Temnospondyli

Temnospondyli 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 specialized forms were ancestral to some modern amphibians, or whether the whole group died out without leaving any descendants.

Evolutionary History
During the Carboniferous, Temnospondyls included primitive medium-sized (Dendrerpeton) or large (Cochleosaurus about 1.5 meters long) semi-aquatic forms. Others, more advanced, were small and resembled newts or salamanders (Limnerpeton), and one group, represented by the genus Branchiosaurus and its relatives, even retained external gills like the modern-day axolotl. During the latest Carboniferous and early Permian, several groups evolved strong, robust limbs and vertebrae, and became adapted to life on land (Cacops) or as large (c. 1.5 meters long) and heavy-bodied semi-aquatic predators (Trematops, Eryops). Others developed long snouts and an astonishing similarity to crocodiles (e.g. Archegosaurus) although lacking armour. This last group included the largest known amphibian, the 9 meter long Prionosuchus of Brazil.

During the later Permian, increasing aridity and more successful reptiles meant the end of the terrestrial forms, but semi- and fully aquatic animals continued to flourish, including the large Melosaurus of Eastern Europe.

As these amphibians continued to flourish and diversify in the lakes and rivers of the late Permian, 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 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 (except the modern Crab-eating Frog). The Capitosauroidea included not only medium-sized but also many giant species, 2.3 to 4 meters 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 any unwary fish or smaller tetrapod that happened to swim past. In the late Triassic (Carnian) these big amphibains were joined by the superficially very similar Metoposauridae (1.5 meters 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 killed all the giant temnospondyls. Only the two Gondwanan families survived; the Brachyopidae and Chigutisauridae. Interestingly, 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 middle 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, this was one of the largest of its kind, as well as the last.

Two types of vertebrae
Originally, Temnospondyls 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.

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. 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. However, at least according to some analyses, the stereospondyls (minus some ambiguous taxa) can still be considered a clade.

Taxonomy
Class Amphibia
 * Order Temnospondyli
 * Saharastega
 * Superfamily Edopoidea
 * Family Cochleosauridae (Chenoprosopidae)
 * Family Edopidae
 * Capetus
 * Iberospondylus
 * Family Colosteidae
 * Family Dendrerpetontidae
 * Superfamily Capitosauroidea
 * Mastodonsauridae
 * Capitosauridae
 * Suborder Euskelia
 * Clade Limnarchia
 * Suborder Dvinosauria
 * Clade Stereospondylomorpha
 * Superfamily Archegosauroidea
 * Family Actinodontidae
 * Family Archegosauridae
 * Family Intasuchidae?
 * Suborder Stereospondyli
 * Family Peltobatrachidae
 * Family Lapillopsidae
 * Family Rhinesuchidae
 * Family Lydekkerinidae
 * Family Sclerothoracidae
 * Infraorder Trematosauria
 * Superfamily Trematosauroidea
 * Superfamily Metoposauroidea
 * Superfamily Plagiosauroidea
 * Superfamily Brachyopoidea
 * SuperFamily Rhytidosteoidea