The Atomic Spud: That's Not a Dinosaur!

Yesterday I was reading one of the How Do Dinosaurs... books to Son of Atomic Spud. (Much to my disappointment, the boy focused on the basketballs and footballs found on nearly each page more than he paid attention to the dinosaurs.) These books are part of a series intended to encourage good behavior in children. Each page features a dinosaur in an everyday situation surrounded by human children and adults; the dinosaur essentially represents a child who likes to pretend that he or she is a dinosaur. Usually the first half of each book shows the dinosaur misbehaving while the second half juxtaposes the proper behavior. This description may make the books sound preachy, but they're actually very well done.

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

There are a few animals that seem to be most often misidentified as dinosaurs:

Dimetrodon

Dimetrodon

Dimetrodon fails the dinosaur test in a number of ways. First of all, this popular sail-backed creature lived during the Permian Period, which was the final geologic period of the Paleozoic Era. Dinosaurs didn't appear until the first period of the following era (i.e., the Triassic Period of the Mesozoic Era). It's believed that the last Dimetrodon went extinct 40 million years before the first dinosaurs walked the earth.

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.

Pterosaurs

Pteranodon

The order Pterosauria includes the well-known genera Pterodactylus and Pteranodon. These flying reptiles lived around the same time as the dinosaurs (i.e., the Triassic Period to the Cretaceous Period). Also, like dinosaurs, the Pterosaurs were part of the Archosaur group. (Modern Archosaurs include crocodilians and birds.) However, Pterosaurs are not classified as dinosaurs, which were exclusively land-dwelling animals. And it's fairly obvious that Pterosaurs didn't have the same upright limbs as dinosaurs.

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.

Elasmosaurus

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.

Mosasaurus

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.