Evolution and systematics

Squamates are the most diverse living clade of reptiles, including about 1,440 genera and 4,450 species of lizards plus 440 genera and 2,750 species of snakes. Although snakes are commonly considered to constitute their own group, they clearly have descended from lizards and are merely limbless lizards. Squamates exhibit more than 70 shared derived traits, which indicate that they are descendants of a common ancestor, forming a large natural monophyletic group. (Snakes and lizards once were classified as different suborders, but since snakes are embedded within lizards, this classification is no longer tenable under the monophyletic standard of modern phylogenetic systematics.)

Sister group

The sister group to squamates is Rhynchocephalia, represented today by only two species of tuatara (Sphenodon) from New Zealand. Superficially, tuatara resemble lizards, in that they have a dorsal crest of scales. They possess two temporal arches (the "diapsid" condition), however, making their skulls quite rigid. They do not have copulatory organs. In tuatara the lower jaw articulates directly with the upper skull, resulting in a narrow gape and slow jaw movements. Tuatara are visual ambush predators that eat large prey, which they pick up with their sticky and fleshy tongues ("lingual prehension"). Because these traits are shared with basal lizards, they are probably ancestral states.

Streptostyly and Jacobson's organ

In all squamates, the lower temporal arch has been lost, and the lower jaw hinges on the quadrate bone, which hangs down from the cranium, a situation known as "streptostyly." This hanging jaw setup provides a mechanical advantage that results in faster jaw opening and closure, a wider gape, and a stronger bite, presumably greatly enhancing prey capture and feeding. All lizards and snakes also have a pair of eversible copulatory organs (hemipenes) at the base of their tails. Another novel and important feature of squamates is a vomero-nasal olfactory organ (Jacobson's organ) in the palate, separate from the nasal capsule. Thus, squamates possess three chemo-sensory systems: (1) taste buds on their tongues, (2) nasal smelling of volatile airborne scents, and (3) vomeronasal analysis of heavy non-airborne chemicals picked up by the tongue and transferred into the Jacobson's organ in the roof of the mouth, where the signal is processed.

Vomerolfaction ability differs greatly among squamates, remaining relatively undeveloped in some basal groups but becoming acutely sensitive in more derived groups, such as varanids and snakes. Ancestral squamates were ambush predators that detected prey by movement using visual cues and had relatively low activity levels and poorly developed chemosen-sory systems. The large clade Iguania retained these ancestral features, whereas in the more advanced Scleroglossa a suite of derived characteristics developed. Ancestral scleroglossans captured and manipulated prey with their jaws (jaw prehension), thus freeing the tongue to evolve along other lines and facilitating the evolution of sensitive vomerolfaction. Active foraging and higher activity levels were further consequences. They also possessed the combined abilities to flex the skull (mesokinesis), and discriminate prey based on chemical cues, ultimately producing more than 20 remaining families of other lizards plus all snakes (13-18 families), making up 80% of all squamates. Most snakes are entirely legless, and no living snake has functional legs (although a fossil did), eardrums, or movable eyelids. Many other lizards, especially burrowers, also have become limbless and lost their eardrums and eyelids.

Fossil record and biogeography

Squamate history dates back at least to the early Jurrasic, if not the late Triassic. Squamata is the sister taxon to

Rhynchocephalia, together making up Lepidosauria. About 200 million years ago (mya), before true lizards existed, one lineage of lepidosaurs gave rise to the ancestor of squamates. No fossils of this common ancestor ("stem group") of all squamates are known. It must have existed during the Upper Tri-assic to the Lower Jurassic, about 200-250 million mya, but a 50-million-year gap in the fossil record precludes exact dating. Most squamate fossils are considerably more recent. Much of early squamate evolution occurred during the Jurassic and Cretaceous on the supercontinent of Pangaea. Diversification of arthropods during the Jurassic provided a literal banquet for terrestrial vertebrates that could find and capture them. Five late Jurassic (about 150 mya) fossil lizards represent ancient extinct lineages. They are scattered across the squamate phylogenetic tree, indicating that early diversification into major clades (Iguania, Gekkota, Scincomorpha, and Anguimorpha) had taken place by the end of the Jurassic. These early lineages gave rise to the currently recognized 23-24 lizard families and 13-18 snake families.

Squamate evolution is intertwined intimately with continental drift. Some squamate families appear to be of Gond-wanan origin, but others arose in the Northern Hemisphere and subsequently dispersed to southern continents. Australia's snakelike pygopodids arose from diplodactylid geckos within the island continent. Four lizard groups reached Australia and underwent extensive adaptive radiations. Some groups, such as skinks and varanids, became more speciose there than they are today in their probable ancestral source areas.

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