Allosauroid theropods were a diverse and widespread radiation of Jurassic-Cretaceous megapredators. Achieving some of the largest body sizes among theropod dinosaurs, these colossal hunters dominated terrestrial ecosystems until a faunal turnover redefined apex predator guild occupancy during the final 20 million years of the Cretaceous. Here we describe a giant new species of allosauroid - Siats meekerorum gen. et sp. nov. - providing the first evidence for the cosmopolitan clade Neovenatoridae in North America. Siats is the youngest allosauroid yet discovered from the continent and demonstrates that the clade endured there into the Late Cretaceous. The discovery provides new evidence for ecologic sympatry of large allosauroids and small-bodied tyrannosauroids. These data support the hypothesis that extinction of Allosauroidea in terrestrial ecosystems of North America permitted ecological release of tyrannosauroids, which went on to dominate end-Cretaceous food webs.
Bibliographical noteFunding Information:
We are grateful to A. Shinya, K. Zahnle, T. Nelson and D. Wagner for preparation; to A. Shinya, L. Herzog, D. Wagner, C. VanBeek, W. Simpson, J. Holstein, J. Ostergaard, J. Smith, T. Gates, C. Kammerer, S. Gieser, S. Clawson and W. Adams for field work; and T. Gates for stratigraphic data. We thank J. Kirkland, S. Madsen and D. Deblieux for advice and information on the field area. We are grateful to P. Sereno, Sandra Chapman and Steve Hutt for access to specimens in their care; R. Benson for matrix files; D. Webb for data assistance. We thank N. Smith and S. Brusatte for discussion and information. M. Donnelly skillfully illustrated the specimen. L.E.Z. was supported by the John Caldwell-Meeker Postdoctoral Fellowship and the Bucksbaum Young Scientist Fellowship during part of this research. Additional financial support provided by the US National Science Foundation (EAR AToL 0228693 and ANT 0838925 PJM); The Field Museum, The FM Women’s Board, NC Museum of Natural Sciences and NC State University. Free online versions of TNT and Mesquite are provided by the Willi Hennig Society and the Free Software Foundation Inc. Fieldwork was conducted under Bureau of Land Management Permits UT08-20S and UT09-014E-Em and we thank the Bureau of Land Management representatives S. Foss and M. Leschin for their assistance.
Copyright 2014 Elsevier B.V., All rights reserved.