Running on Empty: Investigating the Energetics of Theropod Predators and Community Structure of the Latest Cretaceous Frenchman Formation Ecosystem

DECECCHI, T Alexander, University of Pittsburgh Johnstown, Johnstown, PA. BAMFORTH, Emily, Royal Saskatchewan Museum T.rex Discovery Centre, Eastend, Sask. Canada. LARSSON, Hans CE, McGill University, Montreal, QC, Canada.

Abstract

Top predators greatly influence community food webs. Their reach into the food web dictates many properties of ecosystem stability and dynamics. We bring these principles to begin estimating a well-studied terrestrial vertebrate community that lived immediately before the end-Cretaceous mass extinction. The community is in the Frenchman Formation of southern Saskatchewan, Canada. The large theropod, Tyrannosaurus rex, was clearly the top predator in this system. Starting with this taxon, we explore possible food web structures for the Frenchman Formation ecosystem. Reconstructions of large theropod hunting strategies have often been focused on maximizing extreme aspects of prey pursuit such as top speed or agility. Yet much of the daily energy budget for modern predators, especially those that target large vertebrate prey, is focused on search, stalking and low to medium speed pursuit. Here we incorporate these aspects, including shifting speed potential and foraging efficiency in terms of cost of transport across ontogeny, to determine realistic limiting factors impacting the foraging choices of large theropods dinosaurs in the Frenchman Formation ecosystem. We then compared this with speeds estimated for prey species such as the large ornithischians Triceratops and Edmontosaurus and smaller taxa such as Thescelosaurus, using both Froude and mass based reconstructions. We find that at all but the largest size class Tyrannosaurus would likely be faster and more agile than their prey, especially during the “teenage” years with masses between 2000-3000kg. Given the suggested pack hunting in tyrannosaurids and how modern pack hunters can capture prey items even if they do have a higher top speed through strategic such as communication between members of the pack, we suggest that maximizing speed would be of little advantage to Tyrannosaurus. As a result of the pronounced growth spurt in Tyrannosaurus in their "teen" years, this taxon soon reaches sizes where maximum speed is controlled by mass not leg length.  This is unlike albertosaurines which do not show a strongly significant relationship between femoral circumference and hindlimb length as well as a much lesser increase in relative femoral circumference versus length.  The growth spurt would have pushed Tyrannosaurus out of the zone where leg length helps with prey pursuit and into the range where it helps with energy savings quite quickly. We propose it’s not the short periods of high speed chase that define the predation patterns seen in the latest Cretaceous ecosystems, but energetic efficiency over the longer periods of search and stalking matter when reconstructing the paleobiology of the top predators. The hypothesized food web reconstruction thus places deep reaches of Tyrannosaurus into the Frenchman Formation ecosystem. Such generalist predatory strategies are recognized as stabilizing factors in ecosystem stability suggesting the waning days of this dinosaur community were not particularly susceptible to ecosystem failure, placing more emphasis on a catastrophic cause for the end Cretaceous mass extinction.