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Temnospondyli (from Greek temnein, to cut + 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 (Benton 2000, 2004, Laurin 1996, Reisz no date), over whether some specialised forms were ancestral to some modern amphibians, or whether the whole group died out without leaving any descendants.

Classification

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 and more degenerate vertebrae were called "Stereospondyli"; and these two types were considered suborders of the order Temnospondyli.

In the Ratchitomous condition the intercentra were large and wedge-shaped, and the pleuracentra were relatively small blocks that fitted between them. Both elements supported the neural arch, and there were well developoed and interlocking zygapophyses that strengthened the connections between the vertebrae. The strong backbone, plus strong limbs, enabled many ratchitomes to be at least partially, and in some cases fully, terrestrial.

In the stereospondylous condition the pleurocentra have been lost, and the veretbral centra reduced to simple blocks made up of the intercentra only. This weaker type of backbone indicates a more fully aquatic existence (Colbert 1969)

Cladistic Research

More recent research shows that this classification is no longer viable. The basic Ratchitomous condition is primitive for amphibians (tetrapods) in general, the pure stereospondyls seem to have arisen from different ancestors (Carroll 1988), and some temnospondyls have ratchitomous, semi-ratchitomous, and sterospondylous vertebrae at different points the vertebral column if the same individual, and some taxa show intermediate morphologies that don't fit into one or the other category (Steyer and Laurin 2000). However according to some analyses the stereospondyls themselves can still be considered a monophyletic clade (Yates and Warren 2000, Laurin and Steyer 2000)

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 ("Mexican walking fish"). 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 archegosaur 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, along with 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 amphibians to do so. 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 be swimming past. In the late Triassic (Carnian) these big amphibains were joined by the superficially very similar Metoposauridae (1.5 meters long - and distingusihed mainly by the different posiition 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 Chigutosauridae. Interestingly these grew to large size during the Jurassic, with the Brachyopids flourishing in China, and the Chigutosaurs in Gondwana.

The most recent known temnospondyl was the giant Chigutosaur, 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 up to 5 meters in length, this was one of the largest of its kind, as well as the last.

Taxonomy

Class Amphibia

References

External links

Temnospondyls | Carboniferous | Permian | Triassic

Temnospondyli | Temnospondyli

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Temnospondyli".

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