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Fossil range: Early Permian
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Superorder: Reptiliomorpha
Order: Diadectomorpha
Family: Diadectidae
Genus: Diadectes

D. absitus
D. maximus
D. tenuitectus

Diadectes was a genus of large very reptile-like tetrapods that lived during the Early Permian. It is one of the very first herbivorous tetrapods, and also one of the first fully terrestrial animals to attain large size.

Diadectes was a heavily built animal, 1.5 to 3 meters long, with a thick-boned skull, heavy vertebrae and ribs, massive limb girdles and short, robust sprawling limbs. The nature of the limbs and vertebrae clearly indicate a terrestrial animal.

It possesses both amphibian and reptilian characteristics, combining a reptilian skeleton with a more primitive, seymouriamorph and amphibian-like skull. The genus was made the type for the order Cotylosauria, originally considered the most primitive and ancestral lineage of reptiles. More recently, Diadectes has been reclassified as an amphibian-like tetrapod, albeit one very close to the ancestry of the amniotes. It is now classified under the clade and order Diadectomorpha (of which it is the largest known representative), as well as in the revived clade of Cotylosauria.

Among the primitive features, Diadectes has a large otic notch (a feature found in Paleozoic amphibians) with an ossified tympanum.

At the same time its teeth show advanced specialisations for a herbivorous diet that are not found in any other type of early Permian animal. The eight front teeth are spatulate and peg-like, and served as incisors that were used to nip off mouthfuls of vegetation. The broad blunt cheek teeth show extensive wear associated with occlusion, and would have functioned as molars, grinding up the food.

It also had a partial secondary palate, which meant it could chew its food and breath at the same time, something many even more advanced reptiles were unable to do.

Diadectes fossil remains are known from a number of locations across North America, but especially the Texas Red Beds (Witchita and Clear Fork Groups)