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The Temnospondyli (from Greek τέμνειν (''temnein'', "to cut") and σπόνδυλος (''spondylos'', "vertebra")) are a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including fresh water, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis, and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are considered amphibians, many had characteristics, such as scales, claws, and armor-like bony plates, that distinguish them from modern amphibians. Temnospondyls have been known since the early 19th century, and were initially thought to be reptiles. They were described at various times as batrachians, stegocephalians, and labyrinthodonts, although these names are now rarely used. Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Temnospondyli means "cut vertebrae", as each vertebra is divided into several parts. Experts disagree over whether temnospondyls were ancestral to modern amphibians (frogs, salamanders, and caecilians), or whether the whole group died out without leaving any descendants. Different hypotheses have placed modern amphibians as the descendants of temnospondyls, another group of early tetrapods called lepospondyls, or even as descendants of both groups (with caecilians evolving from lepospondyls and frogs and salamanders evolving from temnospondyls). Recent studies place a family of temnospondyls called the amphibamids as the closest relatives of modern amphibians. Similarities in teeth, skulls, and hearing structures link the two groups. ==Description== Many temnospondyls are much larger than living amphibians, and superficially resemble crocodiles. Others are smaller and resemble salamanders. Most have broad, flat heads that are either blunt (brevirostrine) or elongated (longirostrine). The skulls are rounded or triangular in shape when viewed from above, and are usually covered in pits and ridges. The rugged surfaces of bones may have supported blood vessels, which could transfer carbon dioxide to the bones to neutralize acidic buildup in the blood (early semiaquatic tetrapods would have had difficulty expelling carbon dioxide from their bodies while on land, and these dermal bones may have been an early solution to the problem). Many temnospondyls also have canal-like grooves in their skulls called sensory sulci. The sulci, which usually run around the nostrils and eye sockets, are part of a lateral line system used to detect vibrations in water.〔 As semiaquatic animals, most temnospondyls have small limbs with four toes on each front foot and five on each hind foot. Terrestrial temnospondyls have larger, thicker limbs, and some even have claws. One unusual terrestrial temnospondyl, ''Fayella'', has relatively long limbs for its body, and probably lived as an active runner able to chase prey. Homologues of most of the bones of temnospondyls are also seen in other early tetrapods, aside from a few bones in the skull, such as interfrontals, internasals, and interparietals, that have developed in some temnospondyl taxa.〔 Most temnospondyls have tabular horns in the backs of their skulls, rounded projections of bone separated from the rest of the skull by indentations called otic notches; in some temnospondyls, such as ''Zatrachys'', they are pointed and very prominent. Among the most distinguishing features of temnospondyls are the interpterygoid vacuities, two large holes in the back of the palate. Another pair of holes, choanae, are present in front of these vacuities, and connect the nasal passage with the mouth. Temnospondyls often have teeth on their palates, as well as in their jaws. Some of these teeth are so large, they are referred to as tusks. In some temnospondyls, such as ''Nigerpeton'', tusks in the lower jaw pierce the palate and emerge through openings in the top of the skull. Very little is known of the soft tissue of temnospondyls. A block of sandstone, described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania, included impressions of the bodies of three temnospondyls. These impressions show, when alive, they had smooth skin, robust limbs with webbed feet, and a ridge of skin on their undersides. Trackways referable to small temnospondyls have also been found in Carboniferous and Permian rocks. The trackways, called batrachichni, are usually found in strata deposited around freshwater environments, suggesting the animals had some ties to the water. Unlike modern amphibians, many temnospondyls are covered in small, closely packed scales. The undersides of most temnospondyls are covered in rows of large ventral plates. During early stages of development, they first have only small, rounded scales. Fossils show, as the animals grew, the scales on the undersides of their bodies developed into large, wide ventral plates. The plates overlap each other in a way that allows a wide range of flexibility. Later semiaquatic temnospondyls, such as trematosaurs and capitosaurs, have no evidence of scales. They may have lost scales to make movement easier under water or to allow cutaneous respiration, the absorption of oxygen through the skin. Several groups of temnospondyls have large bony plates on their backs. One temnospondyl, ''Peltobatrachus'', has armor-like plating that covers both its back and underside. The temnospondyl ''Laidleria'' also has extensive plating on its back. Most members of the family Dissorophidae also have armor, although it only covers the midline of the back with two narrow rows of plates. Other temnospondyls, such as ''Eryops'', have been found with small, disc-like bony scutes that were in life probably embedded in the skin. All of these temnospondyls were adapted to a terrestrial lifestyle. Armor may have offered protection from predators in the case of ''Peltobatrachus''.〔 The scutes may have provided stability for the spine, as they would have limited flexibility and may have been connected by strong ligaments. Temnospondyls such as ''Sclerothorax'' and ''Eryops'' that may have been at least partly terrestrial also have long neural spines on top of their vertebrae that would have stabilized the spine. Bony scutes are also seen in plagiosaurs, but unlike ''Peltobatrachus'', ''Laidleria'', ''Eryops'', and dissorophids, these animals are thought to have been fully aquatic. Plagiosaurs may have inherited their armor from a terrestrial ancestor, as both ''Peltobatrachus'' and ''Laidleria'' have been considered close relatives of the group.〔 Temnospondyls' vertebrae are divided into several segments. In living tetrapods, the main body of the vertebra is a single piece of bone called the centrum, but in temnospondyls, this region was divided into a pleurocentrum and intercentrum. Two types of vertebrae are recognized in temnospondyls: stereospondylous and rhachitomous vertebrae. In rhachitomous vertebrae, the intercentra are large and wedge-shaped, and the pleurocentra are relatively small blocks that fit between them. Both elements support a spine-like neural arch, and well-developed interlocking projections called zygapophyses strengthen the connections between vertebrae. The strong backbone and strong limbs of many ratchitomous temnospondyls allowed them to be partially, and in some cases fully, terrestrial. In stereospondylous vertebrae, the pleurocentra have been lost entirely, with the intercentra enlarged as the main body of the vertebrae. This weaker type of backbone indicates the stereospondylous temnospondyls spent more time in water. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Temnospondyli」の詳細全文を読む スポンサード リンク
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