Figure 10. Articulated pes of Talos sampsoni (UMNH VP 19479). Digit one reversed from right pes. Abbreviations: DI, digit one; DII, digit two; DIII, digit three; DIV, digit four; MT1, metatarsal one; MT2, metatarsal two; MT3 metatarsal three; MT4, metatarsal four. Scale bar equals 10 mm. doi:10.1371/journal.pone.0024487.g010 morphology exhibited by Talos confirm the presence of a calcaneum in several troodontid species. Interestingly, the preserved fragment of the right astragalus is slightly larger than the corresponding aspect of the left astragalus. We therefore scanned the left tarsal elements with MicroCT to rule out potential pathological modifications resulting from the injury to the second pedal digit. Although it is conceivable that this difference is associated with trauma to the left pes, no evidence of internal post-traumatic reorganization or cortical proliferation was observed in any tarsal element. New Troodontid Theropod from North America Metatarsus. The metatarsus of Talos is asymmetrical—a synapomorphy of Troodontidae [26,32,58]—being comprised of a robust fourth metatarsal, proximally pinched third metatarsal (arctometatarsalian), and markedly compressed, proximodistally abbreviated second metatarsal (Figure 10). The left metatarsus is well preserved, missing only metatarsal I, the proximalmost shaft of metatarsal III, and exhibiting slight damage to the proximal articular surfaces of metatarsals II and IV. The right metatarsus is less complete, preserving metatarsal I, yet missing the proximal and distal ends of metatarsal III and most of the distal aspect of metatarsal II. Relative to Troodon and other derived troodontids, the metatarsus of Talos is significantly more slender in proportion, including Troodon specimens of smaller size and younger ontogenetic stages (e.g., MOR 553E-7.1.9.9) [9]. The extreme slenderness of the metatarsus of Talos relative to Troodon is attributable to reduction of the second metatarsal (in both craniocaudal and transverse dimensions; Figures 4A–B) and the fourth metatarsal in craniocaudal dimension only (Figure 4C). The transverse width of the fourth metatarsal (Figure 4D) is in proportion with the ontogenetic series of Troodon. Metatarsal I is diminutive, with a laterally compressed, attenuated shaft and an asymmetrical, subtriangular, and bulbous distal articular condyle that projects medially, approximately 30˚ from the main axis of the metatarsus (Figure 11G–H). Metatarsal II is markedly reduced in length and breadth, consistent with a reduced capacity for weight bearing as is typical for deinonychosaurians [59]. Its shaft is transversely compressed to such a degree as to be nearly splint-like in relative proportions, exhibiting a midshaft length-to-width ratio approximately one half that of Troodon (e.g., MOR 553S-7.8.91.28, 7.18.92.5). Metatarsal II terminates in a transversely compressed, gently rounded articular surface comprised of a lateral condyle approximately twice the size of the medial condyle (Figures 11A and 11F). In contrast, metatarsal III exhibits proportionally large, subtly ginglymoid articular condyles with a poorly developed extensor fossa (Figures 11I and 11J). As in other deinonychosaurians (e.g., Velociraptor) [51], yet in contrast to Saurornithoides [32], the distal articular surface of metatarsal III is asymmetrical, exhibiting a larger medial condyle (Figure 11I). Proximal to the distal condyles, the shaft of metatarsal III broadens transversely before attenuating to less than 5 mm in size, with a subtriangular cross section throughout (Figure 11J). The dorsolateral aspect of the shaft of metatarsal III just proximal to the distal condyles is sinuous and marked by a pronounced bony ‘‘tab’’ that extends laterally to fill an indentation on the dorsomedial margin of metatarsal IV (Figures 3C and 10). This feature is poorly developed (MOR 748; MOR 553S-7.29.92.113, 8.6.9.406; NMC 12340) on specimens referred to Troodon (except TMP 92.36.575), which instead display a gently concave dorsolateral margin. A bony tab on metatarsal III is apparently also absent on Saurornithoides mongoliensis (AMNH FR 6516) and Sinovenator (IVPP V12615). This feature may be present on the primitive troodontid IGM 100/44, although it does not appear to characterize the better-preserved metatarsal elements of this taxon (fig.2 in [60]). In similar fashion, a bony flange extends from the ventromedial aspect of the shaft of metatarsal IV just proximal to the distal condyles and underlies the shaft of metatarsal III, bracing it ventrally (Figure 11N). On Troodon, this feature is robust and proximally continuous, extending up the shaft to nearly exclude metatarsal III in ventral view and causing the shaft of metatarsal IV to appear strongly kinked laterally at its distalmost extent (MOR 748; TMP 92.36.575). In more primitive troodontids (e.g., PLoS ONE | www.plosone.org 10 September 2011 | Volume 6 | Issue 9 | e24487