| Reptil-like amphibians Temporal range: Early Carboniferous–Middle Triassic Descendant taxon Amniota survives to present |
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|---|---|
| Chroniosuchus, a semi aquatic early reptiliomorph | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Amphibia |
| Subclass: | "Labyrinthodontia" |
| (unranked): | Reptiliomorpha Säve-Söderbergh, 1934 |
| Suborders | |
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to the non-amniote reptile-like labyrinthodont grade. An alternative name, Anthracosauria is commonly used for the group, but is confusingly also used for the "lower" grade of reptiliomorphs by Benton.[1]
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Basal reptiliomorphs were land-based, reptile-like amphibians, in anatomy falling between the mainly aquatic Devonian labyrinthodonts and the first reptiles. University of Bristol paleontologist Professor Michael J. Benton gives the following characteristics for the Reptiliomorpha:[1]
The group differ from the contemporary non-reptiliomorph labyrinthodonts by having a deeper and taller skull, but retained the primitive kinesis (loose attachment) between skull roof and cheek. The deeper skull allowed for laterally placed eyers, contrary to dorsally placed eyers commonly found in amphibians. The skulls of the group are usually found with fine radiating grooves. The back of the skull held a deep otic notch in quadrate bone, likely holding a spiracle rather than a tympanum.[2][3]
The vertebrae showed the typical multi-element construction as seen in labyrinthodonts, but with pleurocentrum being the dominant element. Unlike most labyrinthonts, the body was moderatly deep rather than flat, and the limbs was well-developed and ossified, indicating a predominately terrestrial lifestyle except in secondarily aquatic groups. Each foot held 5 digits, the pattern seen in their amniote descendants.[4] They did however lack the reptilian type of ankle bone that would have allowed the use of the feet as levers for propulsion rather than as holdfasts.[5]
The general build was heavy in all forms, though otherwise very similar to that of early reptiles.[6] The skin, at least in the more advanced forms probably had a water-tight epidermal horny overlay, like seen in today's reptiles, though they lacked horny claws.[7][8] In chroniosuchians and some seymouriamorphans, like Discosauriscus, dermal scales are found in post-metamorphic specimens, indicating they may have had a "knobbly" if not scaly appearance.[9]
They reproduced in amphibian fashion with aquatic eggs that hatched into larvae (tadpoles) with external gills.[10]
During 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 had been considered true reptiles, and it is likely that to a modern observer they would have appeared as large to medium-sized, heavy-set lizards. Several groups however remained aquatic or semiaquatic. The chroniosuchians show the build and presumably habit similar to modern crocodiles as river-side predators, while the Chroniosuchia was either crocodile like or with elongated newt- or eel-like proportions. The two most terrestrially adapted groups were the medium sized insectivorous or carnivorous Seymouriamorpha and the mainly herbivorous Diadectomorpha, with many large forms. The latter group is in most analysis the closest relatives of the Amniotes.[11]
With their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) lead to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians.[12] Whatever the reason, the drive 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. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.[13]
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitating gas exchange between the egg and the atmosphere. 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 Gephyrostegus, Casineria and Westlothiana. Such small animals lay small eggs, 1 cm in diameter or less. Such eggs will have a small enough volume to surface ratio to be able to develop on land without the amnion and chorin actively effecting gas exchange, setting the stage for the evolution of true amniotic eggs.[13] 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.
The name Reptiliomorpha was coined by Professor Gunnar Säve-Söderbergh in 1934 to designate various types of late Paleozoic reptile-like labyrinthodont "amphibians."[14] However Alfred Sherwood Romer used the name Anthracosauria instead. In 1970, the German paleontologist Panchen reverted to Säve-Söderberghs definition,[15] but Romer's terminology is still in use, e.g. Carroll 1988 and 2002, and Hildebrand & Goslow 2001.[16][17][18] Some cladistic also work prefer Anthracosauria.[19]
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.[20]
In 1997 Michel Laurin and Robert Reisz (1997) adapted the term in a cladistic sense.[21] Michael Benton (2000, 2004) made it the sister-clade to Batrachomorpha.[22] However, when considered a linnean ranking, Reptiliomorpha is given the rank of superorder and only includes reptile-like amphibians, not their amniote descendants.[23] 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 is, as Toby White (Palaeos website) puts it, more like dogs than frogs (i.e. mammals but not amphibians).[2] However, given the lack of consensus of the phylogeny of the labyrinthodonts in general, and the origin of modern amphibians in particular, the actual content of the Reptiliomorpha under the latter definition is uncertain.
Classification after Benton (1997):