Mexican beaded lizard beaded lizard under the Convention on International Trade in Endangered Species as an Appendix I animal, making it illegal to export the species. Mexican beaded lizard The Mexican beaded lizard ("Heloderma horridum") is a species of lizard in the family Helodermatidae, one of the two species of venomous beaded lizards found principally in Mexico and southern Guatemala. It and its congener (member of the same genus) the Gila monster ("Heloderma suspectum") are the only lizards known to have evolved an overt venom delivery system. The Mexican beaded lizard is larger than the Gila monster, with duller coloration, black with yellowish
Mexican beaded lizard back to the Cretaceous period. The genus "Heloderma" has existed since the Miocene, when "H. texana" ranged over most of North America. Because the helodermatids have remained relatively unchanged morphologically, they are occasionally regarded as living fossils. Although the beaded lizard appears closely related to the monitor lizards (varanids) of Africa, Asia, and Australia, the wide geographical separation and unique features not found in the varanids indicate the beaded lizard is better placed in a separate family. The species was first described in 1829 by Arend Wiegmann as "Trachyderma horridum", but he renamed it "Heloderma horridum" six months later. Its