Diadectomorpha

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iDiadectomorphs
Fossil range: Late Carboniferous - Early Permian

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Superclass:	Tetrapoda
Superorder:	Reptiliomorpha
(unranked)	Cotylosauria
Order:	Diadectomorpha

Genera

Limnoscelidae
Diadectidae
Tseajaiidae

Diadectomorpha are a clade of large reptile-like tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota. 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.

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 tetrapods, closely related to both reptiles and amphibians.

> A new phylogenetic analysis nests diadectomorphs deep within the Lepidosauromorph lineage of the Amniota (=Reptilia). Within this clade, Cephalerpeton is basal followed by Thuringothyris and the captorhinids. Higher on the tree is an unnamed clade composed of caseids + Milleretta + Oedaleops + Eunotosaurus and Acleistorhinus. Still higher are Belebey + Bolosaurus. At this point the diadectomorphs nest. Procolophon, pareiasaurs + turtles, Lanthanosuchus + Nyctiphruretus and finally all of the smaller, more lizardy lepidosauromophs are more derived clades listed in order of increasing distance from diadectomorphs. From this analysis it seems that within the Lepidosauromorphs there was a gradual and spectral trend to big, bulky, short-toed forms in the Permian that reversed itself in the Triassic resulting in smaller, longer-toed, more agile forms. This analysis, the largest and most inclusive of its kind, further demonstrates that diadectomorphs were not the sister taxa to basal amniotes and were not even close to amphibians. Therefore Carroll's hypothesis regarding a strong reduction in size among adult basal amniotes, such as Cephalerpeton, is once again in favor. The clades Parareptilia and Reptilomorpha no longer appear to be valid.

A taxon formerly associated with therapsids, Tetraceratops, actually nests with Limnoscelis and Tseajaia.

The Diadectomorpha seem to have evolved during late Mississippian times, although they only became common during the Late Pennsylvanian and Early Permian periods.