Diadectes

Diadectes, whose name means "crosswise-biter," is an extinct genus of large reptiliomorphs from the early Permian period, roughly 290 to 272 million years ago. It ranks among the first herbivorous tetrapods and was also one of the earliest fully terrestrial vertebrates to grow to a large size.

This animal was heavily built, reaching up to 3 meters (10 feet) in length. It had a thick-boned skull, heavy vertebrae and ribs, massive limb girdles, and short, sturdy limbs. The structure of its limbs and vertebrae clearly points to a terrestrial lifestyle. While earlier reconstructions suggested a barrel-shaped rib cage, newer fossils indicate the ribs actually stuck out to the sides. High-resolution X-ray microcomputed tomography of *D. absitus* has revealed an endosseous labyrinth in the opisthotic, prootic, and supraoccipital bones, along with a well-preserved vestibule, three semicircular canals, and a developed cochlear recess. The supraoccipital, located medially to the anterior semicircular canal, has a shallow, subarcuate fossa on its ventral surface. An otic tube connects the fenestra vestibuli to the vestibule.

In terms of paleobiology, *Diadectes* shows a mix of reptilian and amphibian traits, combining a reptile-like skeleton with a more primitive, seymouriamorph-like skull. It is classified as a member of the sister group to amniotes. Among its primitive features is a large otic notch—common in labyrinthodonts but absent in reptiles—with an ossified tympanum. At the same time, its teeth show advanced specializations for an herbivorous diet not seen in any other early Permian animal. The eight front teeth are spatulate and peg-like, acting as incisors to nip off vegetation. The broad, blunt cheek teeth show extensive wear from occlusion and functioned as molars for grinding food. A partial secondary palate allowed it to chew and breathe simultaneously, a capability many more advanced reptiles lacked. These traits likely evolved independently as adaptations to a high-fiber diet, and many reptile-like details in its postcranial skeleton may relate to supporting its heavy trunk. These similarities to early amniotes like pelycosaurs and pareiasaurs are likely analogous, not homologous, since the first reptiles evolved from small, swamp-dwelling animals such as *Casineria* and *Westlothiana*. This phenomenon is known as convergent evolution. A juvenile *Diadectes* skeleton has been found with tooth marks on its limb bones, indicating it was preyed upon by species like *Varanops*, *Dimetrodon*, or the amphibian *Acheloma*.

The genus was first named and described by American paleontologist Edward Drinker Cope in 1878, based on part of a lower jaw (AMNH 4360) from the Permian of Texas. Cope noted the teeth had short, compressed crowns with a long axis transverse to the jaw, which inspired the name *Diadectes* ("crosswise biter," from Greek *dia* "crosswise" and *dēktēs* "biter"). He described the animal as "in all probability, herbivorous." The type species name *sideropelicus* (from Greek *sidēros* "iron" and *pēlos* "clay") refers to the Wichita beds in Texas where the fossil was found. *Diadectes* remains are known from several locations across North America, especially the Texas Red Beds (Wichita and Clear Fork).

Many species have been assigned to *Diadectes*, but most are now considered synonyms. Several supposed separate genera of diadectids have also been shown to be junior synonyms. One of these, *Nothodon*, was actually published by Othniel Charles Marsh five days before Cope published *Diadectes*. Despite this, in 1912, Case synonymized the two names and treated *Diadectes* as the senior synonym—a decision followed by later paleontologists even though it violates the International Code of Zoological Nomenclature (ICZN).

A phylogenetic analysis of *Diadectes* and related diadectids was presented in an unpublished 2010 PhD thesis by Richard Kissel. Earlier analyses had found *D. sanmiguelensis* and *D. absitus* to be more basal than other *Diadectes* species, with *Diasparactus zenos* more closely related to the other *Diadectes* species, making the genus paraphyletic. Kissel recovered this paraphyly and proposed the new genus names "Oradectes" for *D. sanmiguelensis* and "Silvadectes" for *D. absitus*. However, names from an unpublished thesis are not valid under the ICZN (nomen ex dissertationae), and as of 2010, these names had not been formally erected. A 2024 paper formally erected the genus *Kuwavaatakdectes* for *D. sanmiguelensis* and also named another species, *D. dreigleichenensis*, which coexisted with *D. absitus* in the Tambach Formation of Germany.