Abstract
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.
Highlights
Modern terrestrial herbivore ecosystems are dominated by mammal faunas that originated with the evolution of ungulate folivores during the middle Eocene [1]
Squamates do not efficiently metabolize plant matter compared with mammals [3], and digestion requires elevated body temperatures which are correlated to large body size [2,4,5,6] and restriction to tropical climates for most taxa [7]
We describe a giant acrodontan lizard from the rich, low-latitude vertebrate fossil record of the late middle Eocene Pondaung Formation of central Myanmar that includes a diversity of eutherian mammals, turtles, squamates and crocodylians recovered from siliciclastic sediments representing fluvial depositional environments [17,18,19,20,21]
Summary
Modern terrestrial herbivore ecosystems are dominated by mammal faunas that originated with the evolution of ungulate folivores during the middle Eocene [1]. The relative paucity and geographical restriction of the squamate fossil record confounds efforts to examine the historical relationship between body size and environment relative to faunal competition, climate and historical contingency in poikilothermic herbivores. Analysis of the new acrodontan’s inferred diet and estimated body mass in the context of the co-occurring fauna and in comparison to modern vertebrate communities allows us test the relative influences of mammalian competition versus climate regime as a regulating mechanism of herbivorous reptile body size by examining herbivore community structure in past and present vertebrate ecosystems and by estimating minimum paleotemperatures necessary to support a giant poikilothermic herbivore based on the mass-specific metabolic relationship between body size and climate in living herbivorous lizards
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