scientific inquiry and remain a continuous point of debate. Currently the diversity of North American troodontids is restricted to three or more recognized Cretaceous forms including Troodon formosus [15–16], Troodon inequalis [4], Geminiraptor suarezarum [17], and the tooth taxon Pectinodon bakkeri [18–19], as well as two putative Late Jurassic taxa, including the tooth taxon Koparion douglassi [20] and a possible new species from the Morrison Formation [21]. Here we report the discovery of a new troodontid theropod— Talos sampsoni—from the Upper Cretaceous Kaiparowits Forma- tion, Grand Staircase-Escalante National Monument, southern Utah, USA. Although an isolated tooth taxon (Pectinodon) [18] was referred to Troodontidae since the coining of Stenonychosaurus inequalis in 1932 [22], Talos represents the first definitive North American troodontid from the Late Cretaceous to be named in over 75 years and sheds new light on the taxonomic and paleobiogeographic diversity of North American dinosaurs of the Western Interior Basin. Institutional Abbreviations AMNH: American Museum of Natural History, New York, New York, USA; IGM: Geological Institute of the Mongolian Academy of Sciences, Ulaan Bataar, Mongolia; IVPP: Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences, Beijing, China; CMN: Canadian Museum of Nature, Ottawa, Ontario, Canada; MOR: Museum of the Rockies, Bozeman, Montana, USA; TMP: Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UMNH: Utah Museum of Natural History, Salt Lake City, Utah, USA. Materials and Methods Taxonomic Considerations Historically, Late Cretaceous North American troodontid materials were classified under the nomina Troodon formosus [23], Polyodontosaurus grandis [24], and Stenonychosaurus inequalis [22]. Since their establishment, these names have been the subject of continuous instability. Romer [25] synonymized Polyodontosaurus (an isolated jaw) with Troodon formosus. Soon thereafter Russell [26] synonymized Polyodontosaurus, and Ornithomimus altus [27] under Stenonychosaurus inequalis based on new materials of the latter, yet noted a lack of sufficient evidence to synonymize these materials with Troodon formosus. Five years later, Barsbold [28] named a second species of Saurornithoides (S. junior), upheld the synonymiza- tions of Russell [26], further noted the distinctiveness of Troodon and Stenonychosaurus, and erected a new higher-level taxon (Saurornithoididae) for Saurornithoides and Stenonychosaurus to the exclusion of T. formosus. Carpenter [18] subsequently coined a new name Pectinodon bakkeri for a sample of troodontid teeth from the Maastrichtian Lance Formation and synonymized Polyodontosaurus, Stenonychosaurus, O. altus, and T. formosus under the new combina- tion Saurornithoides inequalis, to which he referred a partial dentary and basioccipital. Most recently the genus Troodon has taken precedence over Stenonychosaurus in the literature. Based on the discovery of a troodontid dentary (TMP 83.12.11) from the Horseshoe Canyon Formation and a review of North American troodontid materials, Currie [15] synonymized Stenonychosaurus inequalis and Pectinodon bakkeri under Troodon formosus. Thereafter, the name Troodon formosus was applied to nearly all troodontid materials recovered from the Upper Cretaceous of North America [16,29–32]. More recently Currie [4] proposed Troodon inequalis as a more restricted designation for troodontid materials recovered from Dinosaur Provincial Park. New Troodontid Theropod from North America Given the contention regarding the taxonomy of Late Cretaceous North American troodontid materials and the status of Troodon formosus in particular (see 4, 15, 18–19, 26, 33], we recognize the possibility that specimens currently constituting the hypodigm ‘‘Troodon formosus’’ from these and other formations of the Western Interior Basin may represent multiple species and that Troodon formosus, since based on a single tooth [23], may represent a nomen dubium. In order to maintain taxonomic stability until such time as T. formosus is reevaluated and for the purpose of morphological comparison, we herein follow Currie [15], Varricchio et al. [8–9], Varricchio and Jackson [10], and Makovicky and Norell [16] in regarding troodontid specimens from the Oldman, Judith River, Two Medicine, and Dinosaur Park formations as Troodon formosus. Nevertheless, expecting that some of these specimens may be reclassified in the near future, we refer explicitly to specimen numbers when we make comparative statements between Talos sampsoni and Troodon formosus. Histology Petrographic specimens of UMNH VP 19479 were prepared at the Field Museum of Natural History (Chicago, IL). Transverse sections were obtained from the proximal shaft of the femur and fibula, and midshaft of the fibula to minimize damage to the holotype specimen. Casts of pre-sectioned elements and petro- graphic sections are reposited at the UMNH. Sections were embedded in Castalite-APH acrylic-polyester resin, sectioned using a diamond blade, polished successively using 600 and 1,200 grit silicon carbide grinding paper, and micropolished with a de- agglomerated alpha alumina powder (0.3 mm), water, and soap solution. mCT Scanning To assess the degree of pathological modification on UMNH VP 19479, select elements of the left and right ankle and pes were scanned at the Ohio University microCT (OUmCT) Facility (http://www.oucom.ohiou.edu/ou-microct/) on a GE eXplore Locus in-vivo micro-CT scanner. Scan data were acquired at 85 kVp, 400 mA, and a slice thickness of 0.045 mm, with digital post-processing completed in GE MicroView 2.1.2 and AMIRA 4.1 Advanced Graphics Package. Phylogenetic Protocol To investigate its evolutionary relationships, we added Talos sampsoni and the primitive troodontids Mei long [13] and Sinovenator changi [5] to the character matrix of Norell et al. [34]. Character coding, manipulation, and tree visualization were carried out using Mesquite ver. 2.74 [35]. Analyses were executed in the program TNT (Trees using New Technology), provided by the Willi Hennig Society [36]. Most parsimonious trees (MPTs) were obtained via heuristic search methods on 1000 replicates of Wagner trees with random addition sequences and then subject to TBR (tree bisection-reconnection) swapping methods holding 10 trees per replicate. Results were assessed using strict and reduced consensus methods. Ambiguous nodes were collapsed following Rule 1 of Coddington & Sharff [37]. Maximum agreement subtrees [38] were used to identify labile taxa and common topology among all MPTs (most parisimonious trees). Standard bootstrapping (1000 replicates, GC values) [39], symmetrical resampling (1000 replicates, GC values) [40], and Bremer support were conducted to assess node stability. We retained the operational taxonomic units (OTUs) Dromaeosauridae and Avialae yet eliminated the extraneous and generalized OTUs Ornithomimidae and Oviraptorosauridae included in the original Norell et al. study [34] since our intent was not to examine PLoS ONE | www.plosone.org 2 September 2011 | Volume 6 | Issue 9 | e24487