The Evolution of Amphibians, from Ancient to Modern Times
Here’s the strange thing about amphibian evolution: You wouldn’t know it from the small (and rapidly dwindling) population of frogs, toads and salamanders alive today, but for tens of millions of years spanning the late Carboniferous and early Permian periods amphibians were the dominant land animals on earth. Some of these ancient creatures achieved crocodile-like sizes (up to 15 feet long, which may not seem so big today but was positively huge 300 million years ago) and terrorized smaller animals as the “apex predators” of their swampy ecosystems. (See a gallery of prehistoric amphibian pictures.)
Before going further, it’s helpful to define what the word “amphibian” means. Amphibians differ from other vertebrates in three main ways: first, newborn hatchlings live underwater and breathe via gills, which then disappear as the juvenile undergoes a “metamorphosis” into its adult, air-breathing form (juveniles and adults can look very different, as in the case of baby tadpoles and full-grown frogs). Second, adult amphibians lay their eggs in water, which significantly limits their mobility when colonizing land. And third (and less strictly), the skin of modern amphibians tends to be “slimy” rather than reptile-scaly, which allows for the additional transport of oxygen for respiration.
The First Amphibians
As is often the case in evolutionary history, it’s impossible to pinpoint the exact moment when the first tetrapods (the four-legged fish that crawled out of the shallow seas 400 million years ago and swallowed gulps of air with primitive lungs) turned into the first true amphibians. In fact, until recently, it was fashionable to describe these tetrapods as amphibians, until it occurred to experts that most tetrapods didn’t share the full spectrum of amphibian characteristics. For example, three important genera of the early Carboniferous period–Eucritta, Crassigyrinus and Greererpeton–can be variously (and fairly) described as either tetrapods or amphibians, depending on which features are being considered.
It’s only in the late Carboniferous period, from about 310 to 300 million years ago, that we can comfortably refer to the first true amphibians. By this time, some genera had attained relatively monstrous sizes–a good example being Eogyrinus (“dawn tadpole”), a slender, crocodile-like creature that measured 15 feet from head to tail. (Interestingly, the skin of Eogyrinus was scaly rather than moist, evidence that the earliest amphibians needed to protect themselves from dehydration.) Another late Carboniferous/early Permian genus, Eryops, was much shorter than Eogyrinus but more sturdily built, with massive, tooth-studded jaws and strong legs.
At this point, it’s worth noting a rather frustrating fact about amphibian evolution: modern amphibians (which are technically known as “lissamphibians”) are only remotely related to these early monsters. Lissamphibians (which include frogs, toads, salamanders, newts and rare, earthworm-like amphibians called “caecilians”) are believed to have radiated from a common ancestor that lived in the middle Permian or early Triassic periods, and it’s unclear what relationship this common ancestor may have had to late Carboniferous amphibians like Eryops and Eogyrinus.