Labyrinthodontia

Labyrinthodont (Greek, "maze-toothed") is an obsolete term for any member of an extinct superorder or subclass (Labyrinthodontia) of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times (about 350 to 210 million years ago). The name describes the pattern of infolding of the dentine and enamel of the teeth, which are often the only part of the creatures that fossilize. They are also distinguished by a heavy solid skull (and therefore often named "Stegocephalia"), and complex vertebrae, the structure of which is useful in older classifications of the group.

Anatomy

Labyrinthodonts are named for the pattern of infolding of the dentine and enamel of the teeth, that resembles a maze (or labyrinth). Labyrinthodonts could be up to four meters long. They were short-legged and large headed. Their skulls were deep and massive, and their jaws were lined with small, sharp, conical teeth. Also, there was a second row of teeth on the roof of the mouth. In their way of living labyrinthodonts were probably similar to fishes - they laid eggs in the water, where their larvae developed into mature animals.

Characteristically labyrinthodonts have vertebrae made of 4 pieces, an intercentrum, two pleurocentra, and a neural arch/spine. The relative sizes of these pieces distinguishes different groups of labyrinthodonts. They also had special sense organs in the skin, that formed a system for perception of water fluctuations. Some of them possessed well developed gills and many seemingly had primitive lungs. They could breath atmospheric air; that was a great advantage for residents of warm shoals with low oxygen levels in the water. The air was inflated into the lungs by contractions of a special throat sac. Primitive members of all labyrinthodont groups were probably true water predators, and only advanced forms that arose independently in different groups and times, gained an amphibious, semi-aquatic mode of living. Their bulky skeleton and their short limbs suggest that the majority of the labyrinthodonts were slow walkers on land.

Evolution

The Labyrinthodontia evolved from a bony fish group: the Crossopterygii rhipidistia. Nowadays only a few living representatives of these fish remains: two species of coelacanth and six species of lungfish.

The most diverse group of the labyrinthodonts was the Batrachomorpha. Though these animals looked more like crocodiles, they most probably gave rise to the order Anura, the amphibians without tails, which include, in particular, the modern frogs. Batrachomorphs appeared in the Late Devonian, but they had worldwide distribution in the continental shallow basins of the Permian (Platyoposaurus, Melosaurus) and Triassic Periods (Thoosuchus, Benthosuchus, Eryosuchus). Some batrachomorphs existed until the end of the Cretaceous.

Classification

The traditional classification (e.g. Romer 1966, also repeated in Colbert 1969, and Carroll 1988) has three orders:

* Ichthyostegalia (primitive ancestral forms (e.g. "Ichthyostega") - Late Devonian only)
** "While undoubtly amphibians on anatomy and habit, the Ichthyostegalia are ancestral to all tetrapodes, and are not amphibians in the restricted sense."
* Temnospondyli (common, small to large, flat-headed forms with either strong or secondarily weak vertebrae and limbs - mainly Carboniferous to Triassic e.g. "Eryops" from the early Permian is a well-known genus, More recently fossil Jurassic and Cretaceous temnospondyls have been found. Originally considered ancestral to Anura (frogs), may or may not be ancestral to all modern amphibians
** "Temnospondyls are the only "Labyrinthodonts" currently considered to be "true amphibians" in that they are more closely related to modern Lissamphibia than to other tetrapodes."
* Anthracosauria (deep skulls, strong vertebrae but weak limbs, evolving towards and ancestral to reptiles - Carboniferous and Permian - e.g. "Seymouria")
** "The Anthracosauria are thought the direct ancestral to the early reptiles, and thus separate from modern ("true") amphibians."

ystematic overview of Labyrintodonta (Stegocephalia)

(from lobe-fined fish) `- "Eusthenopteron" (advanced lobe-fined fish) `- "Panderichthys" (lobe-fined fish with limb-like fins) `- "Tiktaalik" (transitional fish/amphibian) `- "Acanthostega" (early amphibian with fishlike gills) `- "Ichthyostega" (early amphibian) `- "Crassigyrinus" (early amphibian)
- Loxommatidae(eel-like primitive temnospondyles)
`-Temnospondyls (large, flat-headed stegocephalians) `- Anthracosaurs (reptile-like amphibians)
- Seymouriamorphs (advanced repile-like amphibians)
|- "Westlothiana" (small amphibian or possibly early reptile)
`+ Diadectomorphs (earliest reptiles or sister groups of reptiles)
`- amniotes (i.e. first reptiles) `- Batrachomorpha (modern amphibinas and their extinct relatives) `- Lepospondyls (small stegocephalians) `?- Lissamphibia (modern amphibians

A good summary (with diagram) of characteristics and main evolutionary trends of the above three orders is given in Colbert 1969 pp.102-103.

The grouping "Labyrinthodonts" has since been largely discarded as paraphyletic, that is, artificially composed of organisms that have separate genealogies, and thus not a valid taxon. The groups that have usually been placed within Labyrinthodontia, are currently variously classified as basal tetrapods, non-amniote Reptiliomorpha and as a monophyletic or paraphyletic Temnospondyli, according to cladistic analysis.

References

* Carroll, R. L. (1988), "Vertebrate Paleontology and Evolution", WH Freeman & Co.
* Colbert, E. H., (1969), "Evolution of the Vertebrates", John Wiley & Sons Inc (2nd ed.)
* Romer, A. S., (1947, revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago
* [http://people.eku.edu/ritchisong/342notes1.htm Comparative Vertebrate Anatomy]

External links

* [http://www.palaeos.com/Vertebrates/Units/190Reptilomorpha/Cladogram.html Palaeos.com Cladogram of Reptiliomorpha]
* [http://www-biol.paisley.ac.uk/courses/tatner/biomedia/subunits/subcl98.htm Subclass Labyrinthodontia]

See also

*Tetrapod
*Temnospondyli
*Batrachosauria
*Reptiliomorpha
*Amphibian
*Prehistoric amphibian
*"Ichthyostega"
*"Eryops"
*Prehistoric life