Heloderma
Encyclopedia
Heloderma, the only genus
of the family Helodermatidae, consists of venomous lizard
s 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. Venom glands are located in the lower jaw, unlike snake
s' venom glands, which are located in the upper jaw. Also, unlike snakes, helodermatids lack the musculature to inject venom. 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 lizard
s and agamid
s as well. This controversial new view holds that venom production dates back to the common ancestor of the clade Toxicofera
, which includes all snake
s and 13 families of lizards.
Helodermatids are carnivorous, preying on rodent
s and other small mammal
s, and eating the eggs
of bird
s and reptiles. They are oviparous, laying large clutches
of eggs.
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 osteoderm
s 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 fossil
s. Although the beaded lizard or the Gila monster appear closely related to the monitor lizard
s (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.
, and are well represented in zoo
s 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. & 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. http://www.scribd.com/doc/11116925/Ariano-D-y-G-Salazar-2007-Notes-on-the-Distribution-of-the-Endangered-Lizard-Heloderma-horridum-charlesbogerti-in-the-Dry-Forests-of-Eastern-Gu
Ariano-Sánchez, D. 2006. The Guatemalan beaded lizard: endangered inhabitant of a unique ecosystem. Iguana 13: 178-183. http://www.scribd.com/doc/11117893/Ariano-D-2006-The-Guatemalan-beaded-lizard-Endangered-inhabitant-of-a-unique-ecosystem-Iguana-133-178183
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
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of the family Helodermatidae, consists of venomous lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s native to the southwestern United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
and as far south as Guatemala
Guatemala
Guatemala is a country in Central America bordered by Mexico to the north and west, the Pacific Ocean to the southwest, Belize to the northeast, the Caribbean to the east, and Honduras and El Salvador to the southeast...
. It includes two separate species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, with six subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
. Their closest living relatives are the anguid
Anguidae
The Anguidae is a large and diverse family of lizards native to the northern hemisphere. The group includes the slowworms, glass lizards, and alligator lizards, among others. Anguidae is divided into three subfamilies and contains 94 species in eight genera. Their closest living relatives are the...
lizards.
Helodermatids (or beaded lizards) are large, stocky, slow-moving reptiles that prefer semi-arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
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. Venom glands are located in the lower jaw, unlike snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s' venom glands, which are located in the upper jaw. Also, unlike snakes, helodermatids lack the musculature to inject venom. The venom is typically used only in defense, rather than in subduing prey, and the lizard must chew
Mastication
Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by...
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 lizard
Monitor lizard
Monitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...
s and agamid
Agamidae
Agamids, lizards of the family Agamidae, include more than 300 species in Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards. Phylogenetically they may be sister to the Iguanidae, and have a similar appearance. Agamids usually have...
s as well. This controversial new view holds that venom production dates back to the common ancestor of the clade Toxicofera
Toxicofera
Toxicofera , is a hypothetical clade which represents about 4600 species of extant squamates It encompasses all venomous reptile species, as well as numerous related non-venomous species....
, which includes all snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s and 13 families of lizards.
Helodermatids are carnivorous, preying on rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s and other small mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, and eating the eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
of bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s and reptiles. They are oviparous, laying large clutches
Clutch (eggs)
A clutch of eggs refers to all the eggs produced by birds or reptiles, often at a single time, particularly those laid in a nest.In birds, destruction of a clutch by predators, , results in double-clutching...
of eggs.
Taxonomy
Family Helodermatidae- Genus Heloderma
- Heloderma horridum, beaded lizardBeaded lizardThe Beaded lizard is a species of venomous lizard found principally in Mexico and southern Guatemala. Along with its congener, the Gila monster , it is the only lizard to have evolved an overt venom delivery system, and has long been considered the only venomous lizard...
- Heloderma horridum horridum (WiegmannArend Friedrich August WiegmannArend Friedrich August Wiegmann was a German zoologist. He studied medicine and philology at the University of Leipzig, and afterwards was an assistant to Martin Lichtenstein in Berlin...
, 1829); Mexican beaded lizard - Heloderma horridum alvarezi Bogert & Martin del Campo, 1956; black beaded lizard
- Heloderma horridum exasperatum Bogert & Martin Del Campo, 1956; Rio Fuerte beaded lizard
- Heloderma horridum charlesbogertiHeloderma horridum charlesbogertiThe Motagua Valley beaded lizard, Heloderma horridum charlesbogerti, is a highly endangered subspecies of beaded lizard, a venomous lizard endemic to the dry forests of the Motagua Valley in southeastern Guatemala. It is the only allopatric subspecies, separated from the nearest population of...
Campbell & Vannini, 1988; Motagua Valley beaded lizard
- Heloderma horridum horridum (Wiegmann
- Heloderma suspectum, Gila monsterGila monsterThe Gila monster is a species of venomous lizard native to the southwestern United States and northwestern Mexican state of Sonora...
- Heloderma suspectum suspectum CopeEdward Drinker CopeEdward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...
, 1869; reticulate Gila monster - Heloderma suspectum cinctum Bogert & Martin Del Campo, 1956; banded Gila monster
- Heloderma suspectum suspectum Cope
- Heloderma horridum, beaded lizard
Members of the genus Heloderma have many extinct relatives in the Helodermatidae whose evolutionary history may be traced back to the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, such as Estesia
Estesia
Estesia is an extinct genus of Late Cretaceous helodermatoid lizard found in the Gobi Desert in Mongolia. It was discovered in June 1990 by a joint expedition made up of Mongolian and American palaeontologists, and described in 1992 by Mark Norell, Malcolm McKenna and Michael Novacek...
. The genus Heloderma has existed since the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
, when H. texana lived, and fragments of osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
s from the Gila monster have been found in late Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
(8000-10,000 years ago) deposits near Las Vegas, Nevada
Las Vegas, Nevada
Las Vegas is the most populous city in the U.S. state of Nevada and is also the county seat of Clark County, Nevada. Las Vegas is an internationally renowned major resort city for gambling, shopping, and fine dining. The city bills itself as The Entertainment Capital of the World, and is famous...
. Because the Helodermatids have remained relatively unchanged morphologically
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
, they are occasionally regarded as living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s. Although the beaded lizard or the Gila monster appear closely related to the monitor lizard
Monitor lizard
Monitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...
s (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
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is Heloderma horridum, which was first described in 1829 by Arend Weigmann
Arend Friedrich August Wiegmann
Arend Friedrich August Wiegmann was a German zoologist. He studied medicine and philology at the University of Leipzig, and afterwards was an assistant to Martin Lichtenstein in Berlin...
. 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
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
words hêlos (ηλος)—the head of a nail or stud—and derma (δερμα), meaning skin.
In captivity
H. h. horridum, H. h. exasperatum, and both subspecies of H. suspectum are frequently found in captivityCaptivity (animal)
Animals that live under human care are in captivity. Captivity can be used as a generalizing term to describe the keeping of either domesticated animals or wild animals. This may include for example farms, private homes and zoos...
, and are well represented in zoo
Zoo
A zoological garden, zoological park, menagerie, or zoo is a facility in which animals are confined within enclosures, displayed to the public, and in which they may also be bred....
s 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.
Sources
Ariano-Sánchez, D. 2008. Envenomation by a wild Guatemalan beaded lizard Heloderma horridum charlesbogerti. Clinical toxicology 46 (9): 897-899. http://www.informapharmascience.com/doi/pdf/10.1080/15563650701733031Ariano-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. http://www.scribd.com/doc/11116925/Ariano-D-y-G-Salazar-2007-Notes-on-the-Distribution-of-the-Endangered-Lizard-Heloderma-horridum-charlesbogerti-in-the-Dry-Forests-of-Eastern-Gu
Ariano-Sánchez, D. 2006. The Guatemalan beaded lizard: endangered inhabitant of a unique ecosystem. Iguana 13: 178-183. http://www.scribd.com/doc/11117893/Ariano-D-2006-The-Guatemalan-beaded-lizard-Endangered-inhabitant-of-a-unique-ecosystem-Iguana-133-178183
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