This widespread and diverse group of lizards is often referred to as the old world iguanids. They are widespread throughout the paleotropics and Palearctic, though their range does not extend far into Europe and they seem to have been recently introduced to Madagascar. Agamidae contains two subfamilies, approximately 52 genera, and more than 350 species.
Agamids have experienced independent radiations of form and size on each of Africa, Asia, and Australia. All agamids have well-developed limbs, and many have keeled scales, middorsal crests, and throat flaps or fans. They range in size from tiny (14 mm snout-vent length in Pogona microlepidota) to quite large (145 cm total length in the water dragon Hydrosaurus amboinensis). Diagnostic features for the group are in contention, but may include paired, enlarged sternal fontanelles. Like the chameleons, but unlike other iguanians, agamids have acrodont teeth. Several additional characters are shared with chameleons, including reticular lingual papillae, reduced postfrontals, the expansion of the dentary onto labial face of the coronoid, shortened spenial, loss of pterygoid teeth, and the anterior surangular foramen ventral to the posterior extremity of the dentary.
The agamids are a diverse group of lizards that represent a variety of life history strategies. Among their ranks are terrestrial, tropical forest dwellers; terrestrial desert dwellers; and semiaquatic forms that use water as a refuge. Some agamids dig burrows (e.g. Leiolepis, Uromastyx); others have modified foot scales that allow them to run bipedally across the surface of water (Hydrosaurus). The heavily spined Moloch horridus (thorny devil) can inflate itself with air when stressed. Draco has modified ribs covered in thin skin that acts as an aerofoil, and these lizards glide to safety as an escape response. Many Agama species live in colonies, replete with territorial borders and social hierarchies. Phrynocephalus is live-bearing, but the remaining agamids are oviparous. Agamids are diurnal and visually-oriented, and at least some species can distinguish between colors in the visual spectrum, as well as in ultraviolet wavelengths. In Agama agama, males develop bright breeding coloration, and females respond with arched back and elevated tail only to brightly colored males. In Ctenophorus maculosus, male display coloration is correlated with dominance. Several species of agamids are popular in the pet trade.
Agamids 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. Following a new phylogenetic analysis, Frost and Etheridge (1989) recently split the large family Iguanidae into eight families (including Iguanidae sensu stricto). 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). Frost and Etheridge's (1989) analysis found Agamidae paraphyletic with respect to Chamaeleonidae, although in one topology this is based solely on the questionable identification of the fossil Priscagama as an agamid. They argue that the two agamid subfamilies should be subsumed within a larger Chamaeleonidae, but the literature continues to reflect the traditional classification. Contrary to an Acrodonta clade, some research suggests that Iguanidae sensu lato is paraphyletic with respect to Agamidae. Finally, much work has been done on inter-agamid systematics, the broadest finding being that the two subfamilies are sister to one another (but see Macey, et al. 1997).
In addition to the putative fossil agamid Priscagama, one extinct agamid genus, Mimeosaurus, is known from the Cretaceous.
Cogger, H. G., and R. G. Zweifel, editors. 1998. Encyclopedia of Reptiles and Amphibians, 2nd edition. Academic Press, San Diego.
Cooper, W. E. Jr. and N. Greenberg. 1992. Reptilian coloration and behavior. Pages 298-422 in C. Gans and D. Crews, editors. Hormones, Brain, and Beahvior: Biology of the Reptilia, volume 18, Physiology E. University of Chicago Press, Chicago.
Frost, D. R., and R. Etheridge. 1989. A phylogenetic analysis and taxonomy of Iguanian lizards (Reptilia: Squamata). University of Kansas Museum of Natural History, Miscellaneous publications 81:1-65.
Frost, D. R., and R. Etheridge. 1993. A consideration of iguanian lizards and the objectives of systematics: a reply to Lazell. Herpetological Review 24:50-54.
Lazell, J. D. 1992. The family Iguanidae: Disagreement with Frost and Etheridge (1989). Herpetological Review 23:109-112.
Macey, J. R., A. Larson, N. B. Ananjeva, and T. J. Papenfuss. 1997. Evolutionary shifts in three major structural features of the mitochondrial genome among iguanian lizards. Journal of Molecular Evolution 44:660-674.
Pianka, E. R. 2001. Australia's Thorny Devil. Website at http://uts.cc.utexas.edu/~varanus/moloch.html
Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. 1998. Herpetology. Prentice-Hall, Inc., Upper Saddle River, NJ.
Schwenk, K. 1994. Systematics and subjectivity: the phylogeny and classification of iguanian lizards revisited. Herpetological Review 25:53-57.
Uetz, P., and M. Bartz. 1996. EMBL Reptile Database: Agamidae. (Website.)
Zug, G. R. 1993. Herpetology: an introductory biology of amphibians and reptiles. Academic Press, San Diego.
Heather Heying (author).