Heloderma
Heloderma is a genus of toxicoferan lizards that contains five species, all of which are venomous. It is the only extant genus of the family Helodermatidae. Description. The genus "Heloderma" contains the Gila monster and four species of beaded lizards. The Gila monster is a large, stocky, most of the time slow-moving reptile that prefers arid deserts. Beaded lizards are seen to be more agile and seem to prefer more humid surroundings. The tails of all species of "Heloderma" are used as fat storage organs. The scales of the head, back and tail are bead-like, containing osteoderms  for better protection. The scales of the belly are free from osteoderms. Most species are dark in color, with yellowish or pinkish markings. Venom. The venom glands of "Heloderma" are located at the end of the lower jaws, unlike snakes' venom glands, which are located behind the eyes. Also, unlike snakes, the Gila monster and beaded lizards lack the musculature to inject venom immediately. They have to chew the venom into the flesh of a victim. "Heloderma" venom is used only in defense. Venom glands are believed to have evolved early in the lineage leading to the modern helodermatids, as  their presence is indicated even in the 65-million-year-old fossil genus "Paraderma". In general, one adult helodermatid has approximately 15 to 20 mg of venom, while the estimated lethal dose for humans is 5 to 8 mg. Diet. Helodermatids are carnivorous, preying on rodents and other small mammals, and eating the eggs of birds and reptiles. Reproduction. All species of "Heloderma" are oviparous. The Gila monster typically lays 6 eggs, the beaded lizards up to about 18 eggs . Comparing the different species, all eggs have a similar size, and the same holds true for their hatchlings. Taxonomy. Family  Helodermatidae 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 (8,000-10,000 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 lizards and 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 they are better placed in a separate family. The type species is "Heloderma horridum", which was first described in 1829 by Arend Wiegmann. 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. Conrad, 2008 and Estes et al., 1988 (using morphological data) places Helodermatidae within Varanoidea along with "Lanthanotus borneensis" and "Varanus". However, Estes et al., 1988 understood Helodermatidae  as having split earlier from "Lanthanotus" and "Varanus", whereas Conrad, 2008 groups them at the same branch point. In contrast, molecular studies have identified Heloderma as being within Anguioidea along with Anguidae and Xenosauridae, but specifically sister to Anguidae. Venom. Venom production among lizards was long thought to be unique to this genus, but researchers studying venom production have proposed many others also produce some venom, all placed in the clade Toxicofera, which includes all snakes and 13 other families of lizards. However, except for snakes, helodermatids, and possibly varanids, envenomation is not considered medically significant for humans In captivity . "H. horridum", "H. exasperatum", and "H. suspectum" are frequently found in captivity and are well represented in zoos throughout much of the world. The other two species of "Heloderma", "H. alvarezi" and "H. charlesbogerti", are extremely rare, and only a few captive specimens are known. External links. Schwandt, Hans- Joachim www.heloderma.net 2006 in 6 languages