Dinosaurs didn't spring suddenly into existence two hundred million years ago, huge, toothy, and hungry for grub. Like all living things, they evolved, slowly and gradually, from previously existing creatures--in this case, types of primitive reptiles called archosaurs ("ruling lizards").
On the face of it, archosaurs weren't all that different from dinosaurs. However, these ancient reptiles were much smaller than most dinos, and they had certain giveaway features (splay-footed postures, for example) that set them apart from their more famous descendants. Paleontologists even believe they may have identified the single genus of archosaur from which all dinosaurs evolved: Lagosuchus ("rabbit crocodile"), a quick, tiny reptile that scurried across the forests of the early Triassic.
Archosaurs: Before or After the Permian Extinction?
This, unfortunately, is where we encounter one of those disagreements so common in paleontology. Scientists are unsure whether archosaurs coexisted with therapsids (mammal-like reptiles) in the late Permian period (over 250 million years ago), or whether they appeared on the scene after the Permian Extinction, a geologic upheaveal that killed about three-quarters of all land-dwelling species. This would place the first archosaurs in the early Triassic, a few million years later.
Also--somewhat confusingly--it seems that later archosaurs (such as Desmatosuchus) coexisted with the earliest true dinosaurs (such as Eoraptor and Herrerasaurus). In evolutionary terms, this poses no contradiction: evolved species often wind up living side-by-side with the (unevolved) descendants of their spawning species. But it does pose a problem to paleontologists trying to definitively classify Triassic-period fossils.Whatever the case, it's undeniable that the Permian Extinction opened up huge swaths of evolutionary territory for the archosaurs, some of which evolved over tens of millions of years into the dinosaurs we know and love today. As is the case with all species, the slow course taken by dinosaur evolution depended on a variety of factors, including climate, competition, and the availability of food. For example, it's a sure bet that nature would never have allowed Brachiosaurus to evolve if there weren't tons of vegetation around to supply its dietary requirements.
Paths of Dinosaur Evolution
Archosaurs weren't the only lizard-like creatures roaming the earth before the dinosaurs; there were also therapsids, which looked like a strange hybrid of mammals and reptiles (instead of scales, some therapsids may have had smooth, mammal-like skin). Indeed, these creatures are often referred to as "mammal-like reptiles."
To show the twists and turns evolution can take, consider one of the most famous therapsids, Cynognathus ("dog jaw"). About the size of a large dog, Cynognathus may have been covered with fur, and may have given birth to live young instead of laying eggs. Most astonishingly, it seems likely that Cynognathus was warm-blooded--anticipating the metabolism of mammals that evolved a hundred million years later.
The point of this story is that evolution isn't necessarily a linear process: the same adaptations can appear in widely separated epochs, depending on environmental conditions. Yes, it's a good bet that dinosaurs evolved from archosaurs, but play the tape over again and a whole different race of creatures may have evolved from a descendant like Cynognathus. Dinosaurs might never have existed, and the first true mammals would have appeared on the scene way back in the late Triassic!
Finally, speaking of mammals, it's more than a little ironic that the dinosaurs owed their tens of millions of years of dominance to the Permian Extinction--because the dinos themselves were wiped out by the Cretaceous-Tertiary (K-T) Extinction 65 million years ago, which opened the door for the small, shrew-like mammals that survived to evolve (eventually) into modern humans.
