Mention ancient reptiles, and most people think of dinosaurs. But dinosaurs were actually very advanced reptiles, and only appeared on the earth many millions of years after the reptile class began. Even the popular movie, Jurassic Park, gives a somewhat erroneous view, as the majority of the dinosaurs featured in it actually lived in the Cretaceous period, more recent than the Jurassic.
Reptiles go back to the Carboniferous epoch, before flowering plants, when the earth was dominated by giant ferns, tree-sized club-mosses, and strange plants called seed ferns. The earliest reptiles were hardly distinguishable from amphibians. The Devonian period of the Carboniferous epoch was known for its great amphibians such as Seymouria, large predators occupying a niche similar to the alligators of today. The most important difference between amphibians and reptiles was the development of the shelled egg. This allowed reptiles to live and reproduce away from water.
Early reptiles were still rather amphibian-like in their overall structure: legs splayed out sideways, bellies just barely lifted from the ground, tails dragging behind - in short, a salamander-like gait. This gait is preserved even today in the crocodilians, a very ancient group, much older than dinosaurs, that has changed but little in the intervening epochs. Corcodilians swimming motion is based on waving the body and tail side-to-side, again, just like salamanders, and in fact like the majority of fish. There is thus a definite continuity from the amphibian Seymouria of the Devonian to the mighty reptilian crocodile of today.
As the Devonian gave way to the Permian, reptiles came to predominate. One of the best-known reptiles of this period, featured in many a picture book, is Dimetrodon. This is the famous sail-backed reptile. Its tall, ribbed sail is believed to have been used in temperature control - by turning the sail broadside to the sun, this cold-blooded animal could more quickly absorb enough heat to warm its thick, ponderous body. Somewhat later was Edaphosaurus, another sail-backed reptile, with a more refined sail shape.
What is less well-known about Dimetrodon is that it was what is known as a "mammal-like" reptile; that is, it showed certain characteristics which today are associated with mammals, and is considered to be an evolutionary precursor of mammals. Mammal-like reptiles were the dominant group in Permian times, even as mammals are nowadays. Before the dinosaurs even appeared on the scene, the mammals would already be there. These earliest mammals would have been egg-layers, like the echidnas and platypus of today. The egg-laying mammals, or monotremes, may in a sense be called "reptile-like mammals," and are the nearest living relatives of the mammal-like reptiles of the Permian. Perhaps this accounts for the unusual characteristic of the platypus, in which the males have a venom spur on the ankle. Venom is rare in mammals, but occurs today in four different families of reptiles. It would be interesting to know whether any of the Permian mammal-like reptiles were venomous.
In addition to the Carboniferous forests and earliest reptiles, the Devonian was also the time of the first great flourishing of insects, and insects likely formed the main diet of the smaller early amphibians and reptiles. Larger amphibians like Seymouria, and its reptilian successors the crocodilians, would have been predators on the smaller species, and probably on fishes as well. No doubt many of the mammal-like reptiles were also predators, even as the egg-laying echidnas of today are primitive insect eaters little different from salamanders and small lizards. But in the lush vegetation and tropical climate of those days, herbivory was for the first time becoming a viable option for land-dwelling vertebrates. Certain groups of early reptiles were the first plant-eating vertebrates on land, and in time gave rise to the great plant-eating dinosaurs; this legacy continues to this day - there are no plant-eating amphibians, but the plant-eating habit is found among reptiles, in the turtles and some families of lizards.
Thus we see that the first reptiles on earth were the bridge from the amphibians, not just to modern reptiles, but to mammals as well. Our own mammalian bloodline thus goes back to before the dinosaurs. The dinosaurs, spectacular though they were in their heyday, were but a side branch of evolution, giving rise to the birds (and there never was an Age of Birds); whereas the obscure and all-but-forgotten early reptiles turned out to be the main stream, who sowed the seeds of the Age of Mammals - and hence, ultimately, of Man.