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Heloderma

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For the dermatological condition, see Knuckle pads.

Heloderma
Temporal range: Early Miocene to Recent
Gila monster, Heloderma suspectum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Infraorder: Neoanguimorpha
Clade: Monstersauria
Family: Helodermatidae
Gray, 1837
Genus: Heloderma
Wiegmann, 1829
Species

Heloderma is a genus of toxicoferid lizards that contains five species, all of which are venomous.[1] It is the only extant genus of the family Helodermatidae.

Description

Gila monsters are large, stocky, most of the time slow-moving reptiles that prefer arid deserts. Beaded lizards are seen to be more agile and seem to prefer more humid surroundings.[2][3][4] Their tails are used as fat storage organs. The scales of their head, back and tail look bead-like, containing osteoderms for better protection. The scales on the belly side are free from osteoderms. Most species are dark in color, with yellowish or pinkish markings.[5][6]

Their venom glands are located at the end of their lower jaws, unlike snakes' venom glands, which are located behind their eyes. Also, unlike snakes, beaded lizards lack the musculature to inject venom immediately. The lizards 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.[6][7]

Helodermatids are carnivorous, preying on rodents and other small mammals, and eating the eggs of birds and reptiles. They are oviparous.

The Gila monster typically lays six eggs, the beaded lizards up to about 18 eggs.[6] Comparing the different species, all eggs are similar in size, and the same holds for their hatchlings.

Taxonomy

Cladogram of the Heloderma species
Cladogram of the Heloderma species
Explanation of the numbers
1 Late Eocene (approx. 35 million years)
2 Late Miocene (approx. 10 million years)
3 Pliocene (approx. 4.4 million years)
4 Pliocene (approx. 3 million years)

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.[8] 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.[9]

Mexican beaded lizard skeleton (Museum of Osteology)

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.[10]

Conrad, 2008 and Estes et al., 1988 (using morphological data) places the Helodermatidae within the Varanoidea along with Lanthanotus borneensis and Varanus.[11][12] However, Estes et al., 1988 understood the 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 the Anguioidea along with the Anguidae and the Xenosauridae, but specifically a sister to the Anguidae.[13][14]

Ecology

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.[15] However, except for snakes, helodermatids, and possibly varanids, envenomation is not considered medically significant for humans.

Heloderma suspectum skull with dentition
Heloderma suspectum skull with dentition, photo by A. Laube

In captivity

Hatchling sizes of H. exasperatum and H. suspectum (taxidermy)
Hatchling sizes of H. exasperatum (top) and H. suspectum (taxidermy)

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, though, and only a few captive specimens are known.

  • Heloderma suspectum in captivity
  • Heloderma suspectum with four eggs
    Heloderma suspectum with four eggs
  • Heloderma suspectum with six eggs
    Heloderma suspectum with six eggs
  • A Gila monster hatching
    A Gila monster hatching
  • A group of young Gila monsters
    A group of young Gila monsters
Heloderma skin
Heloderma skin

References

Captured Gila monster
A captured Gila monster
  1. ^ http://herpetology.com/helobite.txt
  2. ^ C. M. Bogert, R. M. Del Campo (1956). "The Gila Monster and its Allies. The relationships, habits, and behavior of the lizards of the family Helodermatidae". Bulletin of the American Museum of Natural History. 109: 1–238.
  3. ^ Beck, D. D. (2005). Biology of Gila Monsters and Beaded Lizards. University Press of California.
  4. ^ Gila Monster and Mexican Beaded Lizard: Helodermatidae - Physical Characteristics
  5. ^ Schwandt, Hans-Joachim (2019). The Gila Monster Heloderma suspectum. Frankfurt/Main: Edition Chimaira. ISBN 978-3-89973-441-6.
  6. ^ a b c Bauer, Aaron M. (1998). Cogger, H.G.; Zweifel, R.G. (eds.). Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. p. 156. ISBN 0-12-178560-2.
  7. ^ Richard L. Cifelli, Randall L. Nydam. 1995. Primitive, helodermatid-like platynotans from the Early cretaceous of Utah. Herpetologica. 51(3):286-291.
  8. ^ King, Ruth Allen; Pianka, Eric R.; King, Dennis (2004). Varanoid Lizards of the World. Bloomington: Indiana University Press. ISBN 0-253-34366-6.
  9. ^ Mattison, Chris (1998). Lizards of the World. London: Blandford. ISBN 0-7137-2357-2.
  10. ^ Wiegmann, A.F.A. (1829). "Über die Gesetzlichkeit in der geographischen Verbreitung der Saurier". Isis. 22 (3–4). Oken: 418–428.
  11. ^ Conrad, Jack L.; Ast, Jennifer C.; Montanari, Shaena; Norell, Mark A. (2011). "A combined evidence phylogenetic analysis of Anguimorpha (Reptilia: Squamata)". Cladistics. 27 (3): 230–277. doi:10.1111/j.1096-0031.2010.00330.x. ISSN 1096-0031.
  12. ^ Estes, Richard (1988). "Phylogenetic relationships within squamata". Phylogenetic Relationships of the Lizard Families: Essays Commemorating Charles L. Camp: 119–281. hdl:10088/6457.
  13. ^ Vidal, Nicolas; Marin, Julie; Sassi, Julia; Battistuzzi, Fabia U.; Donnellan, Steve; Fitch, Alison J.; Fry, Bryan G.; Vonk, Freek J.; Rodriguez de la Vega, Ricardo C.; Couloux, Arnaud; Hedges, S. Blair (2012-10-23). "Molecular evidence for an Asian origin of monitor lizards followed by Tertiary dispersals to Africa and Australasia". Biology Letters. 8 (5): 853–855. doi:10.1098/rsbl.2012.0460. PMC 3441001. PMID 22809723.
  14. ^ Townsend, Ted M.; Larson, Allan; Louis, Edward; Macey, J. Robert (2004-10-01). "Molecular Phylogenetics of Squamata: The Position of Snakes, Amphisbaenians, and Dibamids, and the Root of the Squamate Tree". Systematic Biology. 53 (5): 735–757. doi:10.1080/10635150490522340. ISSN 1063-5157. PMID 15545252.
  15. ^ . Fry, B.; et al. (February 2006). "Early evolution of the venom system in lizards and snakes". Nature. 439 (7076): 584–588. Bibcode:2006Natur.439..584F. doi:10.1038/nature04328. PMID 16292255. S2CID 4386245.
A Chiapas beaded lizard in captivity

Notes

  • Ariano-Sánchez, Daniel (2008). "Envenomation by a wild Guatemalan beaded lizard Heloderma horridum charlesbogerti". Clinical Toxicology. 46 (9): 897–899. doi:10.1080/15563650701733031. PMID 18608297. S2CID 22173811.
  • 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. [1]
  • Ariano-Sánchez, D. 2006. The Guatemalan beaded lizard: endangered inhabitant of a unique ecosystem. Iguana 13: 178-183. [2]
  • 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.
Heloderma exasperatum near Los Alamos
Heloderma exasperatum near Los Alamos
Wikispecies has information related to Helodermatidae.

Schwandt, Hans- Joachim www.heloderma.net 2006 in 6 languages

Wikimedia Commons has media related to Heloderma.

Further reading

  • C. M. Bogert, R. M. Del Campo (1956). The Gila Monster and its Allies. The relationships, habits, and behavior of the lizards of the family Helodermatidae. Vol. 109. Bulletin of the American Museum of Natural History. pp. 1–238.
  • Beck, D. D. (2005). Biology of Gila Monsters and Beaded Lizards. London: University Press of California.
  • Schwandt, Hans-Joachim (2019). The Gila Monster Heloderma suspectum. Frankfurt/Main: Edition Chimaira. ISBN 978-3-89973-441-6.
Wikispecies has information related to Heloderma suspectum
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