| Heloderma | |
|---|---|
 | |
| Gila monster,Heloderma suspectum | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Infraorder: | Neoanguimorpha |
| Clade: | Monstersauria |
| Family: | Helodermatidae |
| Genus: | Heloderma Wiegmann, 1829 |
| Type species | |
| Heloderma horridum Wiegmann, 1829 | |
| Species | |
Heloderma is agenus oftoxicoferan lizards that contains five species, all of which arevenomous.[1] It is the only extant genus of the familyHelodermatidae.
The genusHeloderma contains theGila monster (H. suspectum) and four species of beaded lizards. Their eyes are immobile and fixed in their heads.[2][3] The Gila monster is a large, stocky, mostly slow-moving reptile that prefers arid deserts. Beaded lizards are seen to be more agile and seem to prefer more humid surroundings.[4][5] The tails of all species ofHeloderma are used as fat-storage organs. The scales of the head, back, and tail are bead-like, containingosteoderms for better protection. The scales of the belly are free from osteoderms. Most species are dark in color, with yellowish or pinkish markings.[6][7]
The venom glands ofHeloderma are located at the end of the lower jaws, unlikesnakes' 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 tochew 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 genusParaderma.[7][8] In general, one adult helodermatid has around 15 to 20 mg of venom, while the estimated lethal dose for humans is 5 to 8 mg.[9]
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 cladeToxicofera, which includes allsnakes and 13 other families of lizards.[10] However, except for snakes, helodermatids, and possibly varanids, envenomation is not considered medically significant for humans.
Helodermatids arecarnivorous,preying onrodents and other smallmammals, and eating theeggs ofbirds and reptiles.
All species ofHeloderma areoviparous. The Gila monster typically lays six eggs, and the beaded lizards up to about 18 eggs .[7] Comparing the different species, all eggs have a similar size, as are their hatchlings.
| Explanation of the numbers | |
|---|---|
| 1 | late Eocene (about 35 million years) |
| 2 | late Miocene (about 10 million years) |
| 3 | Pliocene (about 4.4 million years) |
| 4 | Pliocene (about 3 million years) |
Family Helodermatidae
The foursubspecies of beaded lizards were elevated to full species in 2013.[11]
| Image | Species | Taxon author | Common name | Geographic range |
|---|---|---|---|---|
 | H. alvarezi | Bogert and Martin del Campo, 1956 | Chiapan beaded lizard | Mexico: northern Chiapas to extreme northwesternGuatemala |
 | H. charlesbogertiT | Campbell and Vannini, 1988 | Guatemalan beaded lizard | Guatemala: the Motagua Valley |
 | H. exasperatum | Bogert and Martin del Campo, 1956 | Rio Fuerte beaded lizard | Mexico: Rio Fuerte, Rio Mayo, southern Sonora, northern Sinaloa, western Chihuahua and the Sierra Madre Occidental |
 | H. horridumT | Wiegmann, 1829 | Mexican beaded lizard | Mexico: southern Sinaloa to Oaxaca |
 | H. suspectum | Cope, 1869 | Gila monster | Southwest United States,Sonora |
Members of the genusHeloderma have many extinct relatives in the Helodermatidae, whose evolutionary history may be traced back to theCretaceous period, such asEstesia. The genusHeloderma has existed since theMiocene, whenH. texana lived, and fragments ofosteoderms from the Gila monster have been found in latePleistocene (8,000-10,000 years ago) deposits nearLas Vegas, Nevada. Because the helodermatids have remained relatively unchangedmorphologically, they are occasionally regarded asliving fossils.[12] Although the beaded lizards and the Gila monster appear closely related to themonitor lizards (varanids) of Africa, Asia, and Australia, the wide geographical separation and unique features not found in the varanids indicate they are better placed in a separate family.[13]
Thetype species isHeloderma horridum, which was first described in 1829 byArend Wiegmann. Although he originally assigned it the generic nameTrachyderma, he changed it toHeloderma six months later, which means "studded skin", from theAncient Greek wordshêlos (ηλος)—the head of a nail or stud—andderma (δερμα), meaning skin.[14]
Conrad, 2008 and Estes et al., 1988 (using morphological data) place the Helodermatidae withinVaranoidea along withLanthanotus borneensis andVaranus.[15][16] However, Estes et al., 1988 understood the Helodermatidae as having split earlier fromLanthanotus andVaranus, whereas Conrad, 2008 groups them at the same branch point.
In contrast, molecular studies have identifiedHelodermaas being within the Anguioidea along with Anguidae and Xenosauridae, but specifically sister to Anguidae.[17][18]
H. horridum,H. exasperatum, andH. suspectum are frequently found incaptivity and are well represented inzoos throughout much of the world. The other two species ofHeloderma,H. alvarezi andH. charlesbogerti, are extremely rare, and only a few captive specimens are known.
Schwandt, Hans- Joachimwww.heloderma.net 2006 in 6 languages
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