| Lizards Fossil range: Jurassic- Recent | |
|---|---|
 Fossil of Placosauriops, an anguid lizard from the Middle Eocene of Grube Messel, Germany. | |
| Scientific classification
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Lacertilia* | |
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See text. | |
Lizards are a large and widespread group of squamate reptiles, with nearly 5,000 species, ranging across all continents except Antarctica as well as most oceanic island chains.
Evolution and relationships[]
Artist's restoration of the now extinct and fossilized mosasaur Prognathodon, a varanid.
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 amniote was superficially lizard-like, but had a solid, box-like skull, with openings only for eyes, nostrils, etc (termed Anapsid). 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. Those with a single hole, the Synapsids, gave rise to the superficially lizard-like Pelycosaurs which include Dimetrodon and the Therapsids, including the Cynodonts, from which would evolve the modern mammals.
The Diapsids, possessing one temporal fenestra before the eye and one behind it, continued to diversify. One branch, the Archosaurs, retained the basic diapsid skull, and gave rise to a bewildering array of animals, most famous being the crocodilians, pterosaurs, and dinosaurs and their descendants, birds. The ichthyosaurs and plesiosaurs radiated from the same basal Diapsid group.
The smaller Lepidosaurs which would give rise to the lizards began to reduce the skull bones, making the skull lighter and more flexible. 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.
Lizard diversfication[]
Within the Lacertilia are found four generally recognized suborders, Iguania, Gekkota, Amphisbaenia and Autarchoglossa, with the "blind skinks" in the family Dibamidae having an uncertain position. While traditionally excluded from the lizards, the snakes are usually classified as a clade with a similar subordinal rank.[1].