Squamata

Squamata, or the scaled reptiles, is the largest recent order of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the braincase.

Classification
Classically, the order is divided into three suborders:


 * Lacertilia, the lizards;
 * Serpentes, the snakes;
 * Amphisbaenia, the worm lizards.

Of these, the lizards form a paraphyletic group.

Evolution
Squamates are a monophyletic group that is a sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extanct archosaurs. Squamate fossils first appear in the early Jurassic, but a mitochondrial phylogeny suggests that they evolved in the late Permian. The evolutionary relationships within the squamates are not yet completely worked out, with the relationship of snakes to other groups being most problematic. From morphological data, Iguanid lizards have been thought to have diverged from other squamates very early, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence. Because snakes have a faster molecular clock than other squamates, and there are few early snake and snake ancestor fossils, it is difficult to resolve the relationship between snakes and other squamate groups.