Although well known genera like Diadectes first appear in the Late Pennsylvanian, fragmentary remains of possible diadectids are known from much earlier deposits, including a piece of lower jaw found in Mississippian strata from Tennessee.

Case compared diadectids to turtles in 1907, noting their large pectoral girdles, short, strong limbs, and robust skulls.

Case described them as "lowly, sluggish, inoffensive herbivorous reptiles, clad in an armor of plate to protect them from the fiercely carnivorous pelycosaurs.

American paleontologist Edward Drinker Cope named the genus in 1878 on the basis of several vertebrae and teeth from the Early Permian of Texas.

[4] Cope erected the family Diadectidae in 1880 to include Diadectes and Empedocles, a genus he named two years earlier.

[1] In North America, diadectids are known from Texas, Colorado, Utah, New Mexico, Oklahoma, Ohio, West Virginia, Pennsylvania, and Prince Edward Island.

Phanerosaurus was described from several vertebrae near Zwickau by German paleontologist Christian Erich Hermann von Meyer in 1860, but was not recognized as a diadectid until 1925.

[17] In 1998, a new species of Diadectes, D. absitus, was described from the Bromacker sandstone quarry of the Tambach Formation in the Thuringian Forest of central Germany.

[21] In most recent studies of early tetrapod phylogeny, Cotylosauria is no longer recognized and Diadectomorpha is placed as the sister taxon of Amniota.

[22][23] Most phylogenetic studies of the three diadectomorph families – Diadectidae, Limnoscelidae, and Tseajaiidae – have found diadectids and limnoscelids to be more closely related to each other than either is to Tseajaia.

In a 2010 phylogenetic analysis, Diadectidae formed a clade that was characterized by wide cheek teeth with cusps on either side.

He stated in his paper that its assignment to Diadectidae is based only on several isolated maxillae and dentaries containing cheek teeth that only exhibited a resemblance in their molar-like morphology to those in Diadectids.

Furthermore, the appearance of Ambedus pusillus so late in the fossil record also casts a doubt on the fact that it is supposed to represent the basalmost member of the Diadectid lineage.

[14] They underwent a small evolutionary radiation in the Late Carboniferous and Early Permian, diversifying into thirteen species and outnumbering other diadectomorphs, such as the limnoscelids.

[1] During the late Carboniferous and Permian these regions formed a single landmass called Laurasia, which comprised the northern portion of the supercontinent Pangea.

They could not have reached what is now China until the Middle Permian because, prior to that time, the Tethys Sea separated it from the rest of Laurasia.

The group does not seem to have diversified to the same extent in the east as they did in the west given that no diadectids are known from Russia, which has an extensive fossil record of Early and Middle Permian tetrapod assemblages.

Therefore, the ankle structure of diadectids bears a closer resemblance to those of advanced terrestrial vertebrates like mammals and reptiles than those of earlier tetrapods.

This flexibility enabled diadectids to rotate their feet in a forward position while walking, providing greater force when pushing off.

A 2007 study identified two different ichnospecies, Ichniotherium cottae and I. sphaerodactylum, as footprints of the diadectids Silvadectes absitus and Orobates pabsti, respectively.

[27] The close positioning of the footprints attributed to the more advanced diadectides suggests that the animals held their feet almost underneath their bodies, giving them a more efficient gait and to some degree paralleling the stance of mammals more than that of the sprawling amphibians and most reptiles.