Fossil range: Late Cretaceous to Recent
|Gila monster, Heloderma suspectum|
Heloderma, the only genus of the family Helodermatidae, consists of venomous lizards native to the southwestern United States, Mexico and as far south as Guatemala. It includes two separate species, with six subspecies. Their closest living relatives are the anguid lizards.
Helodermatids (or beaded lizards) are large, stocky, slow-moving reptiles that prefer semi-arid habitats. The tails are short and used as fat storage organs. They are covered with small, non-overlapping bead-like scales, with osteoderms on the underside of the body. Both species are dark in color, with yellowish or pinkish markings.
Members of the family are venomous. Unlike snakes, venom glands are located in the lower jaw, and cannot be ejected under pressure. The venom is typically used only in defense, rather than in subduing prey, and the lizard must chew on its victim to work the venom into the flesh. Venom glands are believed to have evolved early in the lineage leading to the modern helodermatids, as there are indications of their presence even in the 65-million-year-old fossil genus Paraderma. Venom production among lizards was long thought to be unique to this genus, but may be present in several monitor lizards and agamids as well. This controversial new view holds that venom production dates back to the common ancestor of the clade Toxicofera, which includes all snakes and 13 families of lizards.
Members of the genus Heloderma have many extinct relatives in the Helodermatidae whose evolutionary history may be traced back to the Cretaceous period, such as Estesia. The genus Heloderma has existed since the Miocene, when H. texana lived, and fragments of osteoderms from the Gila monster have been found in late Pleistocene (8000-10,000 years ago) deposits near Las Vegas, Nevada. Because the Helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. Although the beaded lizard or the Gila monster appear closely related to the monitor lizards (varanids) of Africa, Asia and Australia, the wide geographical separation and unique features not found in the varanids indicates that they are better placed in a separate family.
The type species is Heloderma horridum, which was first described in 1829 by Arend Weigmann. Although he originally assigned it the generic name Trachyderma, he changed it to Heloderma six months later, which means "studded skin", from the Ancient Greek words hêlos (ηλος)—the head of a nail or stud—and derma (δερμα), meaning skin.
H. h. horridum, H. h. exasperatum, and both subspecies of H. suspectum are frequently found in captivity, and are well represented in zoos throughout much of the world. They are often captive bred for the exotic animal trade, and can command high prices. The other two subspecies of H. horridum are extremely rare, and only a few captive specimens are known.
Ariano-Sánchez, D. 2008. Envenomation by a wild Guatemalan beaded lizard Heloderma horridum charlesbogerti. Clinical toxicology 46 (9): 897-899. 
Ariano-Sánchez, D. & G. Salazar. 2007. Notes on the distribution of the endangered lizard, Heloderma horridum charlesbogerti, in the dry forests of eastern Guatemala: an application of multi-criteria evaluation to conservation. Iguana 14: 152-158. 
Ariano-Sánchez, D. 2006. The Guatemalan beaded lizard: endangered inhabitant of a unique ecosystem. Iguana 13: 178-183. 
CONVENTION ON INTERNATIONAL TRADE IN ENDANGERED SPECIES OF WILD FAUNA AND FLORA. 2007. Resume of the 14th Convention of the Parts. The Hague. The Netherlands. [http://www.ssn.org/Meetings/cop/cop14/Other/CoP14_results.pdf
Species: H. horridum - H. suspectum
Heloderma (Wiegmann, 1829)