Eureptilians

Eureptilians are a diverse clade of amniote vertebrates that includes all reptiles and mammals, as well as several extinct lineages. The term "eureptile" was introduced by paleontologist Michael Benton in 1985 to distinguish these animals from earlier amniotes, such as the synapsids and sauropsids, which are now considered part of broader evolutionary groups. Eureptiles first appeared during the late Carboniferous period, around 320 million years ago, and diversified extensively throughout the Permian and Mesozoic eras.

The clade is characterized by several anatomical features, including a more derived skull structure, improved limb positioning for terrestrial locomotion, and enhanced respiratory efficiency. Eureptiles are further divided into two major subgroups: the Lepidosauromorpha, which includes modern lizards, snakes, and tuataras, and the Archosauromorpha, encompassing crocodilians, dinosaurs, birds, and pterosaurs. Mammals evolved from a lineage within the Synapsida, a sister group to eureptiles, rather than directly from eureptiles themselves.

Fossil evidence reveals that eureptiles played a crucial role in the ecological transitions of the late Paleozoic and Mesozoic, occupying niches ranging from predators to herbivores. Notable extinct groups include the pareiasaurs, which were heavily armored herbivores, and the procolophonids, small, lizard-like reptiles. The success of eureptiles contributed to their dominance in terrestrial ecosystems until the Cretaceous-Paleogene extinction event, which wiped out non-avian dinosaurs and many other lineages.

Modern eureptiles—such as snakes, lizards, crocodiles, and birds—continue to thrive, demonstrating the long-term evolutionary success of this clade. Their study provides valuable insights into the diversification of tetrapods and the evolutionary pathways leading to mammals.