even years. The residual haze would have reduced sunlight, killing many plants and other photosynthetic organisms, with rippling effects up the food chain. Some scientists estimate that photosynthesis may have stopped for at least a decade –try to imagine the world surviving without any plants for years at a time! The reduced sunlight may also have brought on a sudden drop in global temperatures. Being large active animals with high energy needs and positioned at the top of prehistoric food chains, dinosaurs were highly susceptible to this series of catastrophes. Smaller, omnivorous terrestrial animals, like mammals, lizards, turtles, or birds, may have been able to survive as scavengers feeding on the carcasses of dead dinosaurs, fungi, roots, and decaying plant matter. , while smaller animals with lower metabolisms were best able to wait the disaster out. The meteorite impact scenario is support by good geological evidence (the impact crater of the right age, and the presence of iridium, tektites, and shocked quartz), and seems to be a reasonable explanation of the patterns of extinctions observed in the fossil record. Many animals would have died in the moments and hours after the Chicxulub impactor struck the earth, either as a result of the shockwaves,
University of Alberta - PALEO 200 6 The Age of Dinosaurs has come to an end: in the years following the Chicxulub impact, photosynthesis is reduced, and large herbivores have starved. With no more carcasses to scavenge, a lone Tyrannosaurusdies at the edge of a lakeshore. Its body will be scavenged by members of the two remaining archosaur lineages: birds and crocodilians. By Jan Sovak. tsunamis, forest fires, or heating of the atmosphere. In the months and years that followed, food chains collapsed as photosynthesis halted. Animals like turtles, champsosaurs, crocodilians, and small mammals and birds could find shelter immediately after the impact, and were not at the top of the food chain, and therefore were more likely to survive. Unfortunately, most dinosaurs were neither of these things –they were the dominant terrestrial herbivores and carnivores, and most (but not all) were too large to find shelter during the impact. Today the dinosaurs are represented only by their descendents, the birds. Learning objective 12.8 –Assess the likelihood that dinosaurs could be brought back from extinction. Will human eyes ever see a living breathing Tyrannosaurusor Triceratops? For those of us living here in the twenty first century, it does not seem likely. You may be familiar with a certain Hollywood franchise that popularized the science fiction premise of cloning dinosaurs from discoveries of their DNA. Unfortunately, DNA is a delicate substance that quickly breaks down over time. It is extremely unlikely that a complete or nearly-complete DNA strand could ever be preserved (even inside the body of a mosquito
University of Alberta - PALEO 200 7 stuck in amber) for 66 million years or more. Some recent discoveries of blood vessels in the Late Cretaceous dinosaurs Tyrannosaurusand Brachylophosaurussuggest that soft tissues and potentially even proteins may be able to survive this long. But such material is still a long ways from what would be needed to even consider cloning a dinosaur. Even if dinosaur DNA was found intact, cloning is a difficult process. Scientists have not yet been able to successfully clone even recently extinct animals and have the clone survive for more than a few minutes. It might be possible to find more recent dinosaur DNA. Remember that birds are one branch of the dinosaur family tree. As such, the DNA of birds contains many of the DNA sequences of their ancestors (but with many of these genes switched off). It has been proposed that a dinosaur could be resurrected by hatching a bird with its advanced DNA sequences turned off and its ancient ancestral sequences turned back on. In this way, perhaps a bird would develop with a long bony tail, teeth, and clawed fingers. But, for the moment, performing such genetic manipulations is well beyond our understanding and technology. Supplementary Materials. Science@NASA –What exploded over Russia?[Video] Recent asteroid impacts visualization–asteroid impacts are surprisingly common! Laelaps –Dinosaurs had the worst luck. [Blog post] Laelaps –Planting the Cenozoic garden. [Blog post] Smithsonian –The top 10 weirdest dinosaur extinction ideas.[Blog post] Io9 –How to survive a mass extinction.[Blog post] Earth Unplugged - Woolly Mammoth: Back from extinction.[Video] TED Talks –Building a dinosaur from a chicken.[video] This one is just for fun: What would have happened if the dinosaurs didn't go extinct? You might be surprised to learn that palaeontologists have given this thought experiment a try a couple of times! Tetrapod Zoology - Dinosauroids Revisited, Revisited.