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. (It's possible that modern lissamphibians branched off from the late Carboniferous Amphibamus, but not everyone subscribes to this theory.)
The Two Types of Prehistoric Amphibians: Lepospondyls and Temnospondyls
As a general (though not very scientific) rule, the amphibians of the Carboniferous and Permian periods can be divided into two camps: small and weird-looking (the lepospondyls), and big and reptile-like (the temnospondyls). The lepospondyls were mostly aquatic or semi-aquatic, and more likely to have the slimy skin characteristic of modern amphibians. Some of these creatures (such as Ophiderpeton and Phlegethontia) resembled small snakes; others (like Microbrachis) were reminiscent of salamanders; and some were simply unclassifiable. A good example of the last is Diplocaulus: this three-foot-long lepospondyl had a huge, boomerang-shaped skull, which might have functioned as an undersea rudder.
Dinosaur enthusiasts should find the temnospondyls easier to swallow. These amphibians anticipated the classic reptilian body plan of the Mesozoic Era (long trunks, stubby legs, big heads, and in some cases scaly skin), and many of them (like Metoposaurus and Prionosuchus) resembled large crocodiles. Probably the most infamous of the temnospondyl amphibians was the impressively named Mastodonsaurus (the name means "nipple-toothed lizard" and has nothing to do with the elephant ancestor), which had an almost comically oversized head that accounted for nearly a third of its 20-foot-long body.
For a good portion of the Permian period, the temnospondyl amphibians were the top predators of the earth's land masses. That all changed with the evolution of the therapsids ("mammal-like reptiles") toward the end of the Permian period; these large, nimble carnivores chased the temnospondyls back into the swamps, where most of them slowly died out by the beginning of the Triassic period. There were a few scattered survivors, though: for example, the 15-foot-long Koolasuchus thrived in Australia in the middle Cretaceous period, about a hundred million years after its temnospondyl cousins of the northern hemisphere had gone extinct.
Introducing Frogs and Salamanders
As stated above, modern amphibians (known as "lissamphibians") branched off from a common ancestor that lived anywhere from the middle Permian to the early Triassic periods. Since the evolution of this group is a matter of continuing study and debate, the best we can do is identify the "earliest" true frogs and salamanders, with the caveat that future fossil discoveries may push the clock back even further. (Some experts claim that the late Permian Gerobatrachus, also known as the Frogamander, was ancestral to these two groups, but the verdict is mixed.)
As far as prehistoric frogs are concerned, the best current candidate is Triadobatrachus ("triple frog"), which lived about 250 million years ago, during the early Triassic period. Triadobatrachus differed from modern frogs in some important ways (for example, it had a tail, the better to accommodate its unusually large number of vertebrae, and it could only flail its hind legs rather than use them to execute long-distance jumps), but its resemblance to modern frogs is unmistakable. The earliest known true frog was the tiny Vieraella of South America, while the first true salamander is believed to have been Karaurus, a tiny, slimy, big-headed amphibian that lived in late Jurassic central Asia.
Ironically--considering that they evolved over 300 million years ago and have survived, with various waxings and wanings, into modern times--amphibians are among the most threatened creatures on the earth today. Over the last few decades, a startling number of frog, toad and salamander species spiraled toward extinction, though no one knows exactly why: the culprits may include pollution, global warming, deforestation, disease, or a combination of these and other factors. If current trends persist, amphibians may be the first major classification of vertebrates to disappear off the face of the earth!
The following is a list of the most important prehistoric amphibians; just click on the links for more information.
Amphibamus One of the first prehistoric amphibians to receive a name.
Archegosaurus This ancient amphibian is known from over a hundred skulls.
Branchiosaurus Not to be confused with the much, much bigger Brachiosaurus.
Cacops A common amphibian of early Permian North America.
Cyclotosaurus This massive amphibian had a worldwide distribution.
Diadectes This amphibian was one of the biggest land animals of its time.
Diplocaulus This bizarre amphibian had a boomerang-shaped skull.
Eocaecilia The earliest caecilian in the fossil record.
Eogyrinus A large, snake-like amphibian of the late Carboniferous.
Eryops A large, crocodile-like amphibian of the Permian period.
Fedexia Yes, this amphibian was named after Federal Express.
Gastric-Brooding Frog This frog gestated its young in its own stomach.
Gerobatrachus Better known as the Frogamander.
Gerrothorax A bizarre amphibian with a football-shaped head.
Golden Toad The poster toad for amphibian extinction.
Karaurus The earliest salamander yet discovered.
Koolasuchus This Australian amphibian outlived its northern cousins.
Mastodonsaurus This impressively named amphibian had nothing to do with mastodons.
Metoposaurus A large amphibian with an amazingly flat head.
Microbrachis This small amphibian spent most of its time in the water.
Ophiderpeton This early amphibian was indistinguishable from a snake.
Phlegethontia A snake-like amphibian of the late Carboniferous.
Platyhystrix This "flat porcupine" was actually an amphibian.
Prionosuchus Probably the largest amphibian that ever lived.
Proterogyrinus A large amphibian with a snake-like tail.
Seymouria This Permian amphibian was once mistaken for a true reptile.
Triadobatrachus The earliest frog ancestor yet discovered.
Vieraella The earliest true frog in the fossil record.
Westlothiana This was once thought to be the first true reptile.