Diadectomorphs possess both amphibian and reptilian characteristics. Originally these animals were included under the orderCotylosauria, and were considered the most primitive and ancestral lineage of reptiles. More recently they have been reclassified as amphibian-grade tetrapods, closely related to the first true amniotes. Contrary to other Reptiliomorphamphibians, the teeth of the Diadectomorpha lacked the infolding of the dentine and enamel that account for the name Labyrinthodontia for the non-amniote tetrapods.
Classification
Diadectomorpha is most commonly given the rank of order when formal taxonomic ranks are applied. It is further divided into three families, representing specialization into different ecological niches. The exact phylogenetic relationship between the three is disputed.
Family Diadectidae is perhaps the best known group, comprising medium to large herbivores. Early members were low-slung, but the latter Diadectes evolved strong, if sprawling legs, paralleling the anatomy of early herbovorious reptiles. The teeth were chisel-like and lacked the typical labyrinthodont infolding of the enamel. The Diadectidae were distributed over most of the Northern parts of Pangaea.
Family Limnoscelidae contained large carnivores or piscivores. The largest genus, Limnoscelis could grow to at least 1.5 meters. The family had pointed and slightly curved teeth with labyrinthodont enamel. Distribution seem to have been restricted to the North America.
Family Tseajaiidae known from a single specimen from North America, were medium-sized, generalized reptiliomorph amphibians. They had blunt teeth and appear to be primarily herbivorous or omnivorous. The known specimen would have been on the order of a meter long.
The reproduction of the Diadectomorphs has been the matter of some debate. If their group lay within the Amniota as has at times been assumed, they would have laid an early version of the amniote egg. Current thinking favours the amniote egg being evolved in very small animals, like Westlothiana or Casineria, leaving the bulky Diadectomorphs just on the amphibian side of the divide. This would indicate the large and bulky Diadectomorphs laid anamniote eggs. However, no unambiguously diadectomorph tadpole is known. Whether this is due to an actual lack of tadpole stage or taphonomy is uncertain. Alfred Romer indicated that the anamniote/amniote divide might not have been very sharp, leaving the question of the actual mode of reproduction of these large animals unanswered. Possible reproductive modes include full amphibian spawning with aquatic tadpoles, internal fertilization with or without ovoviviparity, aquatic eggs with direct development or some combination of these. The reproductive mode might also have varied within the group.
