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Origin of Modern Amphibians
editThe origins of Lissamphibia, the group of modern amphib ians incuding frogs (salientia), salamanders (caudata) and caecilians (gymnomorpha), are currently not well understood. It is generally agreed upon that lissamphibians arose from one or more lineages of Labyrinthodonts, a large group containing nearly all non-lissamphibian and non-amniote tetrapods some time during the carboniferous. Whether lissamphibians constitute a clade within labyrinthodontia is debated, with some authors proposing different origins for batrachia (salamanders + frogs) and gymnomorpha or for procera (caecilans + salamanders) and salientia[1]. Also debated are the specific labyrinthodontian lineages from which amphibians evolved.
Hypotheses on Lissamphibian Origins
editMonophyletic
editMost researchers consider lissamphibians to be monophyletic[2], but the possibility that the group is diphyletic remains. Hypotheses promoting monophyly center around two labyrinthodontia groups, the temnosmondyls and the lepospondyls.
Temnospondyls
editThe majority of researchers currently favor temnospondyls as the ancestors of modern amphibians, with lissamphibians most closely related to amphibamids, a group of dwarfed dissorophoid temnospondyls. This interpretation is based on several lines of data, including
Lepospondyls
editBecause of their generally small size, reduced skull complexity and fully ossified vertebral centra, lepospondyls have sometimes been suggested to give rise to the amphibians. Given the small size of modern amphibians, it is thought that the development of small body size was a key innovation in their evolution.
Diphyletic
editMetamorphosis
editMetamorphosis, defined as a period of rapid and extreme morphological change, appears to be the plesiomorphic state of modern amphibians, though many modern lineages do not undergo metamorphosis. Though metamorphosis is not as pronounced in caecilians as it is in most frogs and salamanders (batrachians), at least some caecilians do undergo a series of rapid developmental changes [3]. Because amphibians are often seen as displaying 'ptrimitive' tetrapod traits, it has often been assumed that such a metamorphosis is ancestral to all modern tetrapods[2]. However, fossil evidence shows no support for metamorphosis in the earliest labarynthodonts, and metamorphosing lineages begin to appear tens of millions of years after the first tetrapods evolved.
- ^ Ruta, M.; Coates, M. I. (2007). "Dates, nodes and character conflict: addressing the lissamphibian origin problem". Journal of Systematic Palaeontology 5 (1): 69–122;. doi:10.1017/S1477201906002008.
- ^ a b Schoch, R. (2009). Evolution of Life Cycles in Early Amphibians. Annu. Rev. Earth Planet. Sci. Annual Review of Earth and Planetary Sciences, doi: 135-162. 10.1146/annurev.earth.031208.100113
- ^ Reiss, J. (1996). Palatal Metamorphosis in Basal Caecilians (Amphibia: Gymnophiona) as Evidence for Lissamphibian Monophyly. Journal of Herpetology, 30(1), 27-27.