We've spent a lot of time so far in this course talking about the things that make dinosaurs different from one another. For example, we've talked about anatomical differences, body coverings, eating behaviors, various locomotion styles, and so on. Dinosaurs evolve from a single common ancestor species. So, how did the different lineages evolve and differentiate from one another? What groups of dinosaurs share certain characteristics and might be more closely related to one another? In this lesson, we are going to discuss dinosaur evolution and dinosaur evolutionary relationships. The Theory of Evolution was first outlined by the British Naturalist, Charles Darwin in 1859. Since Darwin's time, our understanding of the evolutionary processes has deepened considerably. However, the basics of the theory have remained unchanged. Although evolution is commonly misunderstood and misrepresented, the theory is actually quite simple. Darwin's big idea elegantly explains how new species arise. Not from a mysterious or unobservable source, but from the inevitable interactions of three basic characteristics of life. First, if unchecked, organisms produce more offspring than can ultimately be sustained in any environment. Second, organisms tend to resemble their parents. This is because parent organisms pass on some or all of their genetic code to their offspring. Third, occasionally, organisms are produced that have novel traits or novel trait variations. Although Darwin, did not know it, one source of these novelties is genetic mutation. Now, here is how the theory of evolution puts these three basic principles together. Because more organisms are produced, they can survive in an environment some organisms do not survive. This means that organisms of different species must compete for resources. Similarly, organisms of the same species must compete for resources amongst themselves. Most of the time, organisms that have a genetic novelty will be at a slight competitive disadvantage. Because most mutations are detrimental. But every so often, one of those novelties will be an advantage. That means, the organism with the novelty will be more likely to outcompete the organisms that don't have it. And the advantaged organism will have a better chance of surviving and reproducing. Now comes the critical part, because of inheritance, the offspring of the organism with the advantageous genetic novelty are likely to take after their parent. In which case, the offspring who inherit the advantageous novelty will also have a better chance of surviving and reproducing. Eventually, organisms with the advantageous novelty may come to dominate the entire population. This process will naturally repeat itself with more and more advantageous traits, building up in the population. Eventually, enough traits will accumulate. The genetic code of the population will be so different from its ancestors that it will constitute a new species. That's evolution in a nutshell. It is often incorrectly said that evolution is a random process. Just the opposite is true. While genetic mutations may occur randomly, the evolutionary process that weeds out disadvantageous mutations and favors advantageous mutations is a selective process. When we've been talking about dinosaur groups, we've often focused on what makes them different from one another. But we can also look at it from another angle. What do certain species share in common? Lambeosaurus, Corythosaurus, and Parasaurolophus are three Hadrosaurus from the Dinosaur Park Formation of Alberta. The crest has a different shape in each of them. Lambeosaurus has a double crest, while the similar-looking Corythosaurus has a single crest. And Parasaurolophus has a single, elongate crest. But in all three skulls, the crest is formed from the same bones and the crest is hollow, which is characteristics of lambeosaur and hadrosaurs. All the skulls have a wide beak and modified dental battery, characteristic of hadrosaurs. And all skulls have the predentary bone present in ornithischian dinosaurs. Here are the heads of a number of dinosaurs. We're going to draw some circles to show certain groupings. The ankylosaur, euplocephalus, ceratopsian, triceratops, and all of the hadrosaurs have a predentary bone. So we can draw a circle around them. The ankylosaur and ceratopsian don't have the broad beak of the hadrosaurs, so we'll draw a circle around the hadrosaurs and leave the other ornithischians out. Three of the hadrosaurs have a crest, but one doesn't. So we can draw a circle around the crested hadrosaurs. These final three hadrosaurs are called Lambeosaurine Hadrosaurs. Each of them has a hollow crest on the top of its head. The hollow crest of the Lambeosaurine Hadrosaurs is something we can call a shared derived character. A character is a feature that we can describe and label, and has to be heritable. A shared derived character is a feature that is present in two or more species and their common ancestor, but not more distant relatives. Another word for shared derived character is synapomorphy. What is a shared derived character of the circle that includes the ankylosaur, ceratopsian and hadrosaurs but not the theropod? Is it A, large horns and armor, B, a dental battery, C, rear-pointed pubic bone, or, D, fenestra in front of the orbit of the skull? Well, all these features are shared by at least two of the dinosaurs. It is the hip structure, the rear facing pubis that the ankylosaur, ceratopsian, and hadrosaurs all share although theropod does not. In other words, every dinosaur inside the circle is an ornithischian or bird-hipped dinosaur as we learned back in the first module of this course. The Tyrannosaurus is a Saurischian or lizard head dinosaur. So we've drawn circles around different groups of dinosaurs, allowing us to focus in on species with certain shared derived characters or synaphomorphies. Paleontologists use the same method of looking for synaphomorphies to classify different groups of dinosaurs. To explore this concept further, let's take a look at some ceratopsians, a major group of ornithischian dinosaurs. Here, we have Centrosaurus, Chasmosaurus, Triceratops, Styracosaurus, Protoceratops, and Liaoceratops. The frill and pointed beak are each a shared derived character of all the ceratopsians. We'll start by circling the whole group. Horns on the face are a shared derived character of a subset of the ceratopsians, Chasmosaurus, Triceratops, Styracosaurus, and Centrosaurus. We'll put a circle around these four. But the shape of the horns are not the same in all of them. Chasmosaurus and Triceratops have short nose horns and long eyebrow horns. Centrosaurus and Styracosaurus have short eyebrow horns and long nose horns. We'll circle each of these two groups. Protoceratops and Liaoceratops do not have horns on their faces. But they do have frills and pointed beaks. Protoceratops has a longer frill than Liaoceratops. We'll leave these two animals out of the facial horn subset. Liaoceratops is from the early Cretaceous of China. This was a small bodied, early ceratopsian that lacked horns. But it does have a few features that we find in all ceratopsians. The back of the skull is expanded into a shelf, which in later ceratopsians becomes a frill. Over time, the frill evolved into a larger and larger structure like what we see in Protoceratops known from the late Cretaceous of Mongolia. Like Liaoceratops, it doesn't have horns, but you can see how the frill is larger with a pair of openings called fenestrae. Later in their evolution, ceratopsians evolved a nose horn and small bones on the edge of the frill. We call ceratopsians with a nasal horn and these small frill bones ceratopsid ceratopsians. A group of species that shares a common ancestor is called a clade. The nasal horn is a synapomorphy or shared derived character of the ceratopsid clade. Let's spend some more time on this concept of synapomorphy. What is a shared derived character or synapomorphy of pachycephalosaurs? Is it bipedalism? A thick, domed skull? Or an ornithischian hip arrangement? Although pachycephalosaurs have all of these features, it is only the thicken, domed skull that all pachycephalosaurs share. Other dinosaurs do not have this characteristic. Some of the features such as bipedalism and bird-like hips are shared with other dinosaurs and are not exclusive to this group. So the correct answer is B, a thick, domed skull.
