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Temnospondyli

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Temnospondyls
Temporal range: Carboniferous–Early Cretaceous
Skeleton of Eryops.
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Amphibia
Subclass:
Labyrinthodontia
Order:
†(?) Temnospondyli

Zittel, 1888
Suborders

See below

Temnospondyli (from Greek τεμνειν, temnein = "to cut" + σπονδυλως, spondulos = "vertebra") is 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]

However, to settle this, I think it was probably both.

Evolutionary History

Carboniferous and early Permian Temnospondyli

Capetus, a basal temnospondyl.

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 trematopids developed into large (c. 1.5 metres (4.9 ft) long) and heavy-bodied semi-aquatic predators.

Late Permian Temnospondyli

Prionosuchus from the Permian, probably the largest amphibian ever.

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 13.1 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]

Mesozoic survivors

Siderops, a Jurassic temnospondyl.

In 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.

Relationship to modern amphibians

The 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] However, it's possible that they were close relatives of modern amphibians. Or perhaps distant relatives.

Systematics

Two types of vertebrae

Originally, 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]

Taxonomy

Edops, a basal edopoid.
Zygosaurus, a dissorophid.
Sclerothorax, a basal limnarchian.
Dvinosaurus, a dvinosaurid.
Cyclotosaurus, a capitosaurid.

Class Amphibia

Phylogeny

The following cladogram follows the analyses of Holmes, Carroll & Reisz (1998), Yates & Warren (2000), and Sidor et al. (2005).[11][13][14]

Temnospondyli
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The following cladogram is modified from Ruta et al. (2007).[15]

1
7

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

References

  1. ^ 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
  2. ^ Laurin, M. (1996) Terrestrial Vertebrates - Stegocephalians: Tetrapods and other digit-bearing vertebrates, The Tree of Life Web Project
  3. ^ Reisz, Robert, (no date), Biology 356 - Major Features of Vertebrate Evolution - The Origin of Tetrapods and Temnospondyls
  4. ^ Adam Yates, Guide to Wild Dinosaurs, Sterling, 2003, p. 230 gives the length as 2.5 meters. According to Dann's Dinosaurs the total length is estimated at 4 to 5 metres. Walking with Dinosaurs and Oz Fossils both give the length as 5 meters
  5. ^ Anderson J.S., Reisz R.R., Scott D., Fröbisch N.B., & Sumida S.S. (2008): A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature no 453: pp 515-518
  6. ^ Bolt, J. R. (1969): Lissamphibian origins: possible protolissamphibian from the Lower Permian of Oklahoma. Scienc no 166: pp 888-891
  7. ^ Laurin, M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I — systematics, middle ear evolution, and jaw suspension. Annales des Sciences Naturelles, Zoologie, Paris, 13e Série 19: pp 1-42.
  8. ^ Colbert, E. H. (1969). Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
  9. ^ Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, W H Freeman & Co.
  10. ^ a b Laurin, M. and Steyer, J-S (2000) [* Laurin, M. and Steyer, J-S (2000) Phylogeny and Apomorphies of Temnospondyls, The Tree of Life Web Project
  11. ^ a b 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. Zoological Journal of the Linnean Society 128: 77-121.
  12. ^ Schoch, R.R., Fastnacht, M., Fichter, J., and Keller, T. 2007. Anatomy and relationship of the Triassic temnospondyl Sclerothorax. Acta Palaeontologica Polonica 52, 117–136. (PDF)
  13. ^ Holmes, R.B., Carroll, R.L., and Reisz, R.R. (1998). "The first articulated skeleton of Dendrerpeton acadianum (Temnospondyli: Dendrerpentonidae) from the Lower Pennsylvanian locality of Joggins, Nova Scotia, and a review of its relationships." Journal of Vertebrate Paleontology, 18(1): 64-79.
  14. ^ Sidor, C.A., O'Keefe, F.R., Damiani, R.J., Steyer, J.-S., Smith, R.M.H., Larsson, H.C.E., Sereno, P.C., Ide, O., and Maga, A. (2005). "Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea." Nature, 434: 886-889.
  15. ^ Ruta, M. (2007). "A supertree of Temnospondyli: cladogenetic patterns in the most species-rich groups of early tetrapods". Proceedings of the Royal Society B. 274: 3087–3095. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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