This small group of desert-dwelling lizards was formerly, along with six other new families, considered a subfamily of Iguanidae (see Frost and Etheridge 1989). The crotaphytids are represented by approximately 10 species in two genera. Their distribution is limited to the south central United States and northern Mexico.

Crotaphytids are moderately large lizards (100-145 mm snout-vent length), with large heads, and long limbs and tails. Together with the other seven families previously placed in Iguanidae, polychrotids have pleurodont teeth, which distinguishes them from other members of the Iguania (agamids and chamaeleons <<Chamaelonidae>>). The family does not have any commonly recognized synapomorphies, although depending on their closest relatives, the characters of having palatine teeth, posterior coracoid fenestrae, and ribs on the third cervical vertebrae may be evolutionary reversals unique to this family. Crotaphytids also have an S-condition nasal apparatus, which they share only with `iguanids (sensu stricto). The genus Crotaphytus is diagnosed by a single synapomorphy (extensive skull rugosity in older adults), and Gambelia has none. The dearth of synapomorphies for this family suggests to some researchers that these two genera may be relics of a very old group.

Crotaphytids inhabit deserts and plains, which they can run across with exceptional speed. Most Crotaphytus species live exclusively in rocky outcroppings, and exhibit an unusual form of saltatory bipedalism to jump between boulders. Crotaphytids are carnivores. Crotaphytus (collared lizard) species feed primarily on insect and small vertebrates. Species of Gambelia (the leopard lizards) are particularly noted for feeding on other reptiles, especially smaller iguanian lizards. Crotaphytids also emit squealing vocalizations when stressed, which is unknown in other iguanians except some polychrotids. Several species of crotaphytids have females that are brightly colored all the time. In a few species (e.g. Gambelia wislizeni, Crotophytus collaris), the females' orange spots brighten during shortly before oviposition, and fade between clutches. In at least one Crotaphytus species, males have been observed licking females during courtship.

Crotaphytids are unambiguously placed in the Iguania, a group that is sister to all other squamates (lizards and snakes). Within the Iguania, however, relationships are hotly contested. Until recently, almost 1,000 species, including those in Crotaphytidae, were placed in Iguanidae (sensu lato), but Frost and Etheridge's (1989) analysis of iguanian systematics suggested eight monophyletic clades within that large family. They proposed new family status for these eight clades, including Crotaphytidae (and a much reduced Iguanidae (sensu stricto). Most researchers (and Animal Diversity Web) follow this classification, although several formal criticisms have been made (e.g. Lazell 1992, Schwenk 1994, Macey et al 1997). Most researchers agree that the iguanian families that were not previously members of Iguanidae -- Agamidae and Chamaeleonidae -- form the monophyletic group Acrodonta, which is sister to the remaining families (equivalent to Iguanidae sensu lato, of which Crotaphytidae is a member). Among the eight families of Iguanidae sensu lato relationships are not resolved. Possible sister groups to crotaphytids include a clade comprised of oplurids, tropidurids, and phrynosomatids, or a clade comprised of acrodonts and corytophanids.

Fossils are difficult enough to place without pinpointing the particular lineage within iguanians from which they arose. Iguanid (sensu lato) fossils are known from the Eocene in North America. Additionally, one fossil from the Cretaceous, Pristiguana, may be an iguanid (sensu lato), or a teiid.

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Heather Heying (author).