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 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
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]
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]
^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
^Kissel, R. (2010). Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha) (Thesis). Toronto: University of Toronto Press. p. 185.
hdl:
1807/24357.
^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.
^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:
10.1038/19071abstract