The 300 Million Year History of Amphibian Evolution

The 300 Million Year History of Amphibian Evolution Heres the strange thing about amphibian evolution: You wouldnt 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. Before going further, its 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 tadpoles and full-grown frogs. Second, adult amphibians lay their eggs in water, which significantly limits their mobility when colonizing the land. And third, 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, its 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 didnt share the full spectrum of amphibian characteristics. For example, three important genera of the early Carboniferous period- Eucritta, Crassigyrinus, and Greererpeton- can be variously described as either tetrapods or amphibians, depending on which features are being considered. Its 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, its 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 its unclear what relationship this common ancestor may have had to late Carboniferous amphibians like Eryops and Eogyrinus. Its possible that modern lissamphibians branched off from the late Carboniferous Amphibamus, but not everyone subscribes to this theory. Prehistoric Amphibians: Lepospondyls and Temnospondyls As a general rule, the amphibians of the Carboniferous and Permian periods can be divided into two camps: small and weird-looking (lepospondyls), and big and reptilelike (temnospondyls). The lepospondyls were mostly aquatic or semiaquatic, 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. Mastodonsaurus 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 Earths landmasses. 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 (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 to 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, or 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 early Jurassic 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 have 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.