Helodermatid Lizards are the only extant lizards that are truly venomous (Monitor Lizards such as the Komodo Dragon, Varanus komodoensis, deliver a bite laced with harmful bacteria, but are not actually venomous). There are two extant groups of Helodermatid Lizards; the Gila Monster, Heloderma suspectum, and Mexican Bearded Lizard, Heloderma horridum, currently restricted to the southwestern United States, western Mexico and Central America, where they inhabit deserts and semi-deserts as well as dry woodlands and grasslands, and the Bearded Lizards of the genus Pogona, which occupy similar areas in Australia.
The Gila Monster, Heloderma suspectum. A. Holycross/Reptiles of Arizona.
Helodermatid Lizards have distinctive osteoderms (bony plates in their skin) that are circular or hexagonal, dome shaped, and cover their entire body, fusing to the skull on the head. This is not seen in any other form of lizard, making fossil Helodermatid Lizards easy to identify (though these are not numerous, the group having apparently never been numerous).
The skull of The Gila Monster, Heloderma suspectum, showing the fused osteoderms. Will's Skull Page.
In a paper in the March edition of the journal Acta Palaeontologica Polonica, a team of scientists from the Department of Geosciences and the Don Sundquist Center of Excellence in Paleontology at East Tennessee State University led by Jim Mead describe the discovery of Helodermatid Lizard osteoderms from the Miocene-Early Pliocene Gray Fossil Site in Washington County, Tennessee. These are not enough to identify the lizards to species level but are distinctive enough that they can only come from a Helodermatid Lizard.
Osteoderms from the Gray Fossil Site (A-C) and the extant Mexican Bearded Lizard, Heloderma horridum (D-F). A-C₁ and D-F₁ apical view, C₂ & F₂ basal views of C₁ & F₁. From Mead et al. (2012).
The Gray Fossil Site was laid down in a sub-tropical forest at the boundary between the Miocene and the Pliocene. The forest was made up largely of Oaks (Quercus) and Hickory Trees (Carya) with some conifers, Elms (Ulmas), Birches (Betula), Ash (Fraxinus), Hackberry (Celtis), Alder (Alnus) and Willow (Salix). It had an understory of Buttercup Shrub (Corylopsis) and numerous vines including three types of wild grapes (Vitis) and Sinomenium, a woody vine today restricted to lowland tropical and subtropical forests in East Asia.
It is not clear whether the Gray Fossil Site Forest was a wet or dry ecosystem; some of the plant species there, such as Alders, are known to prefer wetlands others, such as grapes, prefer a drier climate. The animal fossils from the site include Alligator, and lungless Plethodontid Salamanders, which would tend to suggest a moister climate, but the presence of Helodermatid Lizard osteoderms, would suggest a drier climate over at least part of the forrest, it the animals had similar environmental preferences to today.
The location of the Gray Fossil Site. From Shunk, Driese & Clark (2006).
The earliest Helodermatid Lizard known is Primaderma nessovi from the Early Cretaceous of Utah. Other Cretaceous forms known are Paraderma bogerti from Wyoming, and Gobiderma pulchrum and Estesia mongoliensis, both from Mongolia. Known Tertiary forms are Eurheloderma from the Palaeocene of Wyoming, Eurheloderma gallicum from the Eocene of France, Lowesaurus matthewi from the Oligocene to Early Miocene of Colorado and Nebraska, Heloderma texana from the Miocene of Texas and some unnamed Miocene remains from Florida.
Fossil Helodermatid Lizard ellements (A) Left maxilla (upper jaw bone) from Eurheloderma gallicum, from the Eocene Phosphorites du Quercy of France. (B) Right maxilla of same. (C) Maxilla of Paraderma bogerti, from the Early Cretaceous of Wyoming. (D) Right frontal (forehead bone) of Lowesaurus matthewi, from the Oligocene White River Formation of Logan County, Colorado. (E) Cranial bone of Heloderma texana, from the Miocene Delaho Formation of Texas. (F) Skull of same. (G) Right maxilla of (D). From Mead et al. (2012).
This suggests that Helodermatid Lizards originated in what is now Eurasia or North America before the breakup of Pangaea in the Jurassic, and that the common ancestor of Australian and North American Helodermatid Lizards lived in the Cretaceous (rather than having a later, Gondwanan origin and reaching North America from the south with the join-up of the Americas at the beginning of the Pliocene). This would suggest that the preference for hot, dry climates is fairly ancient within the group, making it likely that at least part of the Gray Fossil Site Forest had a dry climate.
Ventral view of the skull of the Gila Monster, Heloderma suspectum, with osteoderms in place. From Mead et al. (2012).
The end of the Miocene and beginning of the Pliocene was a time of global cooling, as North and South America were joined by Central America, preventing the flow of ocean currents between the tropical Pacific and Atlantic Oceans (from this time onwards, all flow between Oceans has been in Antarctic waters). It also saw the spreading of grasslands replacing forests in many parts of the world, and the mixing of North and South American faunas.