Also, unlike snakes, the Gila monster and beaded lizards lack the musculature to inject venom immediately.

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.

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

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.

[14] Conrad, 2008 and Estes et al., 1988 (using morphological data) place the Helodermatidae within Varanoidea along with Lanthanotus borneensis and Varanus.

[15][16] 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.