Diapsids are a group of reptiles characterized by having two temporal fenestrae, which are openings in the skull behind the eyes. This group includes modern reptiles like crocodiles, lizards, and birds, as well as extinct species such as dinosaurs. Diapsids are known for their diverse adaptations and evolutionary success in various environments. The diapsid lineage emerged during the late Carboniferous period, around 300 million years ago. They are distinguished from other reptiles, like anapsids, which have no temporal openings. Diapsids have played a significant role in the evolution of vertebrates, leading to many of the reptiles and birds we see today.