outgroup relationships of Troodontidae and higher-level OTUs generally contain a mosaic of character states. We supplemented missing codes for several characters based on recent literature (2, 9, 34), modified one character (32) to capture ingroup rather than outgroup variation, and added 6 characters derived from our observation of Talos and specimens referred to Troodon formosus. The character list and matrix are available as Dataset S1. Nomenclatural Acts The electronic version of this document does not represent a published work according to the International Code of Zoological Nomenclature (ICZN), and hence the nomenclatural acts contained in the electronic version are not available under that Code from the electronic edition. Therefore, a separate edition of this document was produced by a method that assures numerous identical and durable copies, and those copies were simultaneously obtainable (from the publication date noted on the first page of this article) for the purpose of providing a public and permanent scientific record, in accordance with Article 8.1 of the Code. The separate print-only edition is available on request from PLoS by sending a request to PLoS ONE, Public Library of Science, 1160 Battery Street, Suite 100, San Francisco, CA 94111, USA along with a check for $10 (to cover printing and postage) payable to ‘‘Public Library of Science’’. In addition, this published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix ‘‘http://zoobank.org/’’. The LSID for this publication is (urn:lsid:zoobank.org:pub:9CC7B21F-2755-4EBD- AAD3-26EF1D6C3D7B). Fieldwork All necessary permits were obtained for the described field studies. Fieldwork resulting in the recovery of UMNH VP 19479 was conducted with permission of Grand Staircase-Escalante National Monument and the Bureau of Land Management’s National Landscape Conservation System (Assistance Agreement JSA071004). Results Systematic paleontology Dinosauria Owen, 1842 [41]. Theropoda Marsh, 1881 [42]. Coelurosauria von Huene, 1914 [43]. Paraves Sereno, 1997 [44]. Troodontidae Gilmore, 1924 [45]. Talos sampsoni gen. et sp. nov. Talos gen. nov. ZooBank LSID urn:lsid:zoobank.org:act:963F60E2-4BCF- 43EC-9C39-1BC81BE51AD8. Talos sampsoni sp. nov. ZooBank LSID urn:lsid:zoobank.org:act:08E4DCC9-80F3- 4870-826F-1536BFD0A0BD. Figures 1, 3, 5–12. Etymology. Talos, (Greek) referring to the mythological, fleet- footed protector of Crete, often depicted as winged, who succumbed to a wound on the ankle. The name is also a play on the English word ‘‘talon’’ meaning a sharply hooked claw. The specific epithet honors Scott D. Sampson, architect of the Kaiparowits Basin Project. New Troodontid Theropod from North America Figure 1. Skeletal reconstruction of Talos sampsoni (UMNH VP 19479). Talos is known from the upper Campanian Kaiparowits Formation, southern Utah, USA. Skeletal drawing illustrates the distribution of elements preserved with the holotype (in red). Missing elements based on Troodon formosus and/or Saurornithoides mongo- liensis. Drawing Scott Hartman 2010. doi:10.1371/journal.pone.0024487.g001 Holotype. UMNH VP 19479, partial postcranial skeleton, including fragmentary portions of the dorsal, sacral, and caudal axial column, left ulna, additional forelimb fragments, a partial pelvis, and partial left and right hind limbs (Figure 1). Type Horizon, Locality, and Age. UMNH VP 19479 was collected in Grand Staircase-Escalante National Monument from a region known as ‘‘The Blues,’’ within the northern portion of the Kaiparowits Plateau. The specimen was entombed in a light greenish gray siltstone/fine sandstone unit exposed in the middle portion of the ‘‘middle unit’’ [46] of the Kaiparowits Formation, approximately 260 meters above the base (Figure 2). Small freshwater gastropod and bivalve shells (not collected) were observed associated with the skeleton and support the interpretation of the type horizon as overbank or possibly crevasse splay. A couple of small fragments of compactituberculate ornamented (sensu [47]) dinosaur eggshell were also observed at the locality. Small, 1–2 cm diameter calcium carbonate pedogenic nodules present at the bone level, as well as the blocky weathering nature of the host matrix, indicate that the horizon was a hypomature paleosol by the time it was buried deep enough to be isolated from soil forming processes. A bentonite located approximately 185 meters above the base of the Kaiparowits in this area (75 meters below the specimen; Figure 2) yielded a single crystal 40Ar/39Ar date of 75.95 +/2 0.18 Ma [48], indicating a late Campanian age for UMNH VP 19479. Diagnosis. A troodontid dinosaur exhibiting the following combination of characters (autapomorphies marked with asterisk): acetabular margin of ischium strongly concave dorsally; notch between lateral condyle and ascending process of astragalus in lateral view, poorly developed (Figure 3A); intercondylar bridge of astragalus hyperconstricted* (Figure 3B); proximal groove separating astragalar body and ascending process craniocaudally wide and v-shaped* (Figure 3B); cranioproximal groove on astragalar condyles absent; astragalar condyles subequal in cranial extent (Figure 3B); shaft of metatarsal II markedly compressed (midshaft length-to-transverse width ratio 38.8; Figure 4B); metatarsal III distally ginglymoid; pronounced, rounded tab on dorsolateral aspect of metatarsal III proximal to distal condyle (Figure 3C); distal corner of lateral collateral ligament pit on metatarsal IV with small protuberance, creating rounded depression between extensor aspects of distal condyles* (Figure 3D). Remarks. We estimate Talos at approximately two meters in total length, with a body mass of approximately 38 kg based on distal tibia width [49]. Select measurements of UMNH VP 19479 are provided in Table S1. PLoS ONE | www.plosone.org 3 September 2011 | Volume 6 | Issue 9 | e24487