New Troodontid Theropod from North America Figure 3. Skeletal elements of Talos and Troodon illustrating select diagnostic characters of Talos sampsoni (UMNH VP 19479). (A) left astragalus of Talos (on left) and right astragalus of Troodon (on right, reversed, MOR 553S-8.20.92.311) in lateral view, (B) and in proximal view, (C) distal aspect of left metatarsal III of Talos (on left) and Troodon (right two elements, MOR 553S-8.69.406, 7.29.92.113) in extensor view, and (D) distal aspect of left metatarsal IV of Talos (left two elements) and Troodon (on right, reversed, MOR 553S-11.1.01.8) in oblique distal and extensor, and extensor views respectively. Abbreviations: dd, dorsolateral depression on metatarsal IV; dp, dorsolateral tuberosity on metatarsal IV; dg, dorsolateral groove on metatarsal IV; ib, intercondylar bridge on astragalus; ln, lateral notch on astragalus; lt, lateral tab on metatarsal III; pg, proximal groove on astragalus; sc, subequal lateral and medial condyles of astragalus. doi:10.1371/journal.pone.0024487.g003 shaft is generally smooth; however, just distal to the proximal articular facet a flange marks a well-defined origination point for a slight muscle scar that trends along the extensor surface of the shaft for nearly its entire length, as in Troodon formosus (NMC 12340) [26]. A lateral interosseous ligament scar is not visible but can be felt on the shaft. The distal end is transversely expanded, angled at approximately 45 degrees from the transverse plane of the proximal articular facet, and exhibits a weakly convex articular surface (Figure 6B). One ,25 mm fragment of limb bone shaft may represent the only known portion of the radius. Pelvis. The ilia are severely fragmented and crushed, and only a portion of one acetabulum is identifiable. No additional fragments of ilia preserve natural margins. The right and left pubes are represented solely by portions of the shafts (Figure 7F–G), the longest preserving approximately 122 mm of the total length. Proximally the shafts are transversely compressed; distally they transition to a sub-triangular cross-section. Proximal to the pubic apron, each pubic shaft bows laterally to meet the ipsilateral ilium. However, in lateral profile, the pubic shafts appear relatively straight as in other troodontids [5,32] (Figure 7F–G). Due to proximal damage it is unclear if Talos exhibited a near-propubic to propubic pelvis as in most troodontids (e.g., Troodon (MOR 246) Saurornithoides, Sinusonasus [52], and Sinornithoides), or a retroverted pubis as in the basal taxon Sinovenator [5]. The distal aspect of the pubes are also not preserved, thus it cannot be determined if Talos possessed a pubic boot as in Troodon (MOR 553S) or lacks this feature as in Sinovenator [5] and Sinornithoides [53]. Although the absence of this feature in some troodontid specimens has also been attributed to skeletal immaturity [12]. The pubic apron is not preserved; however, broken surfaces reveal that the apron originated from the craniomedial aspect of the shaft as in Sinovenator (IVPP V12615) and Troodon (MOR 553S- 8.3.9.387) (Figure 7F–G). It can also be determined that the pubic apron would have been restricted to less than half the length of the pubes as in Troodon (MOR 553S-8.3.9.387) and unlike the extensive pubic shaft exhibited by Sinovenator. The ischia are represented by nearly complete right and partial left counterparts (Figure 7A–E). The acetabular margin of the proximal ischium is strongly concave rather than relatively flat as observed in Saurornithoides [32] (Figure 7B–C). Whereas its medial surface is flat, the lateral surface of the ischium is concave ventral to the caudodorsal margin. This configuration is observed in Saurornithoides [32], Sinusonasus [52], and Troodon (MOR 553L- 7.27.8.87), yet not in more basal troodontids [53]. The presence of a caudodorsal ridge on the ischial shaft is shared with Saurornithoides and Troodon (MOR 553L-7.27.8.87) (Figure 7C) and may be homologous with the ridge dividing the lateral ischial shaft into cranial and caudal aspects in other paravians (IVPP V12615) [54]. The caudodorsal ridge appears to originate in a slightly more proximal position on Saurornithoides and Talos than it does on Troodon (MOR 553L-7.27.8.87); however because of small sample size this difference may also be attributable to ontogenetic and/or individual variation. In contrast to Sinovenator, Mei, some dromaeosaurids, and the slight protuberance on Troodon (MOR 553L-7.27.8.87), there is no well- defined proximodorsal tuberosity (sensu [55]) on Talos. However, this region is faintly rugose (Figure 7B) and could therefore represent incipient development of the proximodorsal tuberosity related to either ontogenetic stage or phylogenetic position. Although damaged, PLoS ONE | www.plosone.org 5 September 2011 | Volume 6 | Issue 9 | e24487