Lepidosauria
The Lepidosauria (/ˌlɛpɪdoʊˈsɔːriə/, from Greek meaning scaled lizards) is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians.[2] Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of reptiles in the present day.[3] Rhynchocephalia was a formerly widespread and diverse group of reptiles in the Mesozoic Era.[4] However, it is represented by only one living species: the tuatara (Sphenodon punctatus), a superficially lizard-like reptile native to New Zealand.[5][6]
Lepidosauria is a monophyletic group (i.e. a clade), containing all descendants of the last common ancestor of squamates and rhynchocephalians.[7] 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 Archosauria, which includes Aves (birds) and Crocodilia. Testudines (turtles) may be related to lepidosaurs or to archosaurs, but no consensus has been reached on this subject. Lepidosauria is encompassed by Lepidosauromorpha, a broader group defined as all reptiles (living or extinct) closer to lepidosaurs than to archosaurs.
Lizards were originally split into two clades: the Iguania and the Scleroglossa. Snakes and amphisbaenians belong within the clade Scleroglossa. Analysis of teeth has indicated that Iguania is made up of the sister taxa Chamaeleonidae and Agamidae.[8] Snakes are actually a branch within the lizard group. In fact, some lizards, such as the Varanids, are more closely related to snakes than they are to other lizards. Varanids are a diverse group of lizards living from Africa, throughout south, central, and east Asia, as well as the Indo-Pacific islands and Australia.
Snakes currently have about 3,070 extant species, which are grouped into the scolecophidians and the alethinophidians.[9] The scolecophidians comprise about 370 species and are represented by small snakes with a limited gape size.[9] The alethinophidians comprise about 2,700 species and are represented by the more common snakes.[9] As snakes evolved, their gape size increased from the narrowness of the scolecophidians, which allowed for the digestion of larger prey. There are about 600 species of venomous snakes, which all belong to Caenophidia,[9] although the majority of caenophidians are non-venomous colubrids.
While amphisbaenians are mostly limbless, three species have reduced forms of front limbs. Morphological data shows that species with front limbs form a sister group to those that are limbless. This means that the amphisbaenians’ loss of limbs occurred only once.[9]
A genetic study on the genome of the tuatara suggests a divergence rate of around 240 million years ago during the Triassic.[10]
