In diapsid, "double-arched", reptiles, the skull possesses two openings behind the eyesocket and two temporal arches confining these openings. This structure of the skull supplies the most effective use of the external skull surface for the attachment of jaw muscles. The diapsids had arisen in the Carboniferous Period, long before the end of the Paleozoic Era, but their prosperity occured in the Mesozoic Era. The diapsids include the lepidosaurs (lizards, snakes, and their relatives) and the archosaurs (thecodonts and their descendants - crocodiles, pterosaurs, and dinosaurs). The most advanced offshoot of the archosaurian evolution are birds, one of the thriving groups of the Cenozoic fauna.