A New Troodontid Theropod, Talos sampsoni gen. et sp. nov., from the Upper Cretaceous Western Interior Basin of North America Lindsay E. Zanno1,2*, David J. Varricchio3, Patrick M. O’Connor4,5, Alan L. Titus6, Michael J. Knell3 1 Field Museum of Natural History, Chicago, Illinois, United States of America, 2 Biological Sciences Department, University of Wisconsin-Parkside, Kenosha, Wisconsin, United States of America, 3 Department of Earth Sciences, Montana State University, Bozeman, Montana, United States of America, 4 Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, Ohio, United States of America, 5 Ohio Center for Ecology and Evolutionary Studies, Ohio University, Athens, Ohio, United States of America, 6 Grand Staircase-Escalante National Monument, Bureau of Land Management, Kanab, Utah, United States of America Abstract Background: Troodontids are a predominantly small-bodied group of feathered theropod dinosaurs notable for their close evolutionary relationship with Avialae. Despite a diverse Asian representation with remarkable growth in recent years, the North American record of the clade remains poor, with only one controversial species—Troodon formosus—presently known from substantial skeletal remains. Methodology/Principal Findings: Here we report a gracile new troodontid theropod—Talos sampsoni gen. et sp. nov.— from the Upper Cretaceous Kaiparowits Formation, Utah, USA, representing one of the most complete troodontid skeletons described from North America to date. Histological assessment of the holotype specimen indicates that the adult body size of Talos was notably smaller than that of the contemporary genus Troodon. Phylogenetic analysis recovers Talos as a member of a derived, latest Cretaceous subclade, minimally containing Troodon, Saurornithoides, and Zanabazar. MicroCT scans reveal extreme pathological remodeling on pedal phalanx II-1 of the holotype specimen likely resulting from physical trauma and subsequent infectious processes. Conclusion/Significance: Talos sampsoni adds to the singularity of the Kaiparowits Formation dinosaur fauna, which is represented by at least 10 previously unrecognized species including the recently named ceratopsids Utahceratops and Kosmoceratops, the hadrosaurine Gryposaurus monumentensis, the tyrannosaurid Teratophoneus, and the oviraptorosaurian Hagryphus. The presence of a distinct troodontid taxon in the Kaiparowits Formation supports the hypothesis that late Campanian dinosaurs of the Western Interior Basin exhibited restricted geographic ranges and suggests that the taxonomic diversity of Late Cretaceous troodontids from North America is currently underestimated. An apparent traumatic injury to the foot of Talos with evidence of subsequent healing sheds new light on the paleobiology of deinonychosaurians by bolstering functional interpretations of prey grappling and/or intraspecific combat for the second pedal digit, and supporting trackway evidence indicating a minimal role in weight bearing. Citation: Zanno LE, Varricchio DJ, O’Connor PM, Titus AL, Knell MJ (2011) A New Troodontid Theropod, Talos sampsoni gen. et sp. nov., from the Upper Cretaceous Western Interior Basin of North America. PLoS ONE 6(9): e24487. doi:10.1371/journal.pone.0024487 Editor: Carles Lalueza-Fox, Institut de Biologia Evolutiva - Universitat Pompeu Fabra, Spain Received May 16, 2011; Accepted August 11, 2011; Published September 19, 2011 This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Funding: Funding provided by National Science Foundation EAR-0617561 (to PMO) and EAR 0745454, Grand Staircase-Escalante National Monument (GEMNM), the Bureau of Land Management’s National Landscape Conservation System (Assistance Agreement JSA071004), as well as John Caldwell-Meeker Postdoctoral Fellowship (LEZ), Bucksbaum Young Scientist Fellowship (LEZ), Field Museum of Natural History, Ohio University College of Osteopathic Medicine (PMO), and Montana State University (DJV, MJK). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: lzanno@fieldmuseum.org Introduction Troodontids are an intriguing clade of feathered theropods known to possess the smallest body sizes [1–3] and largest relative brain sizes [4] among non-avian dinosaurs, and widely hypoth- esized to be among the closest extinct relatives of birds [2,5–7]. Until recently, the fossil record of troodontids was poorly known. However, the past decade has witnessed the discovery of several remarkable specimens that not only support a close relationship with Avialae but also provide unprecedented glimpses into the biology of non-avian dinosaurs. These include exemplary specimens documenting transitional phases in the evolution of avian reproductive physiology and behavior [8–11], as well as those displaying evidence of an avian-style sleeping posture [12– 13], ‘four-winged’ locomotor capabilities [3], and perhaps most extraordinary, plumage coloration [14]. Recent discoveries notwithstanding, troodontids are among the rarest of dinosaurs. To date, the taxonomic diversity of the clade is confined almost exclusively to Asia, where exceptional preserva- tion permits the documentation of at least 14 species. In contrast, the taxonomic resolution of North American troodontid materials is obfuscated by a paucity of well-preserved skulls and associated skeletons. This preservational bias has resulted in a series of reclassifications and synonymizations that span over 150 years of PLoS ONE | www.plosone.org 1 September 2011 | Volume 6 | Issue 9 | e24487