Diadectomorpha

Diadectomorpha are a clade of large tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian),[1] They have typically been classified as advanced reptiliomorphs ("reptile-like amphibians") close to the ancestry of the Amniota, though some paleontologists consider them basal synapsids.[2][3] They include both large (up to 2 meters long) carnivorous and even larger (to 3 meters) herbivorous forms, some semi-aquatic and others fully terrestrial. The Diadectomorpha seem to have evolved during late Mississippian times, although they only became common after the Carboniferous rainforest collapse and flourished during the Late Pennsylvanian and Early Permian periods.

Diadectomorphs
Temporal range: Late Carboniferous–Wuchiapingian [1]
Skeleton of Diadectes sideropelicus in the American Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Reptiliomorpha
Order: †Diadectomorpha
Watson, 1917
Subgroups

See text.

Anatomy
Life restoration of Limnoscelis

Diadectomorphs possess both amphibian and reptilian characteristics. Originally these animals were included under the order Cotylosauria, and were considered the most primitive and ancestral lineage of reptiles. More recently they have been reclassified as amphibian-grade tetrapods, closely related to the first true amniotes (though they have also been argued to be amniotes more closely related to synapsids than to sauropsids[2][3]). Contrary to other Reptiliomorph amphibians, the teeth of the Diadectomorpha lacked the infolding of the dentine and enamel that account for the name Labyrinthodontia for the non-amniote tetrapods.[4]

Classification

Diadectomorpha is most commonly given the rank of order when formal taxonomic ranks are applied. It is further divided into three families, representing specialization into different ecological niches. The exact phylogenetic relationship between the three is disputed.[5]

  • Family Diadectidae is perhaps the best known group, comprising medium to large herbivores. Early members were low-slung, but the latter Diadectes (from which the whole group take its name) evolved strong, if sprawling legs, paralleling the anatomy of early herbovorious reptiles. The teeth were chisel-like and lacked the typical labyrinthodont infolding of the enamel. The Diadectidae were distributed over most of the Northern parts of Pangaea.
  • Family Limnoscelidae contained large carnivores or piscivores. The largest genus, Limnoscelis could grow to at least 1.5 meters. The family had pointed and slightly curved teeth with labyrinthodont enamel.[6] Distribution seem to have been restricted to the North America.
  • Family Tseajaiidae known from a single specimen from North America, were medium-sized, generalized reptiliomorph amphibians. They had blunt teeth and appear to be primarily herbivorous or omnivorous. The known specimen would have been on the order of a meter (3 ft.) long.[7]

Reproduction and the origin of Amniota
Life restoration of Diadectes

The reproduction of the Diadectomorphs has been the matter of some debate.[8] If their group lay within the Amniota as has at times been assumed, they would have laid an early version of the amniote egg. Current thinking favours the amniote egg being evolved in very small animals, like Westlothiana or Casineria, leaving the bulky Diadectomorphs just on the amphibian side of the divide.[9][10][11][12]

This would indicate the large and bulky Diadectomorphs laid anamniote eggs (in water). However, no unambiguously diadectomorph tadpole is known. Whether this is due to an actual lack of tadpole stage or taphonomy (many diadectomorphs were upland creatures where tadpoles would have a poor probability of being fossilized) is uncertain. Alfred Romer indicated that the anamniote/amniote divide might not have been very sharp, leaving the question of the actual mode of reproduction of these large animals unanswered.[13] Possible reproductive modes include full amphibian spawning with aquatic tadpoles, internal fertilization with or without ovoviviparity, aquatic eggs with direct development or some combination of these. The reproductive mode might also have varied within the group.

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References
  1. Jun Liu and Gabe S. Bever (2015). "The last diadectomorph sheds light on Late Palaeozoic tetrapod biogeography". Biology Letters. 11 (5): 20150100. doi:10.1098/rsbl.2015.0100. PMC 4455737. PMID 25948572.
  2. David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
  3. Jozef Klembara; Miroslav Hain; Marcello Ruta; David S. Berman; Stephanie E. Pierce; Amy C. Henrici (2019). "Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group". Palaeontology. in press. doi:10.1111/pala.12448.
  4. Müller, J. & Reisz, R.R. (2005): An early captorhinid reptile (Amniota: Eureptilia) from the Upper Carboniferous of Hamilton, Kansas. Journal of Vertebrate Paleontology, no 23: pp 561-568
  5. Kissel, R. (2010). Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Toronto: University of Toronto Press. p. 185. hdl:1807/24357.
  6. Williston, S.W. (1911). "A new family of reptiles from the Permian of New Mexico". The American Journal of Science. 4. 33 (185): 378–398. Bibcode:1911AmJS...31..378W. doi:10.2475/ajs.s4-31.185.378.
  7. Time Traveler: In Search of Dinosaurs and Other Fossils from Montana to Mongolia by Michael Novacek
  8. Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
  9. Laurin, M. (2004): The Evolution of Body Size, Cope's Rule and the Origin of Amniotes. Systematic Biology no 53 (4): pp 594-622. doi:10.1080/10635150490445706 article
  10. Smithson, T.R. & Rolfe, W.D.I. (1990): Westlothiana gen. nov. :naming the earliest known reptile. Scottish Journal of Geology no 26, pp 137–138.
  11. Paton, R.L.; Smithson, T.R. & Clack, J.A. (1999): An amniote-like skeleton from the Early Carboniferous of Scotland. Nature no 398, pp 508-513 doi:10.1038/19071 abstract
  12. Monastersky, R. (1999): Out of the Swamps, How early vertebrates established a foothold—with all 10 toes—on land, Science News Volume 155, No 21
  13. Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  • Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
  • Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
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