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Temnospondyli (from Greek τεμνειν, temnein = "to cut" + σπονδυλως, spondulos = "vertebra") are an important and extremely diverse taxon of small to giant primitive amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous. During their evolutionary history they adapted to a very wide range of habitats, including fresh-water aquatic, semi-aquatic, amphibious, terrestrial, and in one group even near-shore marine, and their fossil remains have been found on every continent. Authorities disagree over whether some specialised forms were ancestral to some modern amphibians, or whether the whole group died out without leaving any descendants.[1][2][3]
[edit] Evolutionary History[edit] Carboniferous and early Permian Temnospondyli During the Carboniferous, Temnospondyli included basal medium-sized (Dendrerpeton) or large (Cochleosaurus, about 1.5 metres (4.9 ft) long) semi-aquatic forms. Others such as the amphibamids were smaller and more derived, resembling newts or 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 (ca. 300 Ma), several groups such as the dissorophoids evolved strong, robust limbs and vertebrae and became adapted to life on land while others such as the eryopids and trematopsids developed into large (c. 1.5 metres (4.9 ft) long) and heavy-bodied semi-aquatic predators. [edit] Late Permian Temnospondyli During the later Permian, increasing aridity and more successful reptiles meant the end of the terrestrial temnospondyls, but semi- and fully aquatic animals continued to flourish, including the large Melosaurus of Eastern Europe. Other temnospondyls such as archegosaurids developed long snouts and an astonishing similarity to crocodiles, although they lacked the armour characteristic of the latter group. These temnospondyls included the largest known amphibian, the 9 metres (30 ft) long Prionosuchus of Brazil. As these amphibians continued to flourish and diversify in the lakes and rivers of the late Permian (260.4 - 251.0 Ma), a number of groups became more dependent on life in the water. The vertebrae became weak, the limbs small and vestigal, and the heavy skull large and flat, with the eyes looking upwards. These include the classic Stereospondyli, and other related types. During the Triassic period these animals dominated the fresh-water ecosystems, evolving in a range of both small and large forms. During the Early Triassic(251.0 - 245.0 Ma)one group of successful long-snouted fish eaters, the trematosaurs, even adapted to a life in the sea, the only known amphibians to do so with the exception of the modern Crab-eating frog). Another group, the Capitosauroidea, included not only medium-sized but also many giant species, 2.3 to 4 metres (7.5 to 13 ft) or more in length (e.g. Paracyclotosaurus, Cyclotosaurus), with huge and extraordinarily flat skulls, over a meter long in the largest forms (Mastodonsaurus). These animals seem to have lived on the river bottom, perhaps spending most or all their entire lives in water, and catching their prey by a sudden opening of the upper jaw, sucking in fish or smaller tetrapods that happened to swim past.[citation needed] [edit] Mesozoic survivorsIn the Carnian stage of the late Triassic (228.0 - 216.5 Ma) capitosauroids were joined by the superficially very similar Metoposauridae (1.5 metres (4.9 ft) long—and distinguished mainly by the different position of the eye-sockets), and the curious wide-headed plagiosaurs (about a meter in length), with external gills. The Triassic-Jurassic extinction event (ca. 199.6 Ma) killed all the giant temnospondyls. Only the smaller Brachyopidae and Chigutisauridae survived. These grew to large size during the Jurassic, with the brachyopids flourishing in China, and the chigutisaurs in Gondwana. The most recent known temnospondyl was the giant chigutisaur Koolasuchus, known from the Early Cretaceous of Australia where it seems to have survived in rift valleys that were too cold in the winter for crocodiles, co-existing with dinosaurs. At around 2.5 to 5 meters in length[4], this was one of the largest of its kind, as well as the last. [edit] Relationship to modern amphibiansThe Lissamphibia, the modern amphibians, appear to have risen in the Permian.[5] The root of the group is controversial, but may be with the Temnospondyli. Skull morphology of some of the smaller later form has been compared to those of modern frogs and salamanders. The presence of bicuspid, pedicellate teeth in small, paedomorphic or immature temnospondyls like the genus Doleserpeton has been cited as the most convincing argument in favor of the temnospondyl origin of lissamphibians.[6] Other analysis indicate that another fossil group, the Lepospondyli, are more likely candidates for lissamphibian origin, leaving the Temnospondyli an extinct group.[7] [edit] Systematics[edit] Two types of vertebraeOriginally, Temnospondyli were classified according to the structure of the vertebrae. Earlier forms, with complex vertebrae consisting of a number of separate elements, were called "Rachitomi", and large Triassic aquatic forms with simpler weaker vertebrae were called "Stereospondyli"; and these two types were considered suborders of the order Temnospondyli. In the Rachitomous condition the intercentra were large and wedge-shaped, and the pleurocentra were relatively small blocks that fitted between them. Both elements supported the neural arch, and well-developed interlocking zygapophyses strengthened the connections between the vertebrae. The strong backbone and strong limbs let many ratchitomes be at least partially, and in some cases fully, terrestrial. In the stereospondylous condition the pleurocentra have been lost, and the vertebral centra reduced to simple blocks made up of the intercentra only. This weaker type of backbone indicates a more fully aquatic existence.[8] More recent research has shown that this classification is no longer viable. The basic rachitomous condition is primitive for amphibians (tetrapods) in general. The pure stereospondyls seem to have arisen from different ancestors.[9] Some temnospondyls have rachitomous, semi-rachitomous, and sterospondylous vertebrae at different points the vertebral column of the same individual. Other taxa have intermediate morphologies that do not fit into one or the other category.[10] However, at least according to some analyses, the stereospondyls (minus some ambiguous taxa) can still be considered a clade.[10][11] [edit] Taxonomy     Class Amphibia
[edit] PhylogenyThe following cladogram follows the analyses of Holmes, Carroll & Reisz (1998), Yates & Warren (2000), and Sidor et al. (2005).[11][13][14]
The following cladogram is modified from Ruta et al. (2007).[15]
1 Temnospondyli, 2 Edopoidea, 3 Dvinosauria, 4 Euskelia, 5 Eryopoidea, 6 Dissorophoidea, 7 Limnarchia, 8 Archegosauroidea, 9 Stereospondyli, 10 Rhitidostea, 11 Brachyopoidea, 12 Capitosauria, 13 Trematosauria, 14 Metoposauroidea [edit] References
[edit] External links
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