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Diadectomorpha

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Diadectomorpha is 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 (transitional between "amphibians" sensu lato and amniotes) positioned close to, but outside of the clade Amniota, though some recent research has recovered them as the sister group to the traditional Synapsida within Amniota, based on inner ear anatomy and cladistic analyses.[2][3][4][5] 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 diadectomorphs seem to have originated 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
Skeleton of Limnoscelis paludis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Reptiliomorpha
Order: †Diadectomorpha
Watson, 1917
Subgroups

See text.

Anatomy

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Life restoration of Limnoscelis

Diadectomorphs possessed both amphibian-like and amniote-like 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 either as non-amniote reptiliomorphs lying just outside the clade Amniota, or as early-diverging synapsids (members of the amniote group containing mammals and their extinct relatives).[2][3][5] Contrary to other non-amniote reptiliomorphs, the teeth of the Diadectomorpha lacked the infolding of the dentine and enamel that account for the name Labyrinthodontia for the non-amniote tetrapods.[6]

Classification

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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.[7]

  • The 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 herbivorous reptiles. The teeth were chisel-like and lacked the typical labyrinthodont infolding of the enamel. Diadectids were distributed over most of the Northern parts of Pangaea.
  • The 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.[8] Distribution seem to have been restricted to North America.
  • The family Tseajaiidae, known from a single specimen from North America, were medium-sized, generalized reptiliomorphs. 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.[9]

Below is a cladogram modified from the 2010 analysis:[1]

Diadectomorpha 

Reproduction and the origin of Amniota

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Life restoration of Diadectes

The reproduction of the Diadectomorphs has been the matter of some debate.[10] 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.[11][12][13][14]

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.[15] 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.

Lee and Spencer (1997) argued diadectomorphs probably laid amniote eggs because their adaptations to feed on terrestrial plants rich on fiber mean they were adapted to a niche not seen in unambiguous 'amphibians', and would have required an early acquisition of terrestrial endosymbionts necessary for this diet that supposedly could not have happened if young diadectomorphs were aquatic larvae.[16]

David Berman (2013) ran a phylogenetic analysis, and in this study the analysis resulted in Diadectomorpha being inside amniota as a sister taxon to Synapsida.[17]

References

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  1. ^ a b c 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. ^ a b David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
  3. ^ a b 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. 63: 131–154. doi:10.1111/pala.12448.
  4. ^ Klembara, Jozef; Ruta, Marcello; Hain, Miroslav; Berman, David S. (2021). "Braincase and Inner Ear Anatomy of the Late Carboniferous Tetrapod Limnoscelis dynatis (Diadectomorpha) Revealed by High-Resolution X-ray Microcomputed Tomography". Frontiers in Ecology and Evolution. 9. doi:10.3389/fevo.2021.709766. ISSN 2296-701X.
  5. ^ a b Brocklehurst, N. (2021). "The First Age of Reptiles? Comparing Reptile and Synapsid Diversity, and the Influence of Lagerstätten, During the Carboniferous and Early Permian". Frontiers in Ecology and Evolution. 9: 669765. doi:10.3389/fevo.2021.669765.
  6. ^ 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
  7. ^ Kissel, R. (2010). Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha) (Thesis). Toronto: University of Toronto Press. p. 185. hdl:1807/24357.
  8. ^ 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.
  9. ^ Time Traveler: In Search of Dinosaurs and Other Fossils from Montana to Mongolia by Michael Novacek
  10. ^ Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
  11. ^ 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
  12. ^ 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.
  13. ^ 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
  14. ^ 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
  15. ^ Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  16. ^ Lee, Michael S. Y.; Spencer, Patrick S. (1997-01-01), Sumida, Stuart S.; Martin, Karen L. M. (eds.), "CHAPTER 3 - CROWN-CLADES, KEY CHARACTERS AND TAXONOMIC STABILITY: WHEN IS AN AMNIOTE NOT AN AMNIOTE?", Amniote Origins, San Diego: Academic Press, pp. 61–84, ISBN 978-0-12-676460-4, retrieved 2020-09-18
  17. ^ Berman, David S (2013). "DIADECTOMORPHS,AMNIOTES OR NOT?". New Mexico Museum of Natural History and Science (60): 22.
  • 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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