Reptiles are currently represented by turtles, archosaurs (crocodylians and birds), and lepidosaurs (lizards, snakes and the tuatara). However, these groups represent only a fraction of the known diversity of crown reptiles throughout geological time, and the evolutionary origin of modern reptiles remains a major outstanding topic in vertebrate systematics. The molecular consensus is that turtles are more closely related to archosaurs than to lepidosaurs, but this does not consider the many extinct neodiapsids that also have their origin close to the base of the reptile crown. Great progress has been made on this subject in recent decades through the detailed study of the anatomy and systematics of Permian and Triassic reptile fossils. Nevertheless, there is still no consensus on reptile interrelationships near the origin of the three major modern reptile clades. Here, we review the current state of systematic research for Permo-Triassic lepidosauromorphs, archosauromorphs, pantestudines, sauropterygomorphs, ichthyosauromorphs, thalattosaurs, choristoderes, weigeltisaurids, drepanosauromorphs, and ‘younginiforms’, and provide an outlook for future efforts for these groups. Addressing this systematic question is complicated by the rarity of relevant fossils for some groups and the inherent challenges of studying the phylogenetic relationships of evolutionary radiations. However, increased research efforts, including continuous new fossil discoveries and detailed morphological studies, especially those aided by micro-computed tomography, are likely to help resolve some of these issues. Furthermore, there is a desire for comprehensive morphological character-taxon matrices focusing on this part of the reptile tree, resembling in scope and approach those already available for other major lineages, such as Archosauromorpha and Lepidosauromorpha.
Spiekman et al. (Thu,) studied this question.