Here's a new pair of animals to think about. These are Spinosaurus and Ouranosaurus, both of which have long processes on the vertebrae of their backs that give them a hump, or sail-like structure. If we're trying to group and classify groups of dinosaurs, we might think that having a sail was a shared derived character. And groups Spinosaurus and Ouranosaurus together. But there are some really big differences between Spinosaurus and Ouranosaurus. Spinosaurus has sharp teeth and a three finger hand, and other features that shows that it is clearly a theropod dinosaur. Ouranosaurus has a spike-like thumb and a dental battery, and was an iguanadont ornithischian. The sale of Spinosaurus and Oranosaurus is something called a convergent feature rather than a shared, derived character. These two species did not possess a sail because it was present in their last common ancestor. Instead, each species evolved the sail independently. Convergent evolution is an interesting process that happens when animals are under similar selective pressures. Maybe it's the environment that they live in, or the food they eat. Here are some great examples of convergent evolution. Fast swimming marine predators have a streamline body, like this Ichthyosaur I have here. This is also the case in other animals like the tuna, shark, and dolphin. The pangolin and armadillo eat termites or ants and have a long snout, reduced teeth, a long, sticky tongue, and strong forelimbs for opening burrows. Neither of these two animals are closely related to one another. Animals that live underground and burrow have a short body, small eyes and broad hands. Examples include the mole, and the marsupial mole. Neither of these species are closely related, even though they are both called moles. Wings are an excellent example of convergent evolution. In order to fly, you need wings. Birds, bats, and pterosaurs all evolved wings in very different ways. Bats have a membrane that goes between all five fingers. Pterosaurs also had a wing membrane that was supported by just one finger. Well, birds use feathers to fly. Sometimes we can recognize convergent features easily if we look carefully at the anatomy. It's easy to see birds, bats, and pterosaurs had very different wing structures. Sometimes anatomical distinction is not as easy to see. When we think about the moles, moles and golden moles were thought for a long time to belong to the same clade of mammals. However, more recent research has shown that golden moles are more closely related to elephants than true moles. In all of the cases mentioned in this lesson, similar features evolved in unrelated groups. This happened because similar selective pressures existed, not because the animals shared a common ancestor with that feature. Based on what you know about the relationships of modern animals, mark each of the following as convergent or shared. Bipedalism in humans and birds. Five fingers in humans and bats. Fins in fish and whales. Bipedalism has evolved separately in humans and birds, so it is convergent trait. Both humans and bats and most other mammals have five fingers in their hands. So that trait is shared. Whale flippers are more similar to our hands than they are to the fins of fish. So once again, that trait is convergent. How do we know if a similar-looking structure in two different species is the result of shared ancestry or convergence? We used a technique called Phylogenetic Analysis which creates a family tree of species. This family tree diagram is called a phylogenetic tree or cladogram. When we are looking at a number of species and we want to place them in a cladogram, we assemble a dataset called a character matrix that contains lots of anatomical details of the species we are studying. The anatomical features are called characters and the differences in the anatomical features are called character states. For Spinosaurus and Ouranosaurus we might a character called length of the dorsal vertebra neural spine. The character state could be long for these species, and short for other species. We can easily make other characters. Tooth shape, bipedal or quadrupedal, number of holes behind the eye in the skull, and so on. Phylogenetic analyses work best when there are lots and lots of characters. Let's go through the process of identifying some character states for Protoceratops. Can you correctly identify character states for the following three Protoceratops characters? Brow horns. Beak. Frill. This ceratopsian has a beak and a frill. So, those character states are present. But it doesn't have any brow horns. That character state is absent. Using these same features, as well as numerous others, We can define character states for other ceratopsians. By doing this we can get an idea of what kinds of adaptations evolved at certain times during the evolution of horned dinosaurs. Phylogenetic trees are constructed using the principle of parsimony. Parsimony is the idea that, all other things being equal, the simplest answer is usually the right one. You might hear this referred to as Occam's Razor. In evolutionary terms, what this means is that the family tree with the fewest number of evolutionary changes is probably the right one. It is an interesting challenge to develop a phylogenetic tree with the fewest number of changes. We use a computer program to help us develop phylogenetic trees. As I've mentioned before a phylogenetic tree is a branching diagram showing the relationships between species. Interpreting these diagrams can be a little tricky at first. The important parts of the tree are the nodes and branches. Nodes show us the point at which a species shared a common ancestor. Branches show us how the descendants of that common ancestor diverged from one another to become unique. Using this phylogenetic tree, would you say that pachycephalosaurs are more closely related to ceratopsians or to hadrosaurs? Pachycephalosaurs are more closely related to ceratopsians. They branched off from one another after their common ancestor branched off from the Hadrosaur lineage. So, A is the right answer. Let's look at our sail-backed animals, Ouranosaurus, and Spinosaurus. If we put these animals back on our generalized tree of dinosaur relationships, Ouranosaurus would be over here with the ornithopods, things like Hadrosaurus and Iguanodons. Spinosaurus would be over here with the theropods, or the meat eating dinosaurs. We can see that the sail probably evolved at least twice, just within dinosaurs. In order for the sail to have been a shared derived character, it would have had to have been present in the common ancestor of all dinosaurs and then lost, possibly hundreds of times, in everything, except Spinosaurus, and Ouranosaurus. Another hypothesis could be that Spinosaurus is a weird carnivorous hadrosaur that is more closely related to Ouranosaurus because of the sail. But then we would have to argue that the sharp teeth, absence of predentary bone, pelvis structure, and many other features of Spinosaurus were convergent with theropods, rather than the result of shared ancestry. It is simpler, or more parsimonious, to hypothesize that one feature, the sail,was convergently evolved in Spinosaurus and Ouranosaurus, rather than numerous features being convergent between Spinosaurus and other theropods. This is why we use many characters, rather than just one, to determine evolutionary relationships between species.
