sauropsid

Sauropsida is a clade of amniote vertebrates that includes all living reptiles and birds and their fossil relatives. In many classifications, it is defined as the branch of Amniota that is more closely related to birds than to mammals, thereby excluding the Synapsida (the lineage leading to mammals). The position of turtles has varied in different schemes; they are often treated as sauropsids, though some analyses place them on the stem side of the reptile–bird lineage or within an alternate placement of Diapsida.

Among living sauropsids, two major lineages survive: Lepidosauria (lizards, snakes, and tuataras) and Archosauria (crocodilians and birds, plus all extinct dinosaurs). The sauropsid fossil record includes diverse early reptiles and other amniotes dating to the late Paleozoic, providing the framework for the diversification of reptiles and birds that followed.

Key anatomical and reproductive traits characterize many sauropsids. The skull most commonly retains a diapsid condition, with two temporal openings, though turtles exhibit substantial skull remodeling. Sauropsids typically possess keratinous scales or scutes covering their bodies. Their reproduction involves amniotic eggs with protective membranes; while most are oviparous, some lineages show viviparity or other reproductive adaptations.

Phylogenetically, sauropsids diverged from their sister group, the synapsids, during the late Paleozoic era. The term and its circumscription have evolved with advances in cladistics. Today, Sauropsida is used to refer broadly to reptiles and birds and their extinct relatives, in contrast to Synapsida—the mammal lineage and its extinct relatives.