By Heather Heying
This group of relatively large, dry-adapted lizards is undergoing much systematic revision, and as defined here is equivalent to the previously identified subfamily Iguaninae (see Frost and Etheridge 1989). Under this strict classification, there are eight iguanid genera, and approximately 35 species currently recognized. Iguanids (sensu stricto) are distributed throughout much of tropical America, up into southern temperate North America, throughout the Antilles, on the Gal·pagos, and in Fiji.
The true iguanas are among the largest of the iguanians, ranging from 14 cm snout-vent length (Dipsosaurus) to more than two meters in Iguana iguana. Together with the other seven families previously placed in Iguanidae, iguanids (sensu stricto) have pleurodont teeth, which distinguishes them from other members of the Iguania (agamids and chamaeleons). Several synapomorphies have been proposed for Iguanidae (sensu stricto), including the position of the parietal process of the supratemporal; iguanine caudal vertebrae; and colic septa. Iguanids also have bicuspate teeth and S-condition nasal apparati, though these characters do not diagnose the clade.
Iguanids may be terrestrial (Dipsosaurus, Cyclura), rock-dwelling (Sauromalus, Ctenosaura), or arboreal (Iguana, Brachylophus). Arboreal species leave the trees only rarely, often to lay eggs. Many iguanids are adapted for arid landscapes, which probably established some of the physiological mechanisms that allow the marine iguana, Amblyrhynchus cristatus, to spend much of its time in salt water, diving up to ten meters for the algae that it scrapes off rocks for food. Amblyrhynchus is also unusual in that it basks and sleeps in large groups. Ctenosaura have spiny tails, which they use in defensive behavior. In contrast to their close relatives, all iguanids are herbivores as adults, consuming primarily leaves, fruits, and flowers. Most iguanids are territorial, and male territorial displays, including push-up behaviors, may double as courtship displays. All iguanids are oviparous. Several iguanid species are popular in the pet trade, and some populations have been put at risk as a result of over-zealous collecting of wild animals.
Iguanids 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 were placed in Iguanidae (sensu lato), but Frost and Etheridge's (1989) analysis of iguanian systematics suggested eight distinct clades within that large family. They proposed new family status for these eight clades, including a more restricted 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 only 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). (Although some research suggests that Iguanidae sensu lato is paraphyletic with respect to Agamidae.) Within Iguanidae sensu lato, relationships are not resolved.
Fossils are difficult enough to place without pinpointing the particular lineage within iguanians that they arose from. 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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Contributors
Heather Heying (author).
