Temnospondyli
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Temnospondyli are an important and extremely diverse taxon of small to giant labyrinthodont amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous. 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 descendents.
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 very large Melosaurus of Eastern Europe.
As these amphibians continued to flourish and diversify in the lakes and rivers of the late Permian, one very successful group, the Stereospondyli, 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. 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 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, upto 2.7 to 4 meters or more in length (e.g. Paracyclotosaurus, Cyclotosaurus), with huge and extraordinarily flat skulls, a meter in more 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 extrenal gills.
The Triassic-Jurassic extinction event killed all the giant temnospondyls. Only the two Gondwana families the Brachyopidae and Chigutosauridae survived, and 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 free of crocodiles, co-existing with dinosaurs). At upto 5 meters in length, this was pne of the largest of its kind, as well as the last.
References
- Benton, M. J. (2000), Vertebrate Paleontology, 2nd Ed. Blackwell Science Ltd 3rd ed. (2004) - see also taxonomic hierarchy of the vertebrates, according to Benton 2004
- Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
- Laurin, Michel (1996) Terrestrial Vertebrates - Stegocephalians: Tetrapods and other digit-bearing vertebrates, Tree of Life project
- Reisz, Robert, (no date), Biology 356 - Major Features of Vertebrate Evolution - The Origin of Tetrapods and Temnospondyls
- Yates, A. M. & Warren, A. A. (2000), The phylogeny of the 'higher' temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zool. J. Linnean Soc. 128: 77-121.
External Links
- Temnospondyli - Tree of Life project
- Temnospondyli - Palaeos
- Temnospondyli - Permian Tetrapods
