New Troodontid Theropod from North America Figure 5. Axial elements of Talos sampsoni (UMNH VP 19479). Dorsal vertebra in left lateral (A), caudal (B), and dorsal (E) views; sacral centrum in ventral view (C), and dorsal centrum in ventral view (D). Abbreviations: ipdf, infrapostzygapophyseal fossa; nc, neural canal; ns, neural spine; poz, postzygapophysis; pp, parapophysis; tp, transverse process. Scale bar equals 5 cm. doi:10.1371/journal.pone.0024487.g005 Talos (Figure 8A), Troodon (MOR 553S-7.28.91.239), Saurornithoides [32], and Sinornithoides [12]. As in Saurornithoides [32] and Sinornithoides [12], a ridge originates just distal to this tubercle and extends distally along the lateral aspect of the shaft (lateral ridge, sensu [51]) (Figure 8A). A distinct lateral ridge is present and well developed on some, yet not all individuals of Troodon. The exact morphology of the greater and lesser trochanters and femoral head cannot be assessed due to damage. However, the cross-section approximates the sub-triangular morphology typical of troodontids, exhibiting a concave cranial margin between the femoral head and greater trochanter in proximal view. Tibia. A partial left tibia is preserved (Figure 8B). The proximal aspect of the tibia was not recovered and the remainder of the element was shattered upon surface exposure. A depression on the cranial face of the distal tibia indicates that an extensive ascending process of the astragalus was present and covered the entire transverse surface of the tibia as in Zanabazar [32]. The ascending process of the astragalus was tall (approximately 63 mm in dorsoventral extent), trended medially (as in Troodon, MOR 553- 7.7.91.19), and tapered proximally toward the craniomedial aspect of the tibial shaft (Figure 8B). The medial aspect of the distal tibia is craniocaudally expanded relative to the lateral side, as in theropods generally. There is little evidence of contact with the fibula as in Sinornithoides [53], yet unlike the condition in Zanabazar on which a large, striated fibular fossa is present [32]. The lateral face of the distal tibial shaft is flattened. Fibula. Only a portion of the proximal fibula and a small section of the fibular midshaft are preserved (Figure 8C). The proximalmost aspect of the fibula is damaged and its morphology cannot be assessed. The internal surface is flat and lacks a Figure 6. Left ulna of Talos sampsoni (UMNH VP 19479). Ulna in medial (A) and dorsal (B) views. Abbreviations: er, extensor ridge; lc, lateral aspect of the distal condyle condyle; mc, medial aspect of the distal condyle; paf, proximal articular facet. Scale bar equals 5 cm. doi:10.1371/journal.pone.0024487.g006 PLoS ONE | www.plosone.org 7 September 2011 | Volume 6 | Issue 9 | e24487