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Lizards occur on all continents, except Antarctica, and on most tropical and subtropical oceanic islands. This widespread occurrence denotes their broad ecological, physiological, and behavioural adaptations to extremely hot or cold climates, extremely arid to freshwater and marine habitats, and lowland to high-elevation regions. Their highest species diversity appears to be in semiarid habitats; for example, 53 species of lizards occur at one site in the Great Victoria Desert, Australia, and in some areas, particularly islands, densities can be greater than 3000 per hectare.

Lizards occur on all continents, except Antarctica, and on most tropical and subtropical oceanic islands. This widespread occurrence denotes their broad ecological, physiological, and behavioural adaptations to extremely hot or cold climates, extremely arid to freshwater and marine habitats, and lowland to high-elevation regions. Their highest species diversity appears to be in semiarid habitats; for example, 53 species of lizards occur at one site in the Great Victoria Desert, Australia, and in some areas, particularly islands, densities can be greater than 3000 per hectare.

==Taxonomy==

==Taxonomy==

Reptiles are ectothermic vertebrates with scales or plates, right and left aortic arches, a partially divided heart and a single occipital condyle.  

Reptiles are ectothermic vertebrates with scales or plates, right and left aortic arches, a partially divided heart and a single occipital condyle.  

Lizards are members of the reptilian order Squamata. The nearly 7200 species of lizards are the most diverse and speciose living clade of reptiles; of course this total includes the snakes, which are actually reduced-limbed or limbless lizards. However, because snakes are a monophyletic group arising from within a group of lizards, the taxon Serpentes and its definition delimit a monophyletic group. Herein, the term lizard represents our shared perception and excludes Serpentes. The lizards and their snake descendants are the only living squamates.

Lizards are a very large and widespread group of the reptilian order Squamata. The nearly 7200 species of lizards are the most diverse and speciose living clade of reptiles; of course this total includes the worm lizards and snakes (which are actually reduced-limbed or limbless lizards). However, because snakes are a monophyletic group arising from within a group of lizards, the taxon Serpentes and its definition delimit a monophyletic group. Herein, the term lizard represents our shared perception and excludes Serpentes and Amphisbaenia.  

==Common species==

==Common species==

There are 22 families of lizards, comprising approximately 3,750 species. About 20 species are commonly kept in captivity. The green iguana is the most popular. There are two poisonous species, the Gila monster and the Mexican beaded lizard, although you are unlikely to see them often in practice.

There are 22 families of lizards, comprising approximately 3,800 species. About 20 species are commonly kept in captivity. The green iguana is the most popular. There are two poisonous species, the Gila monster and the Mexican beaded lizard, although you are unlikely to see them often in practice.