Giant lizards occupied herbivorous mammalian ecospace during the Paleogene greenhouse in Southeast Asia

• Published in: Proceedings of the Royal Society. B, Biological sciences Vol. 280; no. 1763; pp. 1 - 7
• Main Authors: Head, Jason J., Gunnell, Gregg F., Holroyd, Patricia A., Hutchison, J. Howard, Ciochon, Russell L.
• Format: Journal Article
• Language: English
• Published: The Royal Society 22-07-2013
• Subjects: Animal physiology; Body size; Evolution; Fauna; Fossils; Herbivores; Lizards; Mammals; Paleoclimatology; Taxa
• ISSN: 0962-8452

Mammals dominate modern terrestrial herbivore ecosystems, whereas extant herbivorous reptiles are limited in diversity and body size. The evolution of reptile herbivory and its relationship to mammalian diversification is poorly understood with respect to climate and the roles of predation pressure and competition for food resources. Here, we describe a giant fossil acrodontan lizard recovered with a diverse mammal assemblage from the late middle Eocene Pondaung Formation of Myanmar, which provides a historical test of factors controlling body size in herbivorous squamates. We infer a predominately herbivorous feeding ecology for the new acrodontan based on dental anatomy, phylogenetic relationships and body size. Ranking body masses for Pondaung Formation vertebrates indicates that the lizard occupied a size niche among the larger herbivores and was larger than most carnivorous mammals. Paleotemperature estimates of Pondaung Formation environments based on the body size of the new lizard are approximately 2–5°C higher than modern. These results indicate that competitive exclusion and predation by mammals did not restrict body size evolution in these herbivorous squamates, and elevated temperatures relative to modern climates during the Paleogene greenhouse may have resulted in the evolution of gigantism through elevated poikilothermic metabolic rates and in response to increases in floral productivity.