Lizards. 3

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains. The group, traditionally recognized as the suborder Lacertilia, is defined as all extant members of the Lepidosauria (reptiles with overlapping scales), which are neither sphenodonts (i.e., tuatara) nor snakes – they form an evolutionary grade. While the snakes are recognized as falling phylogenetically within the Toxicofera clade from which they evolved, the Sphenodonts are the sister group to the Squamates, the larger monophyletic group, which includes both the lizards and the snakes.

Lizards typically have limbs and external ears, while snakes lack both of these characteristics. However, because they are defined negatively as excluding snakes, lizards have no unique distinguishing characteristic as a group. Lizards and snakes share a movable quadrate bone, distinguishing them from the sphenodonts, which have a more primitive and solid diapsid skull. Many lizards can detach their tails to escape from predators, an act called autotomy, but this ability is not shared by all lizards. Vision, including color vision, is particularly well developed in most lizards, and most communicate with body language or bright colors on their bodies as well as with pheromones.

The adult length of species within the suborder ranges from a few cm for some chameleons and geckos to nearly 3 m (9.8 ft) in the case of the largest living varanid lizard, the Komodo Dragon. Some extinct varanids reached great size. The extinct aquatic mosasaurs reached 17 m (56 ft), and the giant monitor Megalania prisca is estimated to have reached perhaps 7 m (23 ft).

Evolution.

The retention of the basic 'reptilian' amniote body form by lizards makes it tempting to assume any similar animal, alive or extinct, is also a lizard. However, this is not the case, and lizards as squamates are part of a well-defined group.

The earliest amniotes were superficially lizard-like, but had solid, box-like skulls, with openings only for eyes, nostrils, termed the anapsid condition. Turtles retain this skull form. Early anapsids later gave rise to two new groups with additional holes in the skull to make room for and anchor larger jaw muscles. The Synapsids, with a single fenestra, gave rise to the superficially lizard-like Pelycosaurs, which include Dimetrodon and the Therapsids, including the Cynodonts, from which the modern mammals would evolve.

The modern Tuatara retains the basic Lepidosaur skull, distinguishing it from true lizards in spite of superficial similarities. Squamates, including snakes and all true lizards, further lightened the skull by eliminating the lower margin of the lower skull opening.

The earliest known fossil remains of a lizard belong to the iguanian species Tikiguania estesi from the Tiki Formation of India, which dates to the Carnian stage of the Triassic period, about 220 million years ago. However, mitochondrial phylogenetics suggests that the first lizards evolved in the late Permian. Most 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.