Like archeologists finding the ruins of a previously unsuspected civilization buried deep beneath the one being excavated, dinosaur enthusiasts are sometimes astonished to learn that entirely different kinds of reptiles once ruled the earth, before the Velociraptors and Stegosauruses we all know and love. For approximately 120 million years--from 350 to 230 million years ago--terrestrial life was dominated by the pelycosaurs, archosaurs, and therapsids (the so-called "mammal-like reptiles") that preceded the dinosaurs.
Stay up to date on the latest dinosaur news--sign up for the free About.com dinosaur newsletter today!
Of course, before there could be therapsids (much less full-blown dinosaurs), nature had to evolve the first genuine 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 amphibians, themselves descended from the proverbial fish that flipped, flopped, and crawled their way out of the oceans 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 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--its name is Greek for "forest mouse"--may well have been the first tetrapod (four-footed animal) to lay eggs and have scaly skin, both of which features would have allowed it to venture farther from the bodies of water to which amphibians were tethered. There's no doubt that Hylonomus evolved from an amphibian species; in fact, scientists believe the elevated oxygen levels of the Carboniferous Period may have helped fuel the development of complex animals in general.
The Rise of the Reptiles
Now came one of those global events that cause some animal populations to prosper, and others to shrivel 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 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--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 on the scene toward the end of the Carboniferous, and persisted well into the Permian, dominating the land for about 40 million years. Probably the most famous pelycosaur was Dimetrodon, a large reptile with a prominent sail on its back (the main function of which was to soak up sunlight and maintain the creature's temperature). Like the later dinosaurs, 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 possible that one fed on the other).
It's impossible to list all the 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, anatomically, by the presence of one hole in the skull behind each eye (technically speaking, all mammals are also synapsids). During the Permian, synapsids coexisted with "anapsids" (reptiles lacking those all-important skull holes). Prehistorica anapsids also reached a striking degree of complexity, as exemplified by such large, ungainly creatures as Scutosaurus. (The only anapsid reptiles living today are the testudines--turtles, tortoises and terrapins.)
Next Page: Therapsids and Archosaurs
