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. (It's possible that modern lissamphibians branched off from the late Carboniferous Amphibamus, but not everyone subscribes to this theory.)