Meeting Abstract

59.5  Jan. 7  Early evolution of Monstersauria (Reptilia, Squamata) CONRAD, J.L.*; NORELL, M.A.; American Museum of Natural History, New York jconrad@amnh.org

Monstersauria (beaded lizards, Gila monsters, and their fossil relatives) has a fossil record that extends back 100 million years (Ma) and includes taxa from North America, Europe, and eastern Asia. Most Cretaceous monstersaurs are known from isolated skull elements. However, new specimens of Estesia mongoliensis and Gobiderma pulchrum, both from the Late Cretaceous of Mongolia, offer new details about the evolution of this clade. Some basal platynotans and varanid relatives show similar osteoderms to those of basal monstersaurs such as G. pulchrum, suggesting that this and some other morphological features usually associated with monstersaurs are plesiomorphic for Varanoidea. Estesia mongoliensis, a proximal outgroup to Heloderma, is essentially identical to Heloderma in the palate morphology despite plesiomorphic features of the skull roof (e.g., complete supratemporal arch). One new specimen of E. mongoliensis shows that the snout was short and rounded as in Heloderma. Importantly, a skeleton of E. mongoliensis reveals no evidence of osteoderms. Gobiderma pulchrum is one of the basal-most known monstersaurs. The new specimen helps demonstrate the plesiomorphic skull shape, short crista cranii, and braincase morphology. Co-occurrence of E. mongoliensis and G. pulchrum in some strata indicate that the two were likely partitioning resources; the former was a savannah monitor-sized predator with venom grooves in its teeth, the latter was a beaded lizard-sized predator without venom. Both lived in dinosaur- and mammal-rich areas. The presence of several late Late Cretaceous fossils suggests that monstersaurs were once a more common faunal component in North America and Asia. It remains unclear whether monstersaurs originated in North America and invaded Eurasia multiple times, or vice versa.