Sauropsida

Sauropsida is a major clade of amniotes that includes reptiles and birds, together with their fossil relatives. In many modern classifications, it is defined as all organisms more closely related to birds than to mammals, making it the sister lineage to Synapsida within Amniota. The crown group typically encompasses Testudines (turtles), Lepidosauria (lizards, snakes, tuataras) and Archosauria (crocodilians, birds and their extinct relatives).

Living sauropsids are diverse in form and ecology, ranging from small desert-dwellers to large aquatic and arboreal species. Within Sauropsida, two major living lineages are Lepidosauria and Archosauria. Birds are part of Archosauria, having evolved from theropod dinosaurs, while turtles have a debated placement but are commonly treated as sauropsids within Eureptilia. The group also includes a large number of extinct lineages that shaped the evolutionary history of terrestrial and aquatic ecosystems.

Fossil evidence indicates that sauropsids first appeared in the late Carboniferous to early Permian. Early members included various primitive eureptiles and parareptiles. The Permian–Triassic periods witnessed extensive diversification, and the Mesozoic Era saw archosaurs become ecologically dominant, giving rise to dinosaurs, pterosaurs and marine reptiles. The rise of birds from theropod dinosaurs in the Late Jurassic represents a major event within Sauropsida, highlighting its long-standing capacity for major evolutionary innovation.

Key features used in sauropsid classification include skull morphology and temporal openings, skin and egg structure, and skeletal anatomy that differentiate them from synapsids. However, definitions of Sauropsida can vary among researchers, with some broader and others more restricted in scope, reflecting ongoing debates about the relationships among early amniotes.