Now that we know the critical features of bird anatomy, we're ready to see how these features first evolved. Let's begin our trek through the dinosaur family tree. Here's Dr. Phil Currie to tell us about the oldest Saurischian dinosaurs. >> The oldest known dinosaurs come to us from Argentina. They include these two fellows, Herrerasaurus and Eoraptor. These dinosaurs are from the Triassic period. And, of course, the Triassic is the earliest of the or middle era. And these dinosaurs, in fact, come from 228 million year old rocks. Those rocks are predated, of course, by earlier times when there must have been dinosaurs, the ones who gave rise to these ones. But we've never yet found earlier dinosaurs. They're there, we just have to find them. The two dinosaurs that I'm looking at basically are carnivorous dinosaurs. You can look at the teeth for example and see that the teeth are flattened from side to side on the front and the back. These are the same kind of teeth that we see in more progressive animals like Tyrannosaurus Rex, for example. However, the relationships of these dinosaurs have been debated. And that's because they represent the time when all dinosaurs were diversifying into the groups that they eventually became. Where they are very obviously different from each other. But back in the Triassic. It's not that obvious. And we can look at Eoraptor, for example, and although it looks like a good carnivorous dinosaur, in fact, there are characters in the teeth at the front of the jaws that suggest this animal may in fact be much more closely related to the earliest. And consequently have nothing to do with the more progressive Theropod dinosaurs like Herresaurus. Neither of these dinosaurs is very large, and there were no large dinosaurs at that time, but there are characters in the skull of Herresaurus in particular that it is a theropod dinosaur. So, for example, we can look at the region between the eye. And the nasal area, and we can see that there is this large sinus. Well, in fact, there are several sinuses like this in the face, which are air-filled and help define all theropod dinosaurs. We can also look at the lower jaw, and we can see that there's a very large opening in the middle of the lower jaw. Which has a joint above it and below it. And this joint, called the intramandibular joint, is found in basically all theropods that evolved after Herrerasaurus. That joint probably represents a safety valve. So that when the animal was feeding if its teeth got stuck in its prey those teeth wouldn't break off because there were some flexibility in the jaw. However, that joint would also allow these animals to have a slightly larger gape and to allow them to perhaps eat and swallow slightly larger prey. When we look at the hands of Herrerasaurus, we can see how primitive this dinosaur is though. This dinosaur has not two fingers that we find in Tyrannosaurus rex, it's got remnants of all five fingers, which is the primitive number of fingers in theropod dinosaurs. The thing about Herrerasaurus though, it's lost all of the phalanges on the fifth finger. All that remains is the metacarpal bone which is in the flat of the hand. The fourth finger is represented by the metacarpal bone, of course, but it also has one single phalanx left. So it's in the process of losing those two fingers, but it still has those fingers represented. It still has three functional fingers and, as you would expect in a carnivorous dinosaur. Those fingers end in sharp claws and the hand is capable of grasping its prey. >> We'd like to introduce you to our interactive phylogenetic tree. It's a tree of life that details many of the specimens you'll learn about in the University of Alberta's paleontology courses including this one. Go to our Coursera home page and find the paleogenetic trait interactive. Have a look at the base of the dinosaur branch of the tree to learn more about early soreianchanz. When we think of carnivorous dinosaurs, we naturally tend to envision large predators ruling over the prehistoric world, and that part of the story is coming. But at their beginning, carnivorous dinosaurs were more modest components of the ecosystems. The early theropod dinosaurs like Eoraptor and Herrerasaurus and their herbivorous cousins are estimated to have accounted for only about 10% of a large bodied terrestrial species that were alive at the time. And, of course, these dinosaurs weren't really all that large. For roughly the first 30 million years, Theropods were out classed, and out numbered by larger carnivores. Primarily Crurotarsans. Crurotarsans are crocodile line archosaurs. These included multi-ton predators, some of which were also bipedal, and similar to theropod dinosaurs in many regards. Alongside such crocodile line competitors, the neotheropda evolved. Neotheropods have skulls with enlarged lacrimals. Lacrimals are bones positioned just in front of the eyes that extended onto the upper skull surface. Such lacrimals may have strengthen the skull, but the more dorsal position is also important. In many later therapods, the lacrimals would evolve into a range of pressed and small horns. Neotherapods also have intramandibular joint in each side of the lower jaw. And the intramandibular joint created a flexible region in the lower jaw. What do you think could have been one possible advantage of a flexible joint on the sides of neotheropod jaws? Is it, A, greater maximum bite force. B, the ability to take expanded bites. C, improved jaw rigidity, or D, a faster bite. B is the correct answer. The intramandibular joint allowed neotheropods to expand their jaws and take bigger bites. However, it was an adaptation with a number of other potential benefits. The intramandibular joint allowed Neotheropods to bite off and swallow larger chunks of meat. It also gave a little flex to their bites and this allowed them to grip and hold onto struggling prey without their lower jaws being easily torqued, or broken. And finally, it may have provided them with the ability to wiggle their lower jaws in feeding and attacking. This would've allowed their lower jaws with their rows of sharp serrated teeth to function like electric carving knives. This increase their injuring and butchering ability. In the pelvis of the neotheropods the number of sake whole vertebrates increase to minimum of five. And attachment surfaces is for the high limb muscles were in large. The feet became functionally tridactyl, meaning only three of the toes touched the ground. And the number of fingers on each hands was reduced to four. Dinosaurs diversified gradually and this shouldn't come as a surprise, that's how evolution works. Bear in mind, the adaptations that distinguished the first theropods from many of their closely related reptilian contemporaries, were subtle. And the late Triassic, neotheropods slowly gained ecological and evolutionary ground. Herbivorous dinosaurs had already become substantially more abundant and included the prosauropods. Prosarapods were the largest herbivorous of their time, and the largest herbivores in earth's history up to that point. Among the first distinct branches of the neotheropods was the Coelophysoidia. Coelophysoids were still relatively small and lightly built. They're an important group to note, because many coelophysis soil such as genus coelophysis are known from large fossil bone beds contain in some instances hundreds of skeletons. These skeletons all belong to the same coelophysis soli species. The bone beds show that although dinosaurs were not the largest predators, in some environments, they were becoming numerous. These crowded bone beds suggest that some celofisoids were gregarious and lived in large packs. At the end of the Triassic period, there was a mass extinction event. Currently, we don't know what caused it. Many of the other large carnivores and herbivores died off, but all the major lineages of dinosaurs survived this extinction event. Of course, mass extinctions are disastrous for many species. But for others, there can be a time of opportunity. In what way might a mass extinction benefit a lineage of animals? Is this because, A, it opens ecological niches that were previously occupied? B, it marks the transition from one geologic period to another. C, it simplifies food webs. Or D, it consolidates mating territory. The correct answers is A. The extinction of several lineages opens new opportunities for the survivors to diversify and occupy those empty niches.
