New Troodontid Theropod from North America Figure 7. Pelvic girdle elements of Talos sampsoni (UMNH VP 19479). Right ischium in proximal (A), medial (B), and lateral (C) views; left ischium in medial (D) and lateral (E) views; right (F) and left (G) pubic shafts in cranial view. Abbreviations: ac, acetabulum; cdr, caudolateral ridge; ip, iliac peduncle; op, obturator process; pa, pubic apron; pdt, proximodorsal tuberosity; pp, pubic peduncle. Scale bar equals 10 cm. doi:10.1371/journal.pone.0024487.g007 pronounced fibular fossa as in Troodon (e.g., MOR 553S- 8.17.92.265) and other paravians. The proximal shaft bows medially to contact the tibia. The shaft is oval in cross-section and narrow in diameter. Tarsus. Tarsal elements of Talos are represented by a nearly complete left calcaneum and a nearly complete left astragalus missing the proximal ascending process, the medial condyle of the right astragalus, and a fragment of the right calcaneum. The astragalus and calcaneum are entirely unfused on both sides. Medial and lateral astragalar condyles are well developed and connected by a highly constricted intercondylar bridge, forming an unusually deep v-shaped outline in cranial (Figure 9B) and proximal (Figure 9D) views. Constriction of the intercondylar bridge is more extreme than that observed in Troodon (e.g., CMN 1560, CMN 199, MOR 553S-11.1.01.1), Zanabazar junior (IGM 100/1), Sinornithoides youngi (IVPP V9612), Sinovenator (IVPP V12615, V12583), or Mei. The astragalar condyles are subequal in their cranial extent (Figures 3B and 9D) unlike Sinovenator, Sinornithoides, and Troodon (e.g., MOR 553S-8.20.92.311, 7.7.91.19), in which the medial condyle extends further cranially than its lateral counterpart. As in Troodon (e.g., MOR 553S-8.20.92.311, 7.7.91.19) and other troodontids [32,57] the medial condyle is transversely wider and less proximally extensive in cranial view (Figures 3B and 9B). This low profile on Talos corresponds to a relatively horizontal proximal margin, similar to Sinovenator (IVPP V12583). In contrast, the proximal margin of the medial condyle of Sinornithoides and Troodon (CMN 1560, CMN 199, MOR 553S-11.1.01.02) is strongly convex and bulbous. As in Sinornithoides and Sinovenator (IVPP V12583), Talos lacks a groove across the craniodorsal face of the astragalus. This differs from Troodon (CMN 1560, CMN 199; MOR 553S-8.20.92.311, 7.7.91.19, 11.1.01.02, 11.1.01.1) and Mei, which exhibit a cranial or craniodorsal groove. In medial view, Troodon possesses a medial condyle with a rounded profile that approaches the base of the ascending process or extends proximal to it. In contrast Talos has an asymmetrical medial condyle that slopes distally from the base of the ascending process (Figure 9A). The groove demarcating the contact between the astragalar body and ascending process forms an unusually wide trough in Talos (Figure 9D) and Troodon (e.g., MOR 553S-8.20.92.311, 7.7.91.19). However, the trough is straight on Troodon, whereas it is v-shaped on Talos due to the hyperconstricted intercondy- lar bridge. This groove is not well defined on the juvenile troodontid specimen IVPP V10597 [57]. In addition, Troodon specimens have a well-developed notch between the lateral condyle and the ascending process that is nearly absent on Talos (Figure 9C). A deeply depressed sulcus excises the cranial surface of the ascending process on Zanabazar [32] and is variably expressed on Troodon (NMC 199; MOR 553S-8.20.92.311, 7.7.91.19), yet is absent on the juvenile troodontid specimen IVPP V20597 [57]. Although the mid-portion of the ascending process is damaged on Talos, preserved morphology suggests the presence of a similar depression (Figure 9B). However, due to damage, it cannot be determined if the ascending process of Talos exhibited the rounded tuberosity that characterizes the cranial surface of the ascending process in Zanabazar [32] and Troodon (e.g., NMC 199; MOR 553S-8.20.92.311, 7.7.91.19). The corresponding aspect of the ascending process of the astragalus just proximal to the medial condyle is markedly thickened on Talos (Figure 9A). In contrast, this area is craniocaudally restricted on Troodon (MOR 553S- 8.20.92.311, 7.7.91.19). The calcaneum is a transversely compressed, medially concave, disk-shaped element that tapers to a point at its caudal aspect (Figure 9E). The peripheral margin is transversely thickened at its cranial extent to meet a corresponding notch in the lateral condyle of the astragalus. As in IVPP V10597 [57], there appears to have been little contact with the fibula. Currie and Peng [57] suggested that the calcaneum in troodontids is lost rather than fused as was reported by Russell [26]. The extremely small and slender nature of the unfused calcaneum of Talos and Sinovenator (IVPP V12583), the presence of a partially fused calcaneum on one large Troodon astragalus (MOR 553S-11.1.01.1), and an unfused theropod calcaneum (CMN 1791) referred to Troodon that compares favorably with the PLoS ONE | www.plosone.org 8 September 2011 | Volume 6 | Issue 9 | e24487