T. rex, Triceratops, Titanosaur–What's the Difference?

NARRATOR: What are the different types of dinosaurs, and how are they related to each other? There are three big groups in the dinosaur family tree, each containing popular dinosaurs you might know.

ROGER BENSON (Curator, Division of Paleontology): There are ornithischian dinosaurs, like Triceratops or Ankylosaurus or Stegosaurus. The sauropodomorph dinosaurs, like Diplodocus, or Apatosaurus, alongside their close relatives like Massospondylus or Plateosaurus. And the third group of dinosaurs are the theropod dinosaurs. They include all living birds. And things like Tyrannosaurus rex, Velociraptor, and Coelophysis.

It’s likely that theropods and sauropodomorphs are related to each other, and that ornithischians are just outside of those. But paleontologists aren’t 100% confident about this. And other relationships have been proposed.

NARRATOR: But for now, we’ll go with the current thinking that theropods and sauropodomorphs are more closely related, in a group known as Saurischians. All of these groups sit under one umbrella, the group Dinosauria. 

BENSON: And that group is recognized based on the fact that all dinosaurs share particular features of their skeletons, mainly related to the hind limb and the pelvis, and they’re the kind of anatomical features that you’d expect in animals that hold their legs directly under their bodies and have legs that are more or less vertical rather than sprawled out to the side. The thigh bone in dinosaurs has a straight shaft, but then a head that turns into meet the hip socket. And the hip socket itself has a hole that goes through the center, but a large buttress that runs over the top to support the pelvis on top of the hind limb against the forces of gravity. 

NARRATOR: This combination leads to dinosaurs holding their legs vertically under their hips–unlike other reptiles of today and the past. Watch how this Komodo dragon walks – its legs go out to the side from its joints, rather than beneath it–not like this ostrich, a living dinosaur.

This might seem like quibbling, or nitpicking, but differences like these don’t arise overnight–they take millions of years to evolve. So when scientists see consistent differences between two groups, they can be sure that those differences reflect real evolutionary changes.

BENSON: We try to understand the relationships among different dinosaur species by studying their anatomy. And we expect that animals that are more closely related to each other will share more features, whereas animals that are more distantly related to each other will share less features.

NARRATOR: We can see this play out in the three major groups of dinosaurs. The ornithischian dinosaurs have the same hip bones as all dinosaurs, but one bone points in a different direction than that of the theropods and sauropodomorphs. The sauropodomorphs differ from ornithischians and theropods by all having small heads and long necks relative to their overall bodies. And the theropod dinosaurs all walked on three toes and two legs, instead of four, and most had blade-like teeth that made them excellent predators. 

BENSON: It’s common for people to use the word dinosaur to refer to any extinct reptiles. But dinosaurs aren’t just any extinct reptiles, they’re a specific group of extinct animals whose only living representatives are birds. 

BENSON: There were a few things that dinosaurs never did. Outside of birds like penguins, they never evolved to live in the oceans. So if you’re looking at a giant extinct reptile that lived in the oceans like a plesiosaur or an ichthyosaur, then you can say that’s not a dinosaur. 

BENSON: Other things that are commonly mistaken for dinosaurs, include the flying reptiles, the pterosaurs. Now, they’re not dinosaurs, but they’re actually quite closely related and they share a handful of features with birds and dinosaurs. Interestingly, they independently evolved flight. So although they fly, they’re not related to birds, which are the flying dinosaurs. 

NARRATOR: And of course, if it wasn’t a reptile at all, then it’s really not a dinosaur.

BENSON: And that includes things like wooly mammoths, which are giant mammals that are actually a type of extinct elephant. It also includes an early relative of mammals called Dimetrodon, which appears extremely reptilian but is actually more closely related to us than it is to dinosaurs. 

NARRATOR: Without dino DNA, the careful study of extinct animals’ anatomy is still the best way to construct the prehistoric family tree.