The diapsids include the most diverse radiation of reptile forms. With roots going back to the Late Carboniferous Period, most Paleozoic diapsids were small, long-legged, swift-running forms that lived in the shadows—or ran away from the shadows, as the case may have been—of the much larger amphibians, anapsids, and synapsids that dominated the terrestrial ecosystems of the Permian. With the coming of the disastrous end-Permian mass extinction, the status of many life-forms was changed forever. That event led the once-lurking diapsids into position to prosper in the Mesozoic Era.
The success of the diapsids is demonstrated by their current numbers. There are about 14,600 species of living diapsids, including birds, crocodilians, lizards, and snakes. As impressive as these numbers are, the most dominant diapsids of all time are no longer with us: the dinosaurs, pterosaurs, mosasaurs, and their extinct ancestors that ruled various spans of the Mesozoic Era. Except for turtles, which are classed as anapsids, the diapsids represent all true reptiles currently known.
The diapsids are divided into two primary groups, the Archo-sauria and Lepidosauria. Each of these groups began to diversify and flourish early in the Triassic Period. These animals were preceded by a variety of diapsid lines that were mostly extinct by the end of the Triassic Period but that also figured importantly in the rise of diapsids as the dominant life forms of the Mesozoic.
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