June 30 , 2016

by Andrew A. Farke

Everyone knows about dinosaurs, and most people who dig deeper quickly learn about the two major groups of dinosaurs: saurischians and ornithischians. Saurischians include the long-necked sauropods such as Apatosaurus, often fearsome theropods like Tyrannosaurus, and of course today's birds. Ornithischians were plant-eaters, with such classics as thumb-spiked Iguanodon, three-horned Triceratops, and the plated Stegosaurus. Both groups are pretty eclectic, yet are traditionally distinguished by the hip region.

Saurischian translates as "lizard hip", and ornithischian translates as "bird hip." The name refers to the orientation of the pelvic bones--in lizards, the front bone called the pubis typically points forward, but in birds the pubis points backward. Yet, things get confusing very quickly. Birds are actually saurischians, and some ornithischians get hips that superficially look very lizard-like, with an extra piece of the pubis projecting forward (see the red bones in the image below).

If your head is spinning, I don't blame you. Thanks to the complexities of evolution and foibles of scientific names, the whole situation is hard to navigate. So, I'm going to give you an even easier way to tell them apart.

Let's forget about the hips. Look at the lower jaw!

Ornithischians have an extra bone at the front of their jaw, called the predentary. Saurischians don't. In ornithischians, the predentary formed part of the toothless beak, and may even have been an independent point of rotation around which the bones behind it could move slightly. Every ornithischian, from Triceratops to Tenontosaurus, has a predentary. They all inherited it from their common ancestor--a shared evolutionary heritage.

So, if you're stuck in a museum and want to distinguish ornithischian from saurischian, look "ahead". Find that lower jaw, and see if it has an extra beaky bone.