The Amphbians. A paraphyletic group of tetrapods, and one of the four traditional tetrapod classes. The lissamphibia is very diverse today, if on the decline. And it's fairly complete, with only one of the four subgroups not surviving to the present. But look back further. The lissamphbians appeared in the late permian, and amphibians go much farther back then that. To the devonian. Looking back, you have the "Ichthyostegalians", on to forms like the throwback Cassigyrinus, among others. From there you find the Temnospondyls, and the Lepospondyls. One of the latter groups lives on in the aforementioned Lissamphbia, but which is not known. Beyond that you have the various non-amniote antracosaurs, such as the Seymouriamorphs, and Diadectomorphs.
Keeping this in mind, our derived forms are nothing in the grand scheme, and given their reduced and derived bodyplans, have little chance of replicating these extinct relatives, save for some of the more conservative salamanders. Except one group. The Anura, the frogs, are perhaps the most divergent from their paleozoic and mesozoic cousins. But one group, the Leiopelmatidae, can still go back. Possesing more vertebra then any other known extant frog species, and still retaining the tail muscles, a more temnospondyl or lepospondyl-like descendant isn't unimaginable. And their insular habitat was the drive for this to happen. On an island with little more then primitve penguins, primitve passerines, and the closest relatives of the lizards, they became the most revalutionary amphibian group on earth. This, is the Archaospondyli.
The name is derived from archaeos, meaning "old; ancient"; and spondylos, meaning "vertebra". This refers to the prescence of developed caudal vertebra, the high number of vertebra, and the overall shape and appearence the creatures, which resembles that of paleozoic "amphibians" superficially.
The earliest ancestors that split from the HE line was a group of fairly conservative, albeit large, leiopelmatids. The spent most of their time in the water, as opposed to some more terrestrial forms of different lineages as well as the descendants of these primitive forms, in the true Archaeospondyli. In the mid-Eocene, one taxon started retaining some juvinile features, noticeably a somewhat notable, still short, tail. The tailed individuals were selected for in certain populations, seing as these ones thrived in fast currents, feeding on fish. Less then five million years later, you get fully tailed forms, thought these still retained their frog like shape. One side-branch from the archaeospondyl-line kept this design in mountain rivers until the Ice Ages, when they along many other leiopelmatids went extinct, these being the Paleocaudidae, a group of conservative paleocaudan group. Paleocauda is the clade that includes all tailed leiopelmatids