Anapsids are a group of reptiles characterized by their skull structure, which lacks temporal fenestrae—openings behind the eyes. This group includes ancient reptiles like turtles and their ancestors, which are believed to have evolved during the late Paleozoic era. Anapsids are distinct from other reptilian groups, such as diapsids and synapsids, which have different skull features. The term "anapsid" is often used to describe the evolutionary lineage that led to modern turtles. While most reptiles have evolved to have openings in their skulls, anapsids retained a more primitive skull structure. This unique feature helps scientists understand the evolutionary history of reptiles and their adaptations over time.