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Reptiliomorpha refers to reptile-like amphibians and the amniotes which evolved from them. The name was once used to refer to reptile-like labyrinthodonts, but this usage of the term is obsolete.
[edit] CharacteristicsUniversity of Bristol paleontologist Professor Michael J. Benton (2000, 2004) gives the following characteristics for the Reptiliomorpha:
[edit] Evolutionary historyDuring the Carboniferous and Permian periods, tetrapods evolved along a number of parallel lines towards a reptilian condition. Some of these tetrapods (e.g. Archeria, Eogyrinus) were elongate, eel-like aquatic forms with diminutive limbs, while others (e.g. Seymouria, Solenodonsaurus, Diadectes, Limnoscelis) were so reptile-like that until quite recently they actually have been considered true reptiles, and it is likely that to a modern observer they would have appeared as small to medium-sized, heavy-set lizards. Basal reptiliomorphs were land-based, reptile-like amphibians. They probably had water-tight skin, covered with horny scales in reptilian fashion, but retained aquatic eggs and larvae (tadpoles). Thus they were reptilian in all but their reproductive biology. A drive to abandon the larval stage and aquatic eggs (possibly due to competition for breeding ponds or to exploit drier environments) led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. True terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack. Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including Casineria and Westlothiana. Although the first amniote probably appeared as early as the latest Mississippian period (Middle Carboniferous), non-amniote (or amphibian) reptiliomorphs continued to flourish alongside their amniote descendants for many millions of years. By the middle Permian the non-amniote terrestrial forms had died out, but several aquatic non-amniote groups continued to the end of the Permain, and in the case of the Chroniosuchids survived the end Permian mass extinction, only to die out at the end of the Early Triassic. Meanwhile, the single most successful daughter-clade of the reptiliomorphs, the amniotes, continued to flourish and to inherit the Earth. [edit] Changing DefinitionsThe name Reptiliomorpha was coined by Professor Gunnar Säve-Söderbergh in 1934 to designate various types of late Paleozoic reptile-like labyrinthodont "amphibians." However Alfred Sherwood Romer used the name Anthracosauria instead, and this has been used until quite recently e.g. Carroll 1988. In 1956 Friedrich von Huene included both amphibians and anapsid reptiles in the Reptiliomorpha. This included the following orders: 1. Anthracosauria, 2. Seymouriamorpha, 3. Microsauria, 4. Diadectomorpha, 5. Procolophonia, 6. Pareiasauria, 7. Captorhinidia, 8. Testudinata. In 1997 Michel Laurin and Robert Reisz (1997) adapted the term in a cladistic sense. Michael Benton (2000, 2004) made it the sister-clade to Batrachomorpha. However, when considered a linnean ranking, Reptiliomorpha is given the rank of superorder and only includes reptile-like tetrapods [Systema Naturae 2000]. More recently Reptiliomorpha has been adopted as the term for the largest clade that includes – according to the technical definitions of the phylocode which only refers to species or genus level organisms – Homo sapiens but not Ascaphus truei (a primitive frog) (International Phylogenetic Nomenclature Meeting 2003). Or, as Toby White (Palaeos website) puts it, more like dogs than frogs (i.e. mammals but not amphibians). [edit] TaxonomyClassification after Benton (1997):
[edit] References and External links
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