Carnosaurs as Apex Scavengers: Agent-based simulations reveal possible vulture analogues in late Jurassic Dinosaurs

Abstract

The Morrison Formation of the Late Jurassic Period is characterized by its diverse assemblage of sauropods, several species of which reached the size of modern cetaceans. While much scientific attention has concerned their biology in life, researchers have yet to examine how their massive carcasses may have influenced the evolution of other dinosaurs in their communities, such as theropods. Theropod consumers local to this faunal system are typically described as powerful apex predators at the top of local food webs, but instead, may have been shaped by competition for carrion resources generated as a byproduct of their giant sauropod neighbors. To test this hypothesis, we wrote a series of agent-based simulations (ABS). Specifically, we simulated allosaurid consumer behavior versus spatially distributed sauropod carcass resources such as could be expected in ecosystems like that represented in the Morrison Formation. We incorporated conditions to test how competition among consumers, seasonality, and predation success influenced carnosaur survival both with and without carrion-abundant systems. Trials of the ABS resulted in a strong selective advantage for allosaurs as obligate scavengers because of the high metabolic and survival costs associated with predation of large vertebrates. Allosaurs with increased predatory success over peers failed to succeed competitively unless the probability of scavenging opportunities fell below a certain threshold and a significant proportion of herbivores were available as prey targets, which might not have been the case in sauropod-dominated systems. Our results may explain why carnosaurs like Allosaurus did not evolve powerful bite forces, binocular vision, or advanced cursorial adaptations. Given the enormous supply of sauropod carrion, they were under no resource-based selective pressure to overpower prey and may have evolved as terrestrial vulture analogues. This also may explain why the absence of sauropods in certain environments led to more obvious predatory adaptations in theropods such as tyrannosaurs. Tyrannosaurs may have been forced to meet their energy budgets by hunting, because non-sauropod carrion production was too low to support them passively.