Understanding Dinosaurs - Part 2: Dinosaur Groups

Part 2: Dinosaur Groups

Last time we talked about the Linnaean classification system, and what it takes to qualify as a dinosaur. Next, we expand on that idea by further dividing up our now defined group. 

The easiest way to look at dinosaurs as a broad group is to divide them into two major orders, Ornithischia and Saurischia. How is your Greek? Not good? Me too. Okay, these two orders are divided up based on hip structures (see I bet you didn't know paleontology was so hip!).

Ornithischia comes from the Greek words meaning 'bird' and 'hip-joint', while Saurischia means 'lizard' 'hip-joint'. Now don't get too attached to these meanings because I will throw a wrench into the whole thing and make it far more complicated to remember. I know, I hear you complaining that I just explained last time, that dinosaurs are not lizards and that is mostly due to their unique hip-joint. Yes, that is true. And saurischian dinosaurs are no exception. The name simply means they have a more lizard-like pelvis than the ornithischians. They both still have a dinosaur pelvis with the same three basic parts and the distinct acetabulum (the fancy hip-socket). But as we look at the two types of hips together, we can see they are arranged differently. Each consists of an illium,an ischium (the part that gives us the 'ischia' part of the name), and the pubis. But while the ornithischians' pubis sits back against the ischium, pointing toward the tail of the creature, the sauruschians' pubis bone juts forward, creating a triangular shape.

This gives us our two basic groups of dinosaurs, bird-hips and lizard-hips. Now comes that wrench I was talking about, and something we will get into far more detail about later. The ornithischians are the smaller group when we look at body plan diversity. We will talk about this group in the next installment. The saurischians, however, are the most diverse. This group contains species varying from T. rex to Brachiosaurus. It also contains birds.

Yes, modern birds are in the lizard-hipped group, and not the bird-hipped group. I told you not to get too attached to those concepts. Birds are an odd thing when it comes to their placement with dinosaurs, and recently they have received some attention by being placed in the Linnaean system as a group of dinosaurs (but there will be an entire post about that topic). For the moment, forget where birds are in the groups. If we look at a bird's hip bones, they do fit the shape shared by ornithischian dinosaurs, which is why we call that order 'bird-hipped'. But modern birds are not related to ornithischian dinosaurs, they instead, evolved from the 'lizard-hipped' dinosaurs. So though, the pelvic structures of birds and ornithischian dinosaurs are similar, they are not interrelated. Rather, bird pelvises started out looking like saurischian dinosaurs, and the pubis bone slowly moved back into a position against the ischium.

Next time: Part 3 - Ornithischian Dinosaurs