The fossil record of Late Paleozoic diapsids is not complete enough to define their evolutionary relationships. Certain anatomical trends can be seen in their development, however, and that development became more and more specialized and advantageous as diapsids of various kinds diversified in the Triassic. Among these adaptations were long legs for running, long necks for added flexibility, lighter skulls, and the specialization of teeth for the consumption of both meat and plants.
One of the earliest known diapsid reptiles is Petrolacosaurus, from the Late Carboniferous of Kansas. This small creature measured a mere 15 inches (40 cm) long, including its tail. It featured a very slight skeleton and a tiny skull with a large orbit for its eyes—an indication of good eyesight. Petrolacosaurus also had long legs, another key difference between diapsids and other amniotes. Keen vision and speed made Petrolacosaurus well suited for giving chase to insects.
The evolutionary relationship between Petrolacosaurus and later diapsids from the Permian is unknown, but it is clear that by the turn of the Triassic Period, several unusual lines of diapsids had found some success living in the shadows of the larger, lumbering amphibians, anapsids, and synapsids of the time.
Coelurosauravus, from the Late Permian of Madagascar, was another small diapsid with a most unusual adaptation. It was one of the first known reptiles to develop a means for gliding through the air, a feature that is seen today in the flying dragon lizard of Southeast Asia. The ribs of Coelurosauravus were elongated out from its body to form a frame for a skin-covered gliding wing. This lightweight creature was about 16 inches (40 cm) long and had a "wingspan" of about 12 inches (30 cm). Although it was a glider and not a powered flier, Coelurosauravus demonstrated that diapsids were highly adaptable and explored a variety of niches, in this case living an elevated lifestyle out of the reach of large, ground-based predators.
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