The presence of an essentially modern crown group squamate so far back in time indicates that the diversification of squamate lineages, which was previously thought to have occurred during the Jurassic and Cretaceous, occurred much earlier than previously theorized. Cryptovaranoides appears to be a highly derived squamate belonging to the group Anguimorpha, which contains many extant lineages such as monitor lizards, beaded lizards and anguids. In 2022, the extinct genus Cryptovaranoides was described from the Late Triassic of England. The study also found that geckos are the earliest crown group squamates, not iguanians. The comparison revealed Megachirella had certain features that are unique to squamates. These data were then compared with a phylogenetic dataset combining the morphological and molecular data of 129 extant and extinct reptilian taxa. The phylogenetic analysis was conducted by performing high-resolution microfocus X-ray computed tomography (micro-CT) scans on the fossil specimen of Megachirella to gather detailed data about its anatomy. Genetic data also suggest that the various limbless groups - snakes, amphisbaenians, and dibamids - are unrelated, and instead arose independently from lizards.Ī study in 2018 found that Megachirella, an extinct genus of lepidosaurs that lived about 240 million years ago during the Middle Triassic, was a stem-squamate, making it the oldest known squamate. Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera. Iguanians were long thought to be the earliest crown group squamates based on morphological data, but genetic data suggest that geckoes are the earliest crown group squamates. Although many of the groups originally recognized on the basis of morphology are still accepted, understanding of their relationships to each other has changed radically as a result of studying their genomes. The relationships of squamates is debatable. Squamates suffered a mass extinction at the Cretaceous–Paleogene (K–PG) boundary, which wiped out polyglyphanodontians, mosasaurs, and many other distinct lineages. Polyglyphanodontia, an extinct clade of lizards, and mosasaurs, a group of predatory marine lizards that grew to enormous sizes, also appeared in the Cretaceous. Other groups like iguanians and varanoids appeared in the Cretaceous. and their overall diversity was established during the mid-Jurassic, with further diversity expansions being mostly the result of added species. Scientists believe crown group squamates probably originated in the Early Jurassic based on the fossil record, The first fossils of geckos, skinks, and snakes appear in the Middle Jurassic. Fossils of rhynchocephalians first appear in the Early Triassic, meaning that the lineage leading to squamates must have also existed at the time. Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to the Archosauria, the clade that contains crocodiles and birds, and their extinct relatives. The only surviving member of the Rhynchocephalia is the tuatara. Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia. The now- extinct mosasaurs reached lengths over 14 m (46 ft).Īmong other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the clade Lepidosauria.Įvolution Slavoia darevskii, a fossil squamate Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko ( Sphaerodactylus ariasae) to the 6.5 m (21 ft) Reticulated python ( Malayopython reticulatus). This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. Members of the order are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. With over 11,500 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Squamata ( / s k w æ ˈ m eɪ t ə/, Latin squamatus, 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles.
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