Lepospondyli is a diverse grouping of early Tetrapod. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco ( Diplocaulus minimus),[Carroll, R. L. (1988) Vertebrate Paleontology and Evolution, WH Freeman & Co.][ Five major groups of lepospondyls are known: Adelospondyli; Aïstopoda; Lysorophia; Microsauria; and Nectridea. Lepospondyls have a diverse range of body forms and include species with newt-like, eel- or snake-like, and lizard-like forms. Various species were aquatic, semiaquatic, or terrestrial. None were large (the biggest genus, the diplocaulid Diplocaulus, reached a meter in length, but most were much smaller), and they are assumed to have lived in specialized ecological niches not taken by the more numerous temnospondyl amphibians that coexisted with them in the Paleozoic.
Lepospondyli was named in 1888 by Karl Alfred von Zittel, who coined the name to include some tetrapods from the Paleozoic that shared some specific characteristics in the notochord and teeth. Lepospondyls have sometimes been considered to be either related or ancestral to modern amphibians][Jason D. Pardo, Matt Szostakiwskyj, Per E. Ahlberg & Jason S. Anderson (2017) Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966>]
Description
All lepospondyls are characterised by having simple, spool-shaped that did not ossify from cartilage, but rather grew as bony cylinders around the notochord. In addition, the upper portion of the vertebra, the neural arch, is usually fused to the centrum (the main body of the vertebra).[Colbert 1969]
Classification
The position of the Lepospondyli within the Tetrapoda is uncertain because the earliest lepospondyls were already highly specialized when they first appeared in the fossil record. Some lepospondyls were once thought to be related or perhaps ancestral to modern (Urodela), but not the other modern amphibians. This view is no longer held and all modern amphibians (frogs, salamanders, and ) are now grouped within the clade Lissamphibia. For a long time, the Lepospondyli were considered one of the three subclasses of Amphibia, along with the Lissamphibia and the Labyrinthodontia.[Romer 1966][Carroll 1988] However, the dissolution of "labyrinthodonts" into separate groups such as Temnospondyli and Anthracosauria has cast doubt on these traditional amphibian subclasses. Much like "Labyrinthodontia", some studies proposed that Lepospondyli is an artificial (polyphyletic) grouping with some members closely related to extinct stem tetrapod groups and others more closely related to modern amphibians or reptiles.[Benton 2000] Early phylogenetic analyses conducted in the 1980s and 1990s often maintained the idea that lepospondyls were paraphyletic, with nectrideans close to colosteids and microsaurs close to temnospondyls, which were considered to be ancestral to modern amphibians.
However, a 1995 paper by Robert Carroll argued that lepospondyls were actually a monophyletic group closer to reptiles. Carroll considered them closer to reptiles than the Seymouriamorpha, but not as close as the Diadectomorpha.
Lepospondyl and tetrapod classification is still controversial, and even recent studies have had doubts about lepospondyl monophyly. For example, a 2007 paper has suggested that adelospondyls are stem-tetrapods close to Colosteidae
Interrelationships
Five main groups of lepospondyls are often recognized: Microsauria, a superficially lizard- or salamander-like and species-rich group; Lysorophia, a group with elongated bodies and very small limbs; Aïstopoda, a group of limbless, extremely elongated snake-like forms; Adelospondyli, a group of presumably aquatic forms that resemble aïstopods, but have more solidly built skulls; and Nectridea, another diverse group that includes terrestrial and aquatic newt-like forms. Microsauria is generally considered paraphyletic; rather than being a monophyletic group, it has been considered an evolutionary grade of basal ("primitive") lepospondyls, although there is growing consensus that a large subset of fossorially-adapted microsaurs, the Recumbirostra, is monophyletic. Lysorophia may belong within the Recumbirostran clade, distinct from other derived lepospondyls. Nectridea may also be paraphyletic, consisting of a range of more anatomically-specialized lepospondyls.
Position within Tetrapoda
The "lepospondyl hypothesis" of modern amphibian origins proposes that lissamphibians are monophyletic (that is, they form their own clade) and that they evolved from lepospondyl ancestors. Two alternatives are the "temnospondyl hypothesis", in which lissamphibians originated within Temnospondyli, and the "polyphyly hypothesis", in which caecilians originated from lepospondyls while frogs and salamanders (collectively grouped within Batrachia) evolved from temnospondyls. Of the three hypotheses, the temnospondyl hypothesis is currently the most widely accepted among researchers. Strong support for this relationship comes from a suite of anatomical features shared between lissamphibians and a group of Paleozoic temnospondyls called .[
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Below is a cladogram from Ruta et al. (2003) that supports the "temnospondyl hypothesis", showing the position of Lepospondyli within crown group Tetrapoda:[
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Gallery
File:Tuditanus1DB.jpg| Tuditanus, a Tuditanidae "microsaur"
File:Pelodosotis1DB.jpg| Pelodosotis, an Ostodolepidae "microsaur"
File:Pantylus1DB.jpg| Pantylus, a Pantylidae "microsaur"
File:Hyloplesion.jpg| Hyloplesion, a "microsaur"
File:Brachydectes NT small.jpg| Brachydectes, a
File:Adelospondylus2.jpg| Adelospondylus, an Adelospondyli
File:Oestocephalus.jpg| Oestocephalus, an early aïstopod
File:Phlegethontia NT small.jpg| Phlegethontia, an aïstopod
File:Diploceraspis12DB.jpg| Diploceraspis, a Diplocaulidae ""
File:Sauropleura.jpg| Sauropleura, a Urocordylidae ""
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Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
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Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd Taxonomic hierarchy
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Colbert, E H. (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
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Laurin, Michel (1996) Terrestrial Vertebrates - Stegocephalians: Tetrapods and other digit-bearing vertebrates
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Romer, A.S., (1966) Vertebrate Paleontology University of Chicago Press, 1933; 3rd ed. 1966.
External links
