New Troodontid Theropod from North America Figure 4. Linear regression of length-to-width ratio in the second and fourth metatarsals of Talos sampsoni and Troodon formosus. Regressions quantify the taxonomic significance of metatarsal slenderness in the holotype of Talos (UMNH VP 19479) based on transverse width of the second metatarsal (p,0.05) and craniocaudal width of the fourth metatarsal (p,0.05), despite its juvenile ontogenetic stage. Troodon specimens represented: metatarsal II (MOR 553S-7.8.91.28; MOR 553S-7.18.92.5; and MOR 748); metatarsal IV (MOR 553S-11.1.01.8; MOR 553S7.28.8.102; MOR 553S-7.1.9.9, MOR 553S-92.260, MOR 748). doi:10.1371/journal.pone.0024487.g004 the overall morphology of the ischial shaft compares closely with Troodon (MOR 553L-7.27.8.87) and Saurornithoides [32], suggesting that the obturator process would have been located near midshaft. A small isolated section of the ischium appears to represent the shaft terminus. However, damage to the periphery of this fragment makes its exact morphology and position indeterminate. Hind limb. Femur. Only a portion of the proximal aspect of the left femur is preserved and the proximal-most aspect of this fragment is eroded (Figure 8A). A robust, crest-like posterior trochanter extends from the caudolateral aspect of the proximal shaft (Figure 8A). The trochanter is notably more robust than on specimens of Troodon, even among larger individuals (MOR 553S-7.28.91.239, 7.16.0.61, MOR 748). Talos appears to have lacked a pronounced fourth trochanter as in Saurornithoides (AMNH FR 6516), Troodon (e.g., MOR 553S- 7.16.0.61; 7.28.91.239), and Sinornithoides [12]. However, due to incomplete preservation of the proximal femur, this cannot be determined with certainty. A small pointed tubercle (homologous with the trochanteric crest) [56] extends from the lateral aspect of the proximal femur in PLoS ONE | www.plosone.org 6 September 2011 | Volume 6 | Issue 9 | e24487