Lizard heads called the parietal eye, a basal ("primitive") feature also present in the tuatara. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps them detect predators stalking it from above. Until 2006 it was thought that among lizards, only the Gila monster and the Mexican beaded lizard were venomous. However, several species of monitor lizards, including the Komodo dragon, produce powerful venom in their oral glands. Lace monitor venom, for instance, causes swift loss of consciousness and extensive bleeding through its
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