What is a dinosaur?

People often mistake the word 'dinosaur' as being a general term for any big, extinct reptile. Earlier this week, we had a great talk from Victoria Arbour, a PhD student from the U of A on pterosaurs. She started out by defining exactly what a dinosaur is, and explaining why a pterosaur is not a dinosaur, and I thought that was a great idea, so here we go. 


The first important thing to understand is the structure of the skull. Amniotes, that is animals that have an amniotic egg (reptiles, mammals, and birds) are divided into three main groups with respect to the 'temporal fenestra', or simply 'holes', in the back of their skull. If you feel above your cheekbone, in your temple, you'll feel that it's not hard and bony, but soft and full of muscle. This is what remains of one of those holes. The first group is the anapsids. These animals have no holes in the back of their skull, and consist mainly of turtles. The second group is the synapsids, which have a single hole in the back of the skull, just like us. This includes mammals, and the extinct reptiles closely related to mammals, like Dimetrodon. That's right, Dimetrodon is not a dinosaur, but more closely related to us and other mammals than a dinosaur! The third group is the diapsids, and like it's name suggests, they have two holes in the back, one on top and one below. All remaining reptiles are diapsids, including dinosaurs, pterosaurs, and birds (which are the direct descendants of dinosaurs, and therefore technically dinosaurs). There is a fourth group you will sometimes hear called the euryapsids, which have only one hole up top (in a different position than the one synapsid hole). These were most likely diapsids that lost the bottom hole over time, and include animals like ichthyosaurs. 

The front hole (on the left) is the nostril, while the large middle hole is for the eye. The holes at the back are the temporal fenestrae. A. Anapsid skull with no holes, B. Synapsid skull with 1 hole, C. Diapsid skull with 2 holes. From Fossil Wiki

So, dinosaurs are diapsids: they have two holes in the back of their head. But so do lots of other reptiles, including pterosaurs. So what makes a dinosaur a dinosaur? Unfortunately, that's a bit of a hard question to answer. First of all, it's important to know that dinosaurs are part of a group called the archosaurs, which includes crocodiles, pterosaurs, and birds. This group is united by a character called the ant-orbital fenestra, which is a small hole in front of the eye (not the nostril), not found in any other group. They are also part of a group called the Ornithodira, which includes just pterosaurs and dinosaurs. Ornithodirans adopted an upright stance, with their legs directly under their body rather than splayed out to the side as seen in modern reptiles. Although pterosaurs do this as well, they have several other characters that separate them from dinos. Now: dinosaurs. Dinosaurs share a number of features that are unique to them. For example, they have a set of jaw muscles that attaches to the top of their skull, not seen in any other group. They also have well developed processes or crests on their humerus (upper arm bone), hip, knee, and ankle. 

Reptilian posture: Top shows a typical reptile with the legs splayed to the side,  while the bottom shows the legs directly under the body, as seen in dinosaurs. Image from Freethought Forum

There are many other characters that dinosaurs share, which are all highly technical and primarily are features of the hind limbs. These features are not found in pterosaurs, which is one of the reasons why pterosaurs, the flying reptiles contemporaneous to the dinosaurs, are not dinosaurs. Pterosaurs also have several features that are not found in the dinosaurs, including an elongated fourth finger that forms their wing, and a new bone called the pteroid. Dinosaurs were (and are still today when including birds) an amazingly diverse group of reptiles. There were huge sauropods that include the largest animals to ever walk on land, and small theropods like Compsognathus, that were close to chicken-size.  Dinosaurs are divided into two main groups: the saurischians (or 'lizard-hipped'), and ornithischians ('bird-hipped'). Saurischians include all of the sauropodomorphs (all large long-necked dinosaurs and their relatives), and the theropods (the carnivores, and other strange creatures), while ornithischians are an almost purely herbivorous group including the hadrosaurs ('duck-billed' dinosaurs), ceratopsians ('horned'), the armoured ankylosaurs and stegosaurs, and many more. Next week, I'll talk a bit about the diversity of dinosaurs, and introduce all the different groups. For now, here's an example of a cladogram, which is kind of like a family tree, for the dinosaurs. 

Cladogram showing the relationships and diversity of dinosaurs. From UCMP Berkeley

References:
National Museum of Natural History