The modern Lizards of the genus Heloderma, from North and Central America, are the only surviving members of a group called the Monstersauria, which dates back to the Cretaceous and is known from Asia and North America. They are related to Slow-worms and Alligator Lizards.
In a paper published in the American Museum Novitates on 25 January 2013, Hongyu Yi of the Division of Paleontology at the American Museum of Natural History and Mark Norell of the Division of Paleontology at the American Museum of Natural History and the Department of Earth and Environmental Sciences at Columbia University describe a Monstersaurid Lizard from the Late Cretaceous of Mongolia.
The new specimen is placed in the species Estesia mongoliensis, which was described by Mark Norell in 1992, though this specimen is more complete than previously described specimens, allowing comparison of the venom delivery system and skull with that of modern Monstersaurs.
The skull of the new Estesia mongoliensis specimen in lateral view. Scale bar is 10 mm. Yi & Norell (2013).
Modern Monstersaurs have deep groves in their side teeth, running from the root to the tip. This is not present in all fossil species from North America, and deep groves similar to those seen in Heloderma do not appear before the Miocene. All North American species posses skulls with fused osteoderms (boney plated that develop in the skin). Previously known Asian Monstersaurs do not possess venom grooves, but also have fused osteoderms, suggesting that the group developed osteoderms, then shallow venom grooves, then deeper grooves. The new Estesia mongoliensis specimen lacks osteoderms, but has shallow, but well developed venom grooves, suggesting that the evolution of the group is more complicated, with venom grooves and/or osteoderms having more than one origin.
See also A giant Monitor Lizard from the Miocene of Samos, Greece and Helodermatid Lizard from the Late Miocene-Early Pliocene of Tennessee.
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