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. (See a complete, A to Z list of non-dinosaur reptiles, a gallery of therapsid pictures, a gallery of archosaur pictures, and a gallery of pelycosaur pictures.)
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 mouse"--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.)
Next Page: Therapsids and Archosaurs