A couple days ago I mentioned the first reptile, a fossil found in a fossil tree in the Joggins formation of Nova Scotia. Before we get into that a little more, let’s define how a reptile differs from an amphibian. Amphibians, as you know, have their feet in both the world of water and the world of land; the name itself means “life of both kinds.” While some amphibians are mostly aquatic and some live largely terrestrial lives, they must return to water to breed.
Reptiles have broken the tie to the water by evolving a way of containing the water world within a package kept on land – the reptilian egg. The word “reptile” is from Latin meaning “creeping.”
Reptiles evolved from amphibians that had some characteristics of reptiles, including strong legs and skins that resisted the loss of moisture. The oldest unquestionable reptile is an animal named Hylonomus, whose name means “forest dweller” or “forest mouse.” The first fossils were found at Joggins, Nova Scotia, with the animals found within fossilized tree stumps, where they had presumably gotten trapped and died.
They were small, maybe 20 centimeters or 8 inches long, and looked a lot like modern lizards. They lived about 312 to 315 million years ago, during the early part of the Pennsylvanian Period. It had a solid skull without the openings typical of amphibians and impressions and footprints that are probably from Hylonomus show it had protective scales rather than smooth, water-permeable skins. It was probably an insectivore.
|Hylonomus - Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL|
The earliest synapsids, once considered to be “mammal-like reptiles,” also developed at about the same time as Hylonomus. They are now considered to be the ancestors of mammals, and it’s thought that reptiles on one hand and mammals on the other evolved at about the same time from an ancestor that laid its eggs on land. The first synapsids may have developed as long ago as 324 million years, putting them into the very late Mississippian Period, but they began to have more mammal-like adaptations in late Pennsylvanian time, about 306 to 312 million years ago, pretty much the same time the first true reptiles were evolving. So rather than thinking it was a smooth progression from amphibians to reptiles to mammals, as I was taught, it seems that the current thinking would have amphibians developing into animals that could lay eggs on land, and those animals fairly quickly branched into the varieties that would lead, respectively, to both reptiles and mammals. The earliest amniotes, animals that lay eggs on land, may be as old as 340 million years, middle Mississippian time. That interpretation is based on a single poor specimen called Casineria from Scotland, but it may represent an animal that was ancestral to all reptiles, mammals, and birds.
Conditions during the Pennsylvanian clearly must have favored the development of land life. The diversification of amphibians into both reptiles and mammals may have been fostered by the abundance of plants in the extensive swamps, a situation that would have created innumerable niches for both small and large animals to thrive. And there were also lots of bugs for those insect-eating early reptiles and mammals to eat.
—Richard I. Gibson
Drawing by Nobu Tamura (http://spinops.blogspot.com) – via Wikipedia under GFDL