The second major group of dinosaurs is the Ornithischians. There are two key traits that distinguish all ornithischians from thischians. First, ornithischians have a hip with a pubic bone that points backwards. Sometimes, the backwards pointing pubis on an ornithischian dinosaur. Does have a projection that points forward. But these forward projections are usually short, so don't let them confuse you. Recall that the backwards pointing pubic bones of Ornithischians resembles those of birds. However, this similarity is actually an evolutionary convergence indicating no close relationship. Birds belong among saurischian dinosaurs, not among the ornithischians. The second trait that distinguishes ornithischians is a special bone in the lower jaw, called the predentary. No other animals have one. The ornithischians were all herbivorous. And both of their key traits were adaptations to a vegetarian lifestyle. The predentory bone helped to form a cropping beak in the front of the mouth. This beak gave ornithischians a tool for slicing off plant stems. And this allowed the teeth in the back of their jaws to become specialized for grinding and chewing. Even with such teeth, plant mater is difficult to digest and their backwards pointing pubis gave ornithischians the extra belly room that they needed for a large digestive tract. >> The ornithischian dinosaurs were all herbivores and the group can be subdivided into five main lineages. First group includes the duck billed hadrosaurs and their earlier relatives the and a variety of small somewhat nondescript plant eaters. The iguanodons and hadrosaurus evolved complex dentations for shearing and grinding plants. And the hadrosaurus also had a very broad beak. Ornithopods started off as bipeds, but the larger hadrosaurus probably walked on all four legs most of the time. The Iguanodon had a characteristic spiky thumb whereas the hadrosaurs basically had a hand that was encased in a mitten of skin. Some Hadrosaurus had elaborate crests on the top of their skulls, while others had flat heads. >> Ornithopods are defined as bird hit dinosaurs that typically walk on two legs and are herbivorous. Which of these animals are ornithopods? A, Edmontosaurus? B, Iguanodon? C, Triceratops. or D, Tyrannosaurus. More than one answer might be correct, so check all of those you think are right. The correct answers are A and B. Ornithopods are typically bipedal herbivorous dinosaurs. Admontosaurus and Iguanodon were both ornithopods. Tyrannosaurus was a carnivore and triceratops had large horns and walked full time on its four legs. >> The second group of ornithischian dinosaurs are the ceratopsians. This group includes the iconic triceratops. These were four legged dinosaurs. And they had relatively short tales, when you compare them to other dinosaurs. The frill at the back of the head was made up of bones from the back of the skull that become very elongated. And in you see the, almost the most extreme form of it. And they're very distinctive. Also, on the frills, you'll see that you can develop. Little horns and spikes and frills and so on. It's a way that they can tell each other apart. Many of the ceratopsians also had horns over their eyes. In the case of chasmosaurus, there are just little bumps but it has an elaborate long horn over the nose. Unlike the ductyl dinosaurs, these animals had very narrow skulls and the beak at the front, was more like a parrot's beak than anything else. But the teeth still had elaborated into banks of teeth for shearing and crushing plant material. The older forms of ceratopsians don't have quite as elaborate frills and horns as these later forms. But still, they do have frills and they have the same kind of beak. >> Pachyrhinosaurus is a common dinosaur found in Alberta and Alaska. Its defining characteristic is a bony mass called a boss. Pachyrhinosaurus had a nasal boss over its nose, and another over the eyes called the superorbital boss. But was it a ceratopsian? Yes. Pachyrhinosaurus was still a ceratopsian, as it had a large frill and parrot-like beak. Sure it doesn't have proper nose and eye horns. But that alone does not exclude it from the group. Even within a single group of dinosaurs, there's a lot of variation, and we need to look at multiple characteristics to decide what group a specific dinosaur belongs to. Even if an animal may be missing a common feature, like horns, it may still belong to that group. >> The third group of ornothician dinosaurs that we're going to talk about are the dome-headed dinosaurs or the Pachycephalosaurus. These dinosaurs are very intelligent looking little animals, because they have a high domed skull. Unfortunately for them, the brains are not very big. In fact most of the depth of that skull is solid bone and we know that these dinosaurs probably used those domes as way of recognizing each other so they could identified which species of dome-headed dinosaurs they were hanging out with. They may have also been used as battering rams. So, when the males were competing for females, for example, they would probably hit each other either on the top of the head or on the sides of the body. The skeletons are relatively primitive looking. They don't have really long legs, so they're not all that fast. Unfortunately, the skeletons are also quite fragile, so we don't tend to find many specimens. With complete skeletons. This is probably the most complete skeletons Stegosaurus was found in the 1920, and for more than 50 years this was the only skeleton we had of this dinosaur. Probably just about everybody has heard of Stegosaurus, the iconic representative of our fourth group of ornithischians, the Stegosaurus. Stegosaurus walked on all four legs, although their front legs were quite a bit shorter than their back legs. The skull was very small compared to the rest of the body, and it was long and low. Their teeth were leaf-shaped, and they had a very narrow beak. The most distinguishing character of stegosaurus, though,. Are the plates or spikes along the back? These are formed from a special type of bone that forms in the skin. And it's called osteoderms. Stegosaurus had large, plate-like osteoderms along the back. Whereas earlier stegosaurus had spikes. Nethertheless. Stegosaurus itself has a long tail, and at the end of the tail are a series of spikes. >> For this quiz, identify each dinosaur that belongs to the stegosaur group of ornithischians. Check all the answers you think are correct. A is spinosaurus. B, ouranosaurus. C, stegosaurus. Or D, anodontosaurus. C is a stegosaur, specifically a stegosaurus. Although the others like Spinosaurus and ouranosaurus had sails along their backs. These solid sails were very different from the individual plates or spines along the back of Stegosaurus. In Anodontosaurus, which is an Ankylosaur, well, we'll be talking about that dinosaur group next. >> The fifth and final group of ornithischian dinosaurs are the Ankylosaurs. Or the armored dinosaurs. Like the stegosaurus, they had osteoderms. But instead of just having a row of osteoderms on top of their backs, Ankylosaurs were covered in osteoderms of many sizes and shapes. Ankylosaurus walked on four legs and had round, rather fat bodies. In fact, this one looks like a coffee table. Their skulls were very blocky, teeth were very small, and leaf shaped. In some but not all Ankylosaurs, the end of the tail was modified to form a tail club with the huge osteoderms at the tip. >> Okay, so now we've examined the two main branches of the dinosaur family tree: the saurischians and the ornithischians. And we've also talked about the two groups of Saurischians, and the five groups of Ornathischians. When you leave this video, you're going to see another puzzle, it will show the family tree of dinosaurs. Your job will be to place the various dinosaurs, and hip arrangements shown in the puzzle, on the correct branches of the family tree. Good luck. Great, now you know the major groupings of dinosaurs. Let's move one from dinosaur skeletons to outward appearances.
