Before the Dinosaurs - Pelycosaurs, Archosaurs, and Therapsids
Dinosaurs weren’t the first four-legged reptiles to prosper on land. The Carboniferous and Permian periods witnessed a large variety of "primitive" terrestrial reptiles, the pelycosaurs, archosaurs and therapsids, many of which boasted surprisingly advanced features.
How Did Dinosaurs Evolve?
Like all living creatures, dinosaurs evolved from preexisting ancestors, in this case a family of prehistoric reptiles known as archosaurs (which also spawned pterosaurs and prehistoric crocodiles). Here's an overview of current thinking about dinosaur evolution.
The First Dinosaurs
Some time during the middle to late Triassic period, about 230 million years ago, the first true dinosaurs evolved from their archosaur ancestors. Here's everything you need to know about these early theropods, including such crucial genera as Eoraptor and Herrerasaurus.
Why Were Dinosaurs So Big?
Species by species, individual by individual, dinosaurs were bigger than any other land-dwelling animals that ever lived. Here are some theories that just might account for dino gigantism.
How Are Dinosaurs Classified?
How do paleontologists classify dinosaurs? Here’s a detailed guide to the major orders, suborders and infraorders, according to the state of current research, including the crucial distinction between ornithischian ("bird-hipped") and saurischian ("lizard-hipped") dinosaurs.