This ever-growing family of diverse lizards was formerly, along with six other new families, considered a subfamily of Iguanidae (see Frost and Etheridge 1989). The tropidurids are represented by more than 280 species, in nine genera and three subfamilies. They are distributed throughout South America, in the West Indies (Leiocephalus only), and on the Galapagos.

Tropidurids from all three radiations (subfamilies) are small to moderate sized (4-15 cm snout-vent length), with spiny scales. Together with the other seven families that were previously part of Iguanidae (sensu lato), tropidurids have pleurodont teeth, which distinguishes them from other members of the Iguania (agamids and chamaeleons). Synapomorphies that diagnose the family include a reduced angular, and a medially incomplete gular fold. In addition, tropidurids have a posterior extension of the splenial, and an enlarged, median sternal fontanelle, characters that might add support for the monophyly of the family, depending on who they are related to. Furthermore, all of the tropidurid subfamilies are diagnosed by multiple shared, derived features.

Tropidurids inhabit a wide range of habitats, including lowland rain forests, savannahs, deserts, and high mountain grasslands above tree line. Some live in rocks, others in sand; still others are terrestrial or arboreal. All are diurnal, and most are oviparous (although Phymaturus species and some Liolaemus are viviparous, especially * are herbivores. Some species feed opportunistically on flowers and fruit. Some Tropidurus species have females that are brightly colored all year round; in these species, males are highly territorial. Other Tropidurus have females that develop bright colors only when gravid. Leiocephalus schreibersii adults are known to cannibalize juveniles, but one study showed that more than half of all adults in one population are not cannibals (Jenssen et al 1989).

Tropidurids 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 Tropiduridae, 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 family status for these eight clades, including Tropiduridae (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, sensu lato, of which Tropiduridae is a member). Among the eight families of Iguanidae sensu lato, which includes Tropiduridae, relationships are not resolved, although several hypotheses posit that tropidurids are closely related to phrynosomatids, oplurids, and perhaps polychrotids. Within Tropiduridae, the subfamilies Tropidurinae and Leiocephalinae are sister taxa, and Liolaeminae is basal.

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).