Helodermatidae
The only living venomous lizards
Helodermatidae consists of one genus and two species, Heloderma suspectum and H. suspectum, which are found in the southwestern United States and Mexico respectively. They live in relatively arid habitats with some rainfall and ground soft enough for digging.
Helodermatids are rather squat lizards, with a maximum length of a little less than 1m, and relatively short tails (25-40 caudal vertebrae) used for fat storage. Helodermatids have short, broad, rounded snouts and powerful jaws. Several other synapomorphies of the group relate to the presence and delivery of venom (unique among lizards). Helodermatids are attractive beasts, with a yellow, pink, or orange colored splotches against an otherwise dark skin thick with osteoscutes. They are somewhat sluggish animals, spending the dry heat of summer and the cold of winter in underground burrows.
Helodermatids emerge in the spring from their burrows to eat eggs and juvenile birds (despite their robust form, they are adequate climbers) and juvenile mammals. Their venom is apparently used for defense rather than for prey immobilization.
Helodermatids are represented in the petroglyphs of the southwestern Indians and the pottery stamps of ancient Mexico. They have no economic importance as the presence of many osteoscutes renders their skins unsuitable for making leather, and their meat as usually (erroneously) regarded as toxic. The venom of helodermatids is primarily neurotoxic, and is delivered via grooves in the teeth of the lower jaw as the animal chews on its victim. Several human deaths from the bite of a Heloderma have been recorded, although the victims have either been in frail health or intoxicated, and bites have always occurred in conjunction with harrassment of these non-agressive animals. Some constituents of heloderma venom are thought to show promise for new treatments for diabetes. The greatest threat to helodermatids is habitat destruction.
Helodermatidae belongs to Varanoidea, a group comprising the New World Helodermatidae, and the Old World Lanthanotidae and Varanidae. Helodermatidae is the sister taxon to Lanthanotus and Varanidae. Varanoids in turn belong to the larger group Anguimorpha, which includes such forms as anguids (alligator lizards) and xenosaurids (knob-scaled lizards).
The earliest known helodermatid-like fossil genera are the Late Cretaceous Estesia, Gobiderma, and Paraderma; later fossil forms include the Tertiary Eurheloderma and Lowesaurus and more recent fossils that have been placed in the genus Heloderma. Gobiderma appears to be the most primitive of this group, and the clade comprising Gobiderma and all its descendants (all the fossils named and the living Helodermatidae) is called Monstersauria.
Ast, J. C. Mitochondrial DNA evidence and evolution in Varanoidea (Squamata). Cladistics, in press.
Estes, R., K. deQuiroz, and J. Gauthier. 1988. Phylogenetic relationships within Squamata. Pp. 119-281 in "Phylogenetic Relationships of the Lizard Families" R. Estes and G. Pregill, Eds., Stanford University Press, CA.
Lidikay, C. E. 1997. Biochemistry of helodermatid venom. Stanislaus Journal of Biochemical Reviews. Electronically published at California State University, Stanislaus.
Norell, M. and Gao, K. 1997. Braincase and phylogenetic relationships of Estesia mongoliensis from the late Cretaceous of the Gobi Desert and the recognition of a new clade of lizards. Am. Mus. Novitates 3211, 1-25.
Pregill, G. K., J. A. Gauthier, and H. W. Greene. 1986. The evolution of helodermatid squamates, with a description of a new taxon and an overview of Varanoidea. Trans. San Diego Soc. Nat. Hist. 21, 167-202.
Rieppel, O. 1980. The phylogeny of anguinomorph lizards. Birkhäuser Verlag, Basel.
Steel, R. 1996. Living dragons: a natural history of the world's monitor lizards. Blandford, London. http://wwwchem.csustan.edu/chem4400/sjbr/venom.htm
Contributors
Jennifer C. Ast (author).