My only real gripe about the books is that they often include prehistoric animals that, although many people seem to think that they're dinosaurs, are not actually dinosaurs. For example, Dimetrodon and various species of pterosaurs often make an appearance, but they simply are not dinosaurs.
Although there are a number of features that distinguish dinosaurs from other prehistoric animals, there are several ways to quickly tell when an animal is not a dinosaur. It's not a dinosaur if:
- It was a flying animal
- It was exclusively aquatic
- It had sprawled legs like a lizard or a crocodile
- It lived before or after the Mesozoic Era, which consisted of the Triassic, Jurassic, and Cretaceous Periods
A less obvious way in which Dimetrodon is not a dinosaur is the fact that its physiology shows it to be a synapsid rather than a sauropsid. Dinosaurs, existing reptiles, and birds are sauropsids while Dimetrodon and mammals are synapsids. In other words, according to current theories of phylogony, Dimetrodon is more closely related to mammals that it is to dinosaurs.
|Dimetrodon: The sprawled legs show that it's not a dinosaur|
The fastest way to tell that Dimetrodon wasn't a dinosaur is the fact that its legs sprawled out from its body. Thanks to the shape of their hip sockets and femurs, dinosaur legs were erect rather than sprawled to the sides. Remember, if it walked like a crocodile, it's not a dinosaur.
It's believed that the pterosaurs' and dinosaurs' last common ancestor lived during the Early Triassic, which ended about 245 million years ago. The first dinosaurs seem to have appeared about 230 million years ago during the Middle to Late Triassic, while the first Pterosaurs appeared about 210 million years ago.
|Pterosaurs: Flying shows that they're not dinosaurs|
Plesiosaurs and Mosasaurs
Like the Pterosaurs, the Plesiosaurs and the Mosasaurs also lived during the time of the dinosaurs. However, since they were specifically adapted to aquatic life, they are not dinosaurs.
While the Mosasaurs don't really resemble dinosaurs and don't seem to be confused with them as often, the fact that the Plesiosaurs aren't dinosaurs might be confusing to some people. In the popular mind, a Plesiosaur like Elasmosaurus looks a lot like a sauropod (e.g., Brachiosaurus, Apatosaurus) but with flippers instead of legs.
Despite their physical similarities, it turns out that the relationship between Plesiosaurs and dinosaurs is even more distant than that between the Pterosaurs and dinosaurs. At least the latter two are both Archosaurs. The Plesiosaurs are part of the Sauropterygian group that first appeared around 245 million years ago and developed alongside the Archosauromorpha group (i.e., the group that eventually gave rise to the Archosaurs).
|A Plesiosaur: The flippers show it's not a dinosaur|
Similarly, the Mosasaurs are only distantly related to dinosaurs. They were part of the Lepidosauromorpha group, of which modern lizards and snakes are a part.
Remember, if it walked like a crocodile, flew, or lived only in the water, it's not a dinosaur.