Like archeologists discovering the ruins of a previously unknown civilization buried deep beneath an ancient city, dinosaur enthusiasts are sometimes astonished to learn that entirely different kinds of reptiles once ruled the earth, tens of millions of years before famous dinosaurs like Tyrannosaurus Rex, Velociraptor, and Stegosaurus. For approximately 120 million years-from the Carboniferous to the middle Triassic periods-terrestrial life was dominated by the pelycosaurs, archosaurs, and therapsids (the so-called "mammal-like reptiles") that preceded the dinosaurs.
Of course, before there could be archosaurs (much less full-blown dinosaurs), nature had to evolve the first true reptile. At the start of the Carboniferous period--the swampy, wet, vegetation-choked era during which the first peat bogs formed-the most common land creatures were prehistoric amphibians, themselves descended (by way of the earliest tetrapods) from the proverbial prehistoric fish that flipped, flopped, and slithered their way out of oceans and lakes millions of years before. Because of their reliance on water, though, these amphibians couldn't stray far from the rivers, lakes, and oceans that kept them moist, and that provided a convenient place to lay their eggs.
Based on the current evidence, the best candidate we know of for the first true reptile is Hylonomus, fossils of which have been found in sediments dating back 315 million years. Hylonomus-the name is Greek for "forest dweller"-may well have been the first tetrapod (four-footed animal) to lay eggs and have scaly skin, features that would have allowed it to venture farther from the bodies of water to which its amphibian ancestors were tethered. There's no doubt that Hylonomus evolved from an amphibian species; in fact, scientists believe that the elevated oxygen levels of the Carboniferous period may have helped fuel the development of complex animals in general.
The Rise of the Pelycosaurs
Now came one of those catastrophic global events that cause some animal populations to prosper, and others to shrivel up and disappear. Toward the start of the Permian period, about 300 million years ago, the earth's climate gradually became hotter and drier. These conditions favored small reptiles like Hylonomus and were detrimental to the amphibians that had previously dominated the planet. Because they were better at regulating their own body temperature, laid their eggs on land, and didn't need to stay close to bodies of water, the reptiles "radiated"-that is, evolved and differentiated to occupy various ecological niches. (The amphibians didn't go away-they're still with us today, in dwindling numbers-but their time in the limelight was over.)
One of the most important groups of "evolved" reptiles was the pelycosaurs (Greek for "bowl lizards"). These creatures appeared toward the end of the Carboniferous period, and persisted well into the Permian, dominating the continents for about 40 million years. By far the most famous pelycosaur (and one that's often mistaken for a dinosaur) was Dimetrodon, a large reptile with a prominent sail on its back (the main function of which may have been to soak up sunlight and maintain its owner's internal temperature). The pelycosaurs made their livings in different ways: for example, Dimetrodon was a carnivore, while its similar-looking cousin Edaphosaurus was a plant-eater (and it's entirely possible that one fed on the other).
It's impossible to list all the genera of pelycosaurs here; suffice it to say that a lot of different varieties evolved over 40 million years. These reptiles are classified as "synapsids," which are characterized by the presence of one hole in the skull behind each eye (technically speaking, all mammals are also synapsids). During the Permian period, synapsids coexisted with "anapsids" (reptiles lacking those all-important skull holes). Prehistoric anapsids also attained a striking degree of complexity, as exemplified by such large, ungainly creatures as Scutosaurus. (The only anapsid reptiles alive today are the Testudines-turtles, tortoises, and terrapins.)
Meet the Therapsids-The "Mammal-Like Reptiles"
The timing and sequence can't be pinned down precisely, but paleontologists believe that sometime during the early Permian period, a branch of pelycosaurs evolved into reptiles called "therapsids" (otherwise known as "mammal-like reptiles"). Therapsids were characterized by their more powerful jaws bearing sharper (and better differentiated) teeth, as well as their upright stances (that is, their legs were situated vertically beneath their bodies, compared to the sprawling, lizard-like posture of earlier synapsids).
Once again, it took a catastrophic global event to separate the boys from the men (or, in this case, the pelycosaurs from the therapsids). By the end of the Permian period, 250 million years ago, over two-thirds of all land-dwelling animals went extinct, possibly because of a meteorite impact (of the same type that killed the dinosaurs 185 million years later). Among the survivors were various species of therapsids, which were free to radiate into the depopulated landscape of the early Triassic period. A good example is Lystrosaurus, which evolutionary writer Richard Dawkins has called the "Noah" of the Permian/Triassic boundary: fossils of this 200-pound therapsid have been found all over the world.
Here's where things get weird. During the Permian period, the cynodonts ("dog-toothed" reptiles) that descended from the earliest therapsids developed some distinctly mammalian characteristics. There's solid evidence that reptiles like Cynognathus and Thrinaxodon had fur, and they may also have had warm-blooded metabolisms and black, wet, dog-like noses. Cynognathus (Greek for "dog jaw") may even have given birth to live young, which by almost any measure would make it much closer to a mammal than to a reptile!
Sadly, the therapsids were doomed by the end of the Triassic period, muscled out of the scene by the archosaurs (of which more below), and then by the archosaurs' immediate descendants, the earliest dinosaurs. However, not all therapsids went extinct: a few small genera survived for tens of millions of years, scurrying unnoticed under the feet of lumbering dinosaurs and evolving into the first prehistoric mammals (of which the immediate predecessor may have been the small, quivering therapsid Tritylodon.)
Enter the Archosaurs
Another family of prehistoric reptile, called the archosaurs, coexisted with the therapsids (as well as the other land reptiles that survived the Permian/Triassic extinction). These early "diapsids"-so-called because of the two, rather than one, holes in their skulls behind each eye socket-managed to out-compete the therapsids, for reasons that are still obscure. We do know that the teeth of archosaurs were more firmly set in their jaw sockets, which would have been an evolutionary advantage, and it's possible that they were quicker to evolve upright, bipedal postures (Euparkeria, for example, may have been one of the first archosaurs capable of rearing up on its hind legs.)
Toward the end of the Triassic period, the first archosaurs split off into the first primitive dinosaurs: small, quick, bipedal carnivores like Eoraptor, Herrerasaurus, and Staurikosaurus. The identity of the immediate progenitor of the dinosaurs is still a matter of debate, but one likely candidate is Lagosuchus (Greek for "rabbit crocodile"), a tiny, bipedal archosaur that possessed a number of distinctly dinosaur-like characteristics, and that sometimes goes by the name Marasuchus. (Recently, paleontologists identified what may well be the earliest dinosaur descended from archosaurs, the 243-million-year-old Nyasasaurus.)
It would, however, be a very dinosaur-centric way of looking at things to write archosaurs out of the picture as soon as they evolved into the first theropods. The fact is that archosaurs went on to spawn two other mighty races of animals: the prehistoric crocodiles and the pterosaurs, or flying reptiles. In fact, by all rights, we should be giving crocodiles precedence over dinosaurs, since these fierce reptiles are still with us today, whereas Tyrannosaurus Rex, Brachiosaurus, and all the rest aren't!