Diadectids have long been considered close relatives of the amniotes, tetrapods that lay eggs on land or retain the fertilized egg within the mother. In 1987, the paleontologist D.M.S. Watson placed the family in the larger group Diadectomorpha, which includes another family of large-bodied diadectomorphs, the Limnoscelidae, as well as the monotypic diadectomorph family Tseajaiidae, represented by the genus "Tseajaia". Throughout the twentieth century, amniotes and diadectomorphs were often grouped together using the old name Cotylosauria, a name originally used for the most basal grade of what was then thought to be reptiles. In the early part of the century, many paleontologists regarded diadectids, along with other cotylosaurs (such as placodonts), to be close relatives of turtles. In most recent studies of early tetrapod phylogeny, Cotylosauria is no longer recognized and Diadectomorpha is placed as the sister taxon of Amniota. However, while the majority of analyses now place diadectids outside Amniota, some have found them to be true amniotes.