Now that you have a better understanding of the different types of bones that make up a skeleton, let's take a look at how we can differentiate dinosaur groups by using our understanding of skeletal anatomy. Dinosaur skeletons come in many shapes and sizes and we can use features of the skeleton to divide dinosaurs into smaller and smaller groups. If we keep doing this, eventually, we can get down to individual species and be able to easily tell them apart. The first major division of dinosaurs is based on the shape of the pelvis. These two groups are called the Saurischia or lizard-hippped dinosaurs and the Ornithischians or bird-hipped dinosaurs. Somewhat confusingly, we'll learn later in the course that birds evolved from the lizard-hipped dinosaurs. Let's take a look at a set of dinosaur hips. All dinosaurs have three pairs of bones that make up their pelvis. This here is a Daspletosaurus pelvis. Phil, what can you tell us about the bone arrangement? >> As you know, Betsy, Daspletosaurus is one of the Tyrannosaurs and Tyrannosaurs have the very typical Saurischian dinosaur hip arrangement. This is the front of the hips here and the back of the hips here. In between the two sides of bones, you have the vertebral column and the vertebral column here is fused together and it forms what we call the sacrum. The sacrum is, in turn, also fused or attached to the ilium depending which animal you're looking at and this whole structure, of course, is designed to support the great weight of the Tyrannosaur. The upper bone, the ilium, is the largest bone in the body and when you look at the muscle attachments on the ilium, you can see that there's a concentration of muscle attachments at the front, which go to the front of the leg and pull the leg forward and another concentration at the back, which attached first to the tail but also down to the back of the leg to pull the leg backwards. The ischium is a smaller bone beneath the illium and it goes down and back and it also is useful for attachment of muscles and orientating muscles in particular directions and in particular, in Tyrannosaurs, you see a very large muscle scar right here. The pubis is the last bone. It goes down and forward and it's very distinctive in Tyrannosaurs because it has this huge boot on the bottom, partially, for muscle attachment, partially, for supporting the weight of the animal when it's squatting on the ground and the pubis is very different in Ornithischian dinosaurs. And that's because the muscles rearrange themselves because they're not moving in the same kind of way and so in an Ornithischian dinosaur, the pubis reorientates itself so that it's back beside the ischium. That's one of the easiest ways to tell an Ornithischian dinosaur from a Saurischian dinosaur. Now here's a familiar beast. This is the skeleton of a Stegosaurus. The location of its hip bones is quite clear. Can you tell if Stegosaurus is an Ornithischian or a Saurischian dinosaur? Check the answer you think is correct. In this Stegosaurus skeleton, the pubis and ischium both point backwards. That makes it an Ornithischian dinosaur. So A is the correct answer. >> Here we are back at the Royal Tyrrell Museum of Paleontology. The Saurischian or lizard-hip dinosaurs can be divided into two major groups. The first are the Sauropods or the long neck dinosaurs. Like we mentioned before, Sauropods were the largest land animals that ever lived. Sauropods had long necks and tails and walked on four legs. Their limb bones are thick and look very much like columns. Their vertebrae are complex and many of them have cavities inside for air sacks. Their skulls were small compared to the rest of their body and their teeth were simple and peg like. The other group of Saurischian dinosaurs were the Theropods and the Theropods were a highly diverse group that included both carnivorous dinosaurs and other forms, including some omnivores and some herbivores. They included huge species like Tyrannosaurus Rex and small species like the Dromaeosaurus and their living representatives are still with us today and right now, they're probably flying through your backyard. All Theropod dinosaurs walked on their hind legs. Carnivorous forms, of course, had very sharply pointed teeth but some forms actually lost their teeth and they had toothless beaks. Theropod dinosaurs, generally, had three clawed fingers but some like Tyrannosaurus Rex lost a finger, so they only had two fingers left. Some really weird Theropod dinosaurs, such as Sciurumimus, had lost all fingers but one and they had a single, well developed claw. Now, Theropods are very diverse in lot of ways. Some have very long snouts, some have very short snouts, some had horns over the eyes, some had horns over the nose and different parts of the bodies changed depending on what the function was required for the habits of those dinosaurs. >> Sauropod and Theropods are the two major groups of Saurischian dinosaurs. However, there is a third, shorter lived and less diverse group, the Prosauropods. Prosauropods were an early group of plant eating dinosaurs and were much smaller than their giant Sauropod cousins but dinosaur size is relative and some Prosauropods still measured over 10 meters in length and weighed over 3 tons. Like Sauropods, Prosauropods had small heads and long necks. Many Prosauropods also had grasping hands and the ability to stand and walk only on their back feet. Although, at the beginning of the age of dinosaurs, Prosauropods were highly successful, the group had a short reign and was soon replaced by more advanced herbivorous dinosaurs. Which dinosaur is mostly related to modern birds? A) Triceratops, B) Giraffatitan, C) Corythosaurus, or D) Tyrannosaurus. There is only one correct answer, so make a considered decision. Even though a Tyrannosaurus is far larger than any living bird, it belongs to the same group of dinosaurs as modern birds do, Theropods. There are many differences between Tyrannosaurus and birds but they do share some characteristics. Both possess feathers, at least, at some point in their growth. Both are bipedal and both have hollow bones containing air sacs. So D is the correct answer.
