New Troodontid Theropod from North America Figure 11. Metatarsus of Talos sampsoni (UMNH VP 19479). Right metatarsal I in lateral (G) and medial (H) views; left metatarsal II in extensor (A), medial (B), flexor (C), lateral (D), proximal (E), and distal (F) views; left metatarsal III in extensor (J), medial (K), and distal (I) views; left metatarsal IV in flexor (N), lateral (O), medial (P), proximal (L), and distal (M) views. Abbreviations: lc, lateral condyle, mc, medial condyle; m3t, tab on extensor surface of metatarsal three; m4t, tab on flexor surface of metatarsal four; MT1, metatarsal one; MT2, metatarsal two; MT3 metatarsal three; MT4, metatarsal four. Scale bar equals 10 mm. doi:10.1371/journal.pone.0024487.g011 Sinornithoides, Sinovenator, Mei) this flange is absent. In Talos, the feature is notably less pronounced, expressed as a convex tab of bone, and does not extend up the ventral shaft to abut metatarsal II. Although the feature is considered taxonomically significant for Talos, its degree of development may also be ontogenetically variable as a single smaller Troodon specimen (MOR 553S- 8.17.92.260) exhibits a similarly reduced condition. In dorsal view, a small protuberance marks the dorsodistal corner of the lateral collateral ligament pit on the fourth metatarsal, creating a notch along the lateral aspect of the lateral distal condyle (Figure 3D). This feature is shared between Talos and Troodon. However, in the latter taxon the notch is more pronounced and extends to the distal margin in dorsal view (MOR 553L-7.28.8.102; MOR 553S-11.1.01.8), whereas in Talos, a bony extension closes off this notch, creating a rounded pit (Figure 3D). Metatarsal IV of both taxa exhibits a strong lateral bow (Figure 11N). Pedal Phalanges. All phalanges of the left foot are preserved except the proximal phalanx of the first pedal digit (although IV-1 and IV-2 are presently inseparable), whereas phalanges II-1, II-3, III-4, and IV-3 are missing from the right foot. Phalanx I-1 is relatively straight shafted with a concave proximal articular facet bearing an asymmetrically oriented, laterally extensive heel, asymmetrical distal condyles, and a deep lateral collateral ligament pit (Figure 12A). Phalanx I-2 trends medially and possesses asymmetrical lateral and medial grooves, the lateral skewed further dorsally (Figure 12A–B). As is characteristic of deinonychosaurians, phalanges of the second digit are modified to support an enlarged, trenchant ungual (Figure 12C–E). Phalanx II-1 of Talos is the most distinctive, bearing a robust and straightened shaft (Figure 12C–D). The proximal articular facet is oval in shape, lacking a medial ridge and exhibiting a rudimentary ventral heel. Just distal to the proximal articular facet the dorsal aspect of the shaft is pinched into a raised ridge (Figures 12E). In marked contrast to the highly symmetrical and circular distal condyles of other troodontids, the craniodorsal aspect of the lateral condyle of phalanx II-1 is flat, creating a distally pointed profile in lateral view (Figure 12D). The medial PLoS ONE | www.plosone.org 11 September 2011 | Volume 6 | Issue 9 | e24487