Dinosaurs are split into two main categories.

 

Dinosaurs are split into two main categories.

To put it simply, either a dinosaur is a saurischian (Greek for “lizard-hipped”), or they’re an ornithischian (Greek for “bird-hipped”).

Funnily enough, the lizard-hipped dinosaurs are more commonly related to modern-day birds, while the bird-hipped dinosaurs all went extinct!

Dinosaurs (leaving out the birds) evolved into many different groups over their roughly 170 million year existence, from approximately 235 million years ago to 66 million years ago. The first true dinosaur was probably a small, bipedal animal of carnivorous or omnivorous diet, similar to Eoraptor from Argentina. The earliest true dinosaur fossils date to the early part of the Late Triassic, but it is very unlikely that we have found the actual very first dinosaurs, and the group probably originated in the Middle Triassic or Early Triassic.

We have a general picture of the closest relatives to dinosaurs. The closest major group to dinosaurs appears to be the flying pterosaurs. In a wider sense, dinosaurs belong to a group called Archosauria. Archosaurs, the “ruling reptiles”, are today represented by crocodilians and birds. Therefore, when paleontologists want to think about aspects of dinosaurs that didn’t fossilize, like behavior and soft tissues, they can consider birds, which are living dinosaurs, and crocodilians, which are their next-closest relatives. The only problem is that many dinosaurs likely had lifestyles and anatomies that were nothing like either modern crocodiles or birds, but these comparisons can at least narrow down the possibilities.