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Reptiliomorpha

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Reptiliomorphs
Temporal range: 340–230 Ma Descendant taxon Amniota survives to present
Fossil skeletons of the reptiliomorph Seymouria
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: 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]

Characteristics

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]

  • narrow premaxillae (less than half the skull width)
  • vomers taper forward
  • phalangeal formulae (number of joints in each toe) of foot 2.3.4.5.4–5

Cranial morphology

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 eyes, contrary to dorsally placed eyes commonly found in amphibians. The skulls of the group are usually found with fine radiating grooves. The quadrate bonein the back of the skull held a deep otic notch, likely holding a spiracle rather than a tympanum.[2][3]

Postcranial skeleton

The vertebrae showed the typical multi-element construction as seen in labyrinthodonts. According to Benton, in the vertebrae of "anthracosaurs" (i.e. Embolomeri) the intercentrum and pleurocentrum may be of equal size, while in the vertebrae of seymouriamorphs the pleurocentrum is the dominant element and the intercentrum is reduced to a small wedge. The intercentrum gets further reduced in the vertebrae of amniotes, where it becomes a thin plate or disappears altogether.[4] Unlike most labyrinthodonts, the body was moderately deep rather than flat, and the limbs were 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.[5] 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.[6]

Physiology

The general build was heavy in all forms, though otherwise very similar to that of early reptiles.[7] 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.[8][9] 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.[10]

They reproduced in amphibian fashion with aquatic eggs that hatched into larvae (tadpoles) with external gills.[11]

Evolutionary history

Two Archeria, an aquatic reptiliomorph
Seymouriashadow.
Diadectes, an advanced reptiliiomorph, variously classified as a reptile or amphibian

Early reptiliomorphs

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.[12]

From aquatic to terrestrial eggs

Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) led 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.[13] 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.[14]

Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitate 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.

Origin of amniotes

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 Solenodonsaurus, 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.[14] Although the first amniote probably appeared as early as the Middle Mississippian sub-epoch, 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.

Changing definitions

The name Reptiliomorpha was coined by Professor Gunnar Säve-Söderbergh in 1934 to designate various types of late Paleozoic reptile-like labyrinthodont "amphibians". In his view, the amphibians had evolved from fish twice, with one group composed of the ancestors of modern salamanders and caecilians and the other, which Säve-Söderbergh referred to as Reptiliomorpha, consisting of the ancestors of anurans (frogs) and amniotes.[15]

Alfred Sherwood Romer rejected Säve-Söderbergh's theory of a biphyletic amphibia and used the name Anthracosauria to describe the labyrinthodont lineage from which amniotes evolved. In 1970, the German paleontologist Alec Panchen took up Säve-Söderberghs name for this group as having priority,[16] but Romer's terminology is still in use, e.g. by Carroll (1988 and 2002) and by Hildebrand & Goslow (2001).[17][18][19] Some writers preferring phylogenetic nomenclature use Anthracosauria.[20]

In 1956, Friedrich von Huene included both amphibians and anapsid reptiles in the Reptiliomorpha. This included the following orders: Anthracosauria, Seymouriamorpha, Microsauria, Diadectomorpha, Procolophonia, Pareiasauria, Captorhinidia, Testudinata.[21]

In 1997, Michel Laurin and Robert Reisz (1997) adapted the term in a sense compatible with the Phylocode.[22] Michael Benton (2000, 2004) made it the sister-clade to Lepospondyli.[1] However, when considered in a Linnean framework, Reptiliomorpha is given the rank of superorder and includes only reptile-like amphibians, not their amniote descendants.[23] More recently, in accordance with the Phylocode, Reptiliomorpha has been adopted as the term for the largest clade that includes Homo sapiens but not Ascaphus truei (a primitive frog)[24][25] or is, as Toby White (Palaeos website) puts it, more like dogs than frogs (i.e. more like mammals than like 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.

Taxonomy

Classification after Ruta et al. (2003):[26]

File:Discosauriscus BW.jpg
Discosauriscus.
Gephyrostegus.
Limnoscelis.
Diplovertebron.

Inclusion of lepospondyls

Several paleontological studies indicate that amniotes and diadectomorphs share more recent common ancestor with lepospondyls than with seymouriamorphs, Gephyrostegus and Embolomeri (e.g. Laurin and Reisz, 1997,[27] 1999;[28] Ruta, Coates and Quicke, 2003;[26] Vallin and Laurin, 2004;[25] Ruta and Coates, 2007[29]). Assuming that the clade Lissamphibia (containing modern amphibians) isn't descended from lepospondyls but from a different group of tetrapods, e.g. from temnospondyls,[26][29][30] it would mean that Lepospondyli belonged to Reptiliomorpha sensu Laurin (2001), as it would make them more closely related to amniotes than to lissamphibians. On the other hand, if lissamphibians are descended from lepospondyls,[25][27][28] then not only Lepospondyli would have to be excluded from Reptiliomorpha sensu Laurin (2001), but seymouriamorphs, Gephyrostegus and Embolomeri would also have to be excluded from this clade, as they were more distantly related to amniotes than lepospondyls were.

Complicating the picture is the question of whether the lepospondyls form a phylogenetic unit at all, or is a wastebasket taxon containing the paedamorphic forms and tadpoles of various labyrinthodonts, notably the reptile-like amphibians.[31] The cladistic analysis of Gerobatrachus (Anderson et al., 2008) suggests that salamanders and frogs have evolved from temnospondyls, while caecilians evolved from lepospondyls (which were recovered as the sister group of diadectomorphs; the amniotes weren't included in the analysis), rendering Lissamphibia itself polyphyletic.[32] This analysis was subsequently criticized by Marjanović and Laurin, whose own analyses (Marjanović and Laurin 2008, 2009) recovered monophyletic Lissamphibia;[33][34] lissamphibian monophyly was also confirmed by later analysis conducted by Hillary C. Maddin, Farish A. Jenkins Jr and Jason S. Anderson (2012).[30] The uncertainty over the phylogeny of the reptiliomorphans reflects the lack of consensus on origin of lissamphibians and relationship between the various labyrinthodonts in general.

Phylogeny

The following cladogram simplified after an analysis of stem amniotes presented by Ruta et al. in 2003.[26]

 Crown group Tetrapoda 
"Anthracosauria"

References

  1. ^ a b c Benton, M. J. (2000), Vertebrate Paleontology, 2nd Ed. Blackwell Science Ltd 3rd ed. 2004 – see also taxonomic hierarchy of the vertebrates, according to Benton 2004
  2. ^ a b Palaeos Reptilomorpha
  3. ^ Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation. Biological Journal of the Linnean Society No 11: pp 19–76 Abstract
  4. ^ Chapter 4: "The early tetrapods and amphibians." In: Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd.
  5. ^ Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  6. ^ Palaeos Reptilomorpha: Cotylosauria
  7. ^ Romer, A.S. (1946). "The primitive reptile Limnoscelis restudied". American Journal of Science. 244: 149–188.
  8. ^ R. L. Paton, T. R. Smithson and J. A. Clack, "An amniote-like skeleton from the Early Carboniferous of Scotland", (abstract), Nature 398, 508-513 (8 April 1999)
  9. ^ Maddin & al. (2009): The anatomy and development of the claws ofXenopus laevis (Lissamphibia: Anura) reveal alternate pathways of structural evolution in the integument of tetrapods. Journal of Anatomy, no 214 (4): pp 607-19 Abstract
  10. ^ Laurin, M. (1996). "A Reevaluation of Ariekanerpeton, a Lower Permian Seymouriamorph (Vertebrata: Seymouriamorpha) from Tadzhikistan". Journal of Vertebrate Paleontology. 16 (4): 653–665.
  11. ^ Špinar, Z. V. (1952): Revision of some Morovian Discosauriscidae. Rozpravy ustrededniho Uštavu Geologickeho no 15, pp 1–160
  12. ^ Laurin, M. (1996): Phylogeny of Stegocephalians, from the Tree of Life Web Project
  13. ^ Duellman, W.E. & Trueb, L. (1994): Biology of amphibians. The Johns Hopkins University Press
  14. ^ a b Carroll R.L. (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) Origins of the higher groups of tetrapods — controversy and consensus. Ithaca: Cornell University Press, pp 331-353.
  15. ^ Säve-Söderbergh, G. (1934). "Some points of view concerning the evolution of the vertebrates and the classification of this group". Arkiv för Zoologi. 26A: 1–20.
  16. ^ Panchen, A. L. (1970). Handbuch der Paläoherpetologie - Encyclopedia of Paleoherpetology Part 5a - Batrachosauria (Anthracosauria), Gustav Fischer Verlag - Stuttgart & Portland, 83 pp., ISBN 3-89937-021-X.
  17. ^ Carroll, R. L., (1988): Vertebrate paleontology and evolution. W. H. Freeman and company, New York.
  18. ^ Carroll, R. (2002): ""Palaeontology: Early land vertebrates". Nature 418: 35-36.
  19. ^ Hildebrand, M.; Goslow, G. E., Jr (2001). Analysis of vertebrate structure. Principal ill. Viola Hildebrand. New York: Wiley. p. 429. ISBN 0-471-29505-1. {{cite book}}: Cite has empty unknown parameter: |1= (help)CS1 maint: multiple names: authors list (link)
  20. ^ Gauthier, J., Kluge, A.G., & Rowe, T. (1988): "The early evolution of the Amniota". In The phylogeny and classification of the tetrapods, no 1: amphibians, reptiles, birds. Edited by M.J. Benton. Clarendon Press, Oxford, pp. 103–155.
  21. ^ Von Huene, F., (1956), Paläontologie und Phylogenie der niederen Tetrapoden, G. Fischer, Jena.
  22. ^ Second circular of the first International Phylogenetic Nomenclature Meeting, 2003
  23. ^ Systema Naturae (2000) / Classification Superorder Reptiliomorpha
  24. ^ Laurin, M. (2001). "L'utilisation de la taxonomie phylogénétique en paléontologie: avantages et inconvénients". Biosystema. 19: 197–211.
  25. ^ a b c Vallin, Grégoire; Laurin, Michel (2004). "Cranial morphology and affinities of Microbrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians". Journal of Vertebrate Paleontology. 24 (1): 56–72. doi:10.1671/5.1.
  26. ^ a b c d Marcello Ruta, Michael I. Coates and Donald L. J. Quicke (2003). "Early tetrapod relationships revisited" (PDF). Biological Reviews. 78 (2): 251–345. doi:10.1017/S1464793102006103. PMID 12803423. Cite error: The named reference "RCQ03" was defined multiple times with different content (see the help page).
  27. ^ a b Laurin, M. (1997). "A new perspective on tetrapod phylogeny". Amniote Origins: Completing the Transition to Land. Academic Press. pp. 9–60. ISBN 0-12-676460-3, 9780126764604. {{cite book}}: Check |isbn= value: invalid character (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |editors= ignored (|editor= suggested) (help)
  28. ^ a b Laurin, M. (1999). "A new study of Solenodonsaurus janenschi, and a reconsideration of amniote origins and stegocephalian evolution". Canadian Journal of Earth Sciences. 36 (8): 1239–1255. doi:10.1139/e99-036. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  29. ^ a b Ruta, M. (2007). "Dates, nodes and character conflict: addressing the lissamphibian origin problem". Journal of Systematic Palaeontology. 5 (1): 69–122. doi:10.1017/S1477201906002008. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  30. ^ a b Hillary C. Maddin, Farish A. Jenkins Jr and Jason S. Anderson (2012). "The Braincase of Eocaecilia micropodia (Lissamphibia, Gymnophiona) and the Origin of Caecilians". PLoS ONE. 7 (12): e50743. doi:10.1371/journal.pone.0050743.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  31. ^ White, T. & Kazlev, M. A.: Lepospondyli: Overview, from Palaeos website.
  32. ^ Anderson J. S., Reisz R. R., Scott D., Fröbisch N. B., & Sumida S. S. (2008): A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature No 453, pp 515–518 doi:10.1038/nature06865 PMID 18497824
  33. ^ David Marjanović and Michel Laurin (2008). "A reevaluation of the evidence supporting an unorthodox hypothesis on the origin of extant amphibians". Contributions to Zoology. 77 (3): 149–199.
  34. ^ David Marjanović and Michel Laurin (2009). "The origin(s) of modern amphibians: a commentary". Evolutionary Biology. 36 (3): 336–338. doi:10.1007/s11692-009-9065-8.
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