The Lepidosauria (, from Greek meaning scaled lizards) is a superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata, comprising lizards and snakes, contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day. Rhynchocephalia was a formerly widespread and diverse group of reptiles in the Mesozoic Era. However, it is represented by only one living species: the tuatara (Sphenodon punctatus), a superficially lizard-like reptile native to New Zealand.
Lepidosauria is a monophyletic group (i.e. a clade), containing all descendants of the last common ancestor of squamates and rhynchocephalians. Lepidosaurs can be distinguished from other reptiles via several traits, such as large keratinous scales which may overlap one another. Purely in the context of modern taxa, Lepidosauria can be considered the sister taxon to Archelosauria, which includes Testudines (turtles), Aves (birds) and Crocodilia (crocodilians). Lepidosauria is encompassed by Lepidosauromorpha, a broader group defined as all reptiles (living or extinct) closer to lepidosaurs than to archosaurs.
Evolution
Lepidosauromorpha is thought to have split from their sister group, Archelosauria, during the Permian period. The earliest members of Lepidosauromorpha date to the Early Triassic. The oldest known definitive lepidosaur is the rhynchocephalian Agriodontosaurus from the Helsby Sandstone Formation of the United Kingdom, dating to the upper Anisian stage of the Middle Triassic, approximately . The next earliest rhynchocephalian, Wirtembergia, is known from the Ladinian stage of the Middle Triassic. Sophineta is known from older rocks in the Early Triassic, but its exact placement within the broader clade Lepidosauromorpha is uncertain and it may not be a true lepidosaur. While the lepidosaur Megachirella may represent a stem-group squamate from the Middle Triassic, the earliest modern members of the group are known from the Middle Jurassic. Squamates underwent a great radiation in the Cretaceous, while rhynchocephalians declined during the same time period.
Description
Extant reptiles are in the clade Diapsida, named for the two pairs of temporal fenestrae present on the skull behind the eye socket. Until recently, Diapsida was said to be composed of Lepidosauria and their sister taxa Archosauria. The Lepidosauria is then split into Squamata and Rhynchocephalia. More recent morphological studies and molecular studies also place turtles firmly within Diapsida, even though they lack temporal fenestrations.
thumb|The quadrate bone is particularly elongated in snakes, to facilitate [[cranial kinesis]]
The reptiles in the Lepidosauria can be distinguished from other reptiles by a variety of characteristics. Lepidosaurs are suggested to be distinguished from more primitive lepidosauromorphs by the development of a conch on the quadrate, allowing for the development of a tympanic membrane in the ear (a trait lost in the tuatara, but present in early rhynchocephalians), as well as the development of a subolfactory process on the frontal bones of the skull. thumb|left|Schematic skull of a [[squamate showing the location of major dermal bones]]
The group Squamata This is made possible by a loose connection between the quadrate and its neighboring bones. Without this, snakes would not be able consume prey that are much larger than themselves. Amphisbaenians are mostly legless like snakes, but are generally much smaller. Three species of amphisbaenians have kept reduced front limbs and these species are known for actively burrowing in the ground. The tuatara and some extinct rhynchocephalians have a more rigid skull with a complete lower temporal bar closing the lower temporal fenestra formed by the fusion of the jugal and quadrate/quadratojugal bones, similar to the condition found in primitive diapsids. However early rhynchocephalians and lepidosauromorphs had an open lower temporal fenestra, without a complete temporal bar, so this is thought to be a reversion rather than retention. The temporal bar is thought to stabilise the skull during biting.
Male squamates have evolved a pair of hemipenises instead of a single penis with erectile tissue that is found in crocodilians, birds, mammals, and turtles. The hemipenis can be found in the base of the tail. The tuatara does not have a hemipenis, but instead has shallow paired outpocketings of the posterior wall of the cloaca. The tuatara lays eggs that are usually about one inch in length and which take about 14 months to incubate. A reptile will increase three to twentyfold in length from hatching to adulthood.
Most lepidosaurs rely on camouflage as one of their main defenses. Some species have evolved to blend in with their ecosystem, while others are able to change their skin color to blend in with their current surroundings. The ability to autotomize the tail is another defense that is common among lepidosaurs. Other species, such as the Echinosauria, have evolved the defense of feigning death.
While birds, including raptors, wading birds and roadrunners, and mammals are known to prey on reptiles, the major predator is other reptiles. Some reptiles eat reptile eggs, for example the diet of the Nile monitor includes crocodile eggs, and small reptiles are preyed upon by larger ones. The range of the tuatara has already been minimized by the introduction of cats, rats, dogs, and mustelids to New Zealand. The eradication of the mammals from the islands where the tuatara still survives has helped the species increase its population. An experiment observing the tuatara population after the removal of the Polynesian rat showed that the tuatara expressed an island-specific increase of population after the rats' removal. However, it may be difficult to keep these small mammals from reinhabiting these islands.
Habitat destruction is the leading negative impact of humans on reptiles. Humans continue to develop land that is important habitat for the lepidosaurs. The clear-cutting of land has also led to habitat reduction. Some snakes and lizards migrate toward human dwellings because there is an abundance of rodent and insect prey. However, these reptiles are seen as pests and are often exterminated.