Phrynosomatids are a remarkably diverse group of lizards that have frequently served as subjects of research in ecology and life history theory. Phrynosomatids, which were previously identified as sceloporines, are represented by nine genera, and more than 120 species. Members of group are found throughout most of the continental United States, all of Mexico, and in northern Central America.

Phrynosomatids are relatively small (most < 10 cm snout-vent length) lizards, and represent a diversity of forms. Together with the other seven families previously part of Iguanidae (sensu lato), phrynosomatids have pleurodont teeth, which distinguishes them from other members of the Iguania (agamids and chamaeleons). Several synapomorphies diagnose the family, including the absence of pterygoid teeth; reduction of the clavicular flange; the posterior process of interclavicle invested by the sternum anteriorly; a sink-trap nasal apparatus (with an elongate septomaxilla); an enlarged posterior lobe of the hemipenes; and completely divided m. retractor lateralis posterior. In addition, the sand lizards (genera Uma, Callisaurus, and Holbrookia), together with Phrynosoma, form a monophyletic group diagnosed by the absence of lacrimals and postfrontals.

Few life history characters unite the phrynosomatids, beyond a vague preference for arid landscapes. Phrynosoma species are extremely spiny, cryptic, and solitary desert dwellers. They exhibit a defense behavior in which they squirt blood at potential predators from sinuses in their orbits. Sceloporus, by far the most speciose phrynosomatid genus, includes terrestrial, arboreal and rock-dwelling species. Some Sceloporus and Phrynosoma are ovoviviparous; all other phrynosomatids are oviparous. Uma species dive in loose sand. Several species of phrynosomatids have brightly colored gravid females that are known to aggressively reject courtship attempts by back-arching and sidlehopping. Additional species have males with bright coloration that is predictive of social behavior (e.g. attack rate). Because of their high population densities, short generation time, and accessibility to North American researchers, phrynosomatids (especially Sceloporus) have been well-studied, and figure prominently in research on life history theory, physiological ecology, hybrid zones and chromosome evolution.

Phrynosomatids 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 Phrynosomatidae, 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 Phrynosomatidae (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 Phrynosomatidae is a member). Among the eight families of Iguanidae sensu lato, relationships are not resolved, although several hypotheses posit that phrynosomatids are closely related to tropidurids, oplurids, and perhaps polychrotids.

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).