Temnospondyl

Temnospondyli (from Greek τέμνειν (temnein, "to cut") and σπόνδυλος (spondylos, "vertebra")) is a diverse subclass 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 armour-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.

Temnospondyli was named by the German palaeontologist Karl Alfred von Zittel in his second edition of Handbuch der Palaeontologie, published in 1888. However, temnospondyl remains have been known since the early part of the 19th century. The earliest described temnospondyl was Mastodonsaurus, named by Georg Friedrich Jaeger in 1828 from a single tooth that he considered to belong to a reptile. Mastodonsaurus means "breast tooth lizard" after the nipple-like shape of the tip of the tooth. The naming of these first specimens was disputed. Leopold Fitzinger named the animal Batrachosaurus in 1837. In 1841, the English palaeontologist Richard Owen referred to the genus as Labyrinthodon to describe its highly folded or labyrinthine teeth. Owen thought that the name Mastodonsaurus "ought not to be retained, because it recalls unavoidably the idea of the mammalian genus Mastodon, or else a mammilloid form of the tooth... and because the second element of the word, saurus, indicates a false affinity, the remains belonging, not to the Saurian, but to the Batrachian order of Reptiles." Owen recognized that the animal was not a "saurian" reptile,  yet he also referred Jaeger's Phytosaurus to the genus. Although the two genera have similarly sized conical teeth, Phytosaurus was later found to be a crocodile-like reptile. Additional material, including skulls, firmly placed Labyrinthodon as an amphibian. Jaeger also named Salamandroides giganteus in 1828, basing it on partial occiput, or back portion of the skull. In 1833, he described a complete skull of S. giganteus that had the same teeth as his Mastodonsaurus, making it the first known complete skull of a temnospondyl. Because Mastodonsaurus was named first, it has precedence over the other names as a senior subjective synonym. Batrachosaurus is still used as the name of an unrelated brachyopid temnospondyl.

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 build up 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, all known temnospondyls have small limbs with no more than 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.

Carboniferous and Early Permian Temnospondyls first appeared in the Early Carboniferous around 330 million years ago (Mya). During the Carboniferous, temnospondyls included basal medium-sized forms such as Dendrerpeton or large semiaquatic forms such as Cochleosaurus. Other, more derived temnospondyls, such as the amphibamids, were smaller and more terrestrial. They resembled salamanders, and some taxa, such as the genus Branchiosaurus, even retained external gills like the modern-day axolotl. During the latest Carboniferous and Early Permian around 300 Mya, several groups, such as the dissorophids and trematopids evolved strong, robust limbs and vertebrae and became adapted to life on land while others such as the eryopids, developed into large semiaquatic predators. The dvinosaurs, a group of small aquatic temnospondyls, evolved from terrestrial ancestors in the Late Carboniferous. Late Permian During the Late Permian, increasing aridity and the diversification of reptiles contributed into a decline in terrestrial temnospondyls, but semiaquatic and fully aquatic temnospondyls continued to flourish, including the large Melosaurus of Eastern Europe. Other temnospondyls, such as archegosaurids, developed long snouts and a close similarity to crocodiles, although they lacked the armor characteristic of the latter group. These temnospondyls included the largest known batrachomorph, the 9-m-long Prionosuchus of Brazil. Mesozoic As temnospondyls continued to flourish and diversify in the Late Permian(260.4 - 251.0 Mya), a major group called Stereospondyli became more dependent on life in the water. The vertebrae became weak…

Metabolism and gas exchange A study on the fully aquatic Archegosaurus shows that its heat balance, gas exchange, osmoregulation, and digestion were more similar to those of fish than those of modern aquatic amphibians like salamanders. Feeding Although the earliest temnospondyls were primarily semiaquatic, they had the ability to feed on land. Later, eryopoids and dissorophoids, some well adapted to terrestrial life, also fed on land. Some eryopoids became better adapted toward life in water and shifted their diets toward aquatic organisms. The first primarily aquatic feeders were archegosaurs in the Permian. Trematosaurs and capitosaurs became independently aquatic and also returned to this type of feeding. Most aquatic stereospondyls have flattened heads. When feeding, they probably opened their mouths by lifting their skulls instead of lowering their lower jaws. The jaw mechanics of the plagiosaurid Gerrothorax is well known, and is one of the most highly adapted. Gerrothorax is thought to have lifted its skull to around 50° above horizontal through the flexing of the atlanto-occipital joint between the occipital condyles of the skull and the atlas vertebra of the neck. As the skull is raised, the quadrate bone pushes forward and causes the lower jaw to protrude outward. Other stereospondyls probably also lifted their skulls, but they are not as well adapted for such movement. D.M.S. Watson was the first to suggest skull lifting as a means of feeding in temnospondyls. He envisioned that Mastodonsaurus, a much larger temnospondyl than Gerrothorax, was able to make the same movement. Paleontologist A.L. Panchen also supported the idea in 1959, suggesting that Batrachosuchus also fed in this way. a group that includes frogs, and classified Batrachia within Reptilia. What are today classified as reptiles (lizards, snakes, crocodilians and turtles) were called saurian reptiles.
 * 1) Owen placed Labyrinthodon in Batrachia,