Lizards, snakes, and two species of Sphenodon—the lizardlike tuatara of New Zealand—are the living descendants of the lepido-saurs, another early branch of the diapsid family tree. The earliest members of this group, the sphenodontians, appeared in the Late Triassic, grew diverse during the Jurassic Period, but then dwindled in numbers, probably due to increasing competition from lizards. The sphenodontian Planocephalosaurus, from the Late Triassic of England, was about eight inches (20 cm) long and appeared much like the living tuatara, but Planocephalosaurus has several anatomical features that distinguish it from lizards, including teeth that are fused to the jaw and cannot be replaced like the teeth of all other reptiles. The tuatara, like the living crocodile and the lobe-finned coelacanth fish, is in many ways a living fossil, having survived generation after generation, for millions of years, with very few evolutionary changes.

Latter forms of sphenodontians from the Mesozoic Era took a variety of novel forms, including one genus, Pleurosaurus of Germany, that become aquatic.

The most successful lepidosaurs emerged after the Triassic Period. The lizards arose during the Jurassic Period, and the snakes arose during the Cretaceous. Another extraordinary lepidosaur form, the mosasaurs, were fierce, fully aquatic predators that ruled the Late Cretaceous seas of North America and elsewhere. Some mosasaurs grew to over 49 feet (15 m) long.

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