FOSSIL VERTEBRATES/Palaeozoic Non-Amniote Tetrapods 475 been considered the earliest stem amniote for a while, although it has recently been supplanted by Casineria Its elongate body and short limbs may suggest, alternatively, that it belongs to a further group, the microsaurs, but its poor skull preservation does not yet allow the resolution of this question Most other late Vise´ an forms actually span the boundary into the early Namurian, but deserve to be treated as Early Carboniferous forms A gap in the fossil record in the mid–late Namurian seems to represent some kind of faunal turnover, as there is only one genus that has a representative both before and after the interval A distinctively Late Carboniferous fauna emerges during the later Namurian The mid-Carboniferous forms include the bizarre aquatic predator, Crassigyrinus scoticus (Figure 6), which features an enormous gape, huge palatal teeth, and a snout with a large midline foramen at the front, whose function is obscure Crassigyrinus was an elongate animal with disproportionately small fore limbs, and was possibly an analogue of a modern moray eel It is known from a few localities in Scotland The colosteids form a group that had members in Scotland and the USA They were permanently aquatic predators that retained a number of primitive features in their anatomy, and probably had persistent gillbars Members include Pholidogaster, Greererpeton (Figure 6), and Colosteus Their relationship with other Carboniferous groups is disputed, but that is true both of Crassigyrinus and another group that appears in the mid-Carboniferous, the baphetids (formerly known as loxommatids) Tetrapods of the Late Carboniferous Temnospondyls, anthracosaurs, and baphetids that originated in the mid-Carboniferous survived to radiate in the Late Carboniferous, producing large crocodile-like predatory forms characteristic of the coal swamps of the time Baphetids are striking for their curious keyhole-shaped eye-sockets (Figure 5); the purpose of the forwardly directed extension is obscure Very little is known about their postcranial skeletons, because, although their skulls are well known, only two have postcranial material associated with them Anthracosaurs (Figures and 7) retain many primitive characters of the skull, but these are shared with later amniote-like forms, and they also share their type of vertebral construction (called gastrocentrous) with more terrestrially adapted forms, including amniotes By contrast, most early temnospondyls retained a primitive style of vertebral construction (called rhachitomous), associated with an aquatic life style and seen in Acanthostega Their skulls, however, share features later seen in modern amphibians; large vacuities in the roof of the mouth (although covered with skin in life) might have been part of a breathing and hearing mechanism similar to that found in modern amphibians today Temnospondyls also show other similarities to frogs and salamanders in their life histories Several families are known to have had aquatic larvae with external gills and that metamorphosed into more terrestrial adults, and some, it is suspected, had members that remained as permanently aquatic larval-like forms, as in some modern salamanders, such as the axolotl These larval temnospondyls are collectively known as ‘branchiosaurs’ (Figure 7), but the term embraces several different families By contrast, some temnospondyls became much more fully terrestrial One group, the dissorophoids, produced some of the most terrestrial of temnospondyls, while, at the same time, including forms that are apparently the most closely related to frogs (Figures and 8) The three groups described above (together with colosteids and a few others) were, at one time, lumped together as ‘labyrinthodonts’, but in fact these animals may not be particularly closely related to each other – indeed they may belong to opposing sides of the dichotomy leading to the two modern tetrapod groups These groups filled the large-predator niche at the same time as several other groups filled the role of small aquatic or terrestrial nibblers of invertebrates These forms, often collectively known as ‘lepospondyls’, include the microsaurs (both aquatic forms with elongate bodies and small short-bodied terrestrial forms that appear superficially amniotelike), the adelogyrinids and aăstopods (Figure 5) (elongate and almost or completely lacking limbs), and the nectrideans (united by specialized vertebrae in the tail that formed a vertically flattened and stiffened but laterally flexible and powerful organ for swimming) The skulls of all these groups are specialized and difficult to compare with each other or with non-lepospondyls This is one reason why their relationships to each other and to non-lepospondyls are still highly controversial Some studies have suggested that these forms are in fact closer to modern amphibians and amniotes than are temnospondyls or anthracosaurs Tetrapods of the Late Carboniferous are found almost universally distributed over a wide area of Central and Western Europe and eastern North America, representing the known extent of the coal forest Although genera may differ, families produced their equivalents in widely spread localities, suggesting a uniformity of habitat and climate over many millions of years Finds have usually been found in