We refer to the Triassic, Jurassic, and Cretaceous periods as “the Age of Dinosaurs,” but they weren’t the only animals alive then.
Dimetrodons look like dinosaurs, but they’re actually more closely related to humans than to T. rex. They lived before the dinosaurs did, and they’re part of the group that branched off and eventually became mammals. We can tell by looking at the openings in their skulls– they have an extra hole by their ears that dinosaurs don’t have.
(image by Dmitry Bogdanov)
Pterosaurs, the flying reptiles, lived at the same time as the dinosaurs, but they’re actually only distantly related to each other. One way that we can tell is by looking at their leg bones– dinosaurs had different features on their limbs to help make them stronger.
Mosasaurs and other swimming reptiles aren’t dinosaurs either. They’re on a different branch of the family tree, more closely related to crocodiles and Komodo dragons than to dinosaurs. Their limbs sprawled out to the sides, while dinosaurs carried their legs directly beneath them. (Think about the difference between how a lizard’s legs are splayed out, but a bird’s legs are carried straight beneath their bodies.)
When paleontologists determine where an animal falls on the tree of life, they look at lots of specific little traits, like the structure of specific bones, to see just where they fit. These differences, like holes in the skull and the shape of the leg bones, might not seem important to the untrained eye, but they’re key for scientists to determine how animals are related to each other.
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