remained small in primitive forms, vanishing entirely in the more advanced ones. The intercentra bore the weight of the animal, being large and forming a complete ring. All were more or less flat-headed with either strong or secondarily weak vertebrae and limbs. There were also fully aquatic forms, like the Dvinosauria, and even marine forms such as the Trematosauridae. The Temnospondyli may have given rise to the modern frogs and salamanders in the late Permian or early Triassic. Lepospondyli A small group of uncertain origin, the Lepospondyli evolved mostly small species that can be found in European and North American Carboniferous and early Permian strata. They are characterized by simple spool-shaped vertebrae formed from a single element, rather than the complex system found in other labyrinthodont groups. Most were aquatic and external gills are sometimes found preserved. The Leposondyli were generally salamander-like, but one group, the Aïstopoda, was snakelike with flexible, reduced skulls, though whether the families belong with the other lepospondyls is uncertain. Some microsaur lepospondyls were squat and short-tailed and appear to have been well adapted to terrestrial life. The best known genus is Diplocaulus, a nectridean with a boomerang-shaped head. The position of Lepospondyli in relation to other labyrinthodont groups is uncertain, and it is sometimes classified as a separate subclass. The teeth were not labyrinthodont, and the group has classically been seen as separate from the Labyrinthodontia. There is some doubt as to whether the lepospondyls form a phylogenetic unit at all, or is a wastebin taxon containing the padamorphic forms and tadpoles of other labyrinthodonts, notably the reptile-like amphibians, or even very small primitive amniotes with reduced skulls. Evolutionary history The labyrinthodonts have their origin in the early middle Devonian (398–392 Mya) or possibly earlier. They evolved from a bony fish group: the fleshy-finned Rhipidistia. The only other living group of Rhipidistans alive today are the lungfish, the sister group of the landliving vertebrates. Earliest traces of the land-living forms are fossil trackways from Zachełmie quarry, Poland, dated to 395 million years ago, attributed to an animal with feet very similar to Ichthyostega. Swamp predators By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. The early labyrinthodonts were wholly aquatic, hunting in shallow water along tidal shores or weed filled tidal channels. From their piscine ancestors, they had