Figure 2. Stylized stratigraphic section of the middle and lower units of the Kaiparowits Formation [46]. Section indicates relative chronostratigraphic positions of Talos sampsoni UMNH VP 19479, troodontid indet. UMNH VP 16303, and Hagryphus giganteus (UMNH VP 12765). Radiometric dates derived from 40Ar/39Ar dating of bentonites [46], undated bentonite horizons also shown. doi:10.1371/journal.pone.0024487.g002 At face value, the pes of Talos appears notably more gracile than Troodon. To quantify this observation we investigated proportional differences between metatarsal elements of Talos and a growth series of Troodon by calculating linear regressions and p-values (F test) for length-to-width ratios of metatarsals II and IV (Figure 4A–D). Although sample sizes were limited (3–5 representative elements per regression), Troodon specimens representing smaller and larger elements were available in all cases. The length-to-transverse width (p,0.05) and length-to-craniocaudal width (0.05,p,0.1) ratios of metatarsal II and length-to-craniocaudal width of metatarsal IV (p,0.05) of Talos are markedly lower than estimated for comparable metatarsals of Troodon. However, the length-to-transverse width ratio of metatarsal IV falls within the variation observed for Troodon. This finding supports the interpretation that discrepancy in the slenderness of metatarsal II of Talos as compared to Troodon is a distinguishable feature and is not simply attributable to ontogenetic variation. These data also confirm that the metatarsus of Talos is significantly more gracile than that of Troodon, even among smaller and more immature individuals. Description and Comparisons Axial Column. The axial column is represented by a single, nearly complete dorsal vertebra, two fragmentary dorsal centra (deriving from the middle and caudal aspects of the series), an unfused sacral centrum, a caudal vertebral fragment, multiple proximal chevron fragments, and additional poorly preserved surface fragments of dorsal and sacral(?) vertebrae. New Troodontid Theropod from North America The oval centrum of the nearly complete dorsal vertebra together with a lack of a hypapophysis suggests a middle position in the dorsal series. This centrum is amphicoelous and taller than wide, bearing a degree of ventral constriction consistent with its interpretation as a mid-dorsal. Postmortem distortion and root rot has obscured much of the cranial and right lateral aspects of this vertebra, including the shape of the transverse processes, both prezygapophyses, and associated laminae. An isolated dorsal centrum, identified as belonging to the caudal part of the series, is platycoelous and spool-shaped, lacking any ventral constriction. As in Troodon (e.g., MOR 553S-8.20.92.305) and Saurornithoides [2], yet unlike other troodontids [2], the neural arch of the mid- dorsal appears pneumatic (Figure 5B). However, the degree and position of pneumatic features is difficult to quantify due to incomplete preservation. Whereas pneumatic foramina appear to be present within infrapostzygapophyseal fossae, it is unclear if the infraprezygapophyseal fossae are also perforated. The ventral surfaces of the transverse processes and all preserved dorsal centra are apneumatic as in Troodon, Saurornithoides, Byronosaurus, Sinove- nator, and Mei [32,50]. The single preserved mid-dorsal neural spine is long as in Troodon (MOR 553S-8.20.92.305) (Figure 5A–B) and unlike the relatively short, fan-shaped spines of basal troodontids [5,13]. The neural spine tapers to a dorsocaudal point in lateral profile (Figure 5A), a morphology not exhibited by other troodontids. It is possible that this is a preservational artifact related to weathering of the distalmost spine, although it may also prove diagnostic with the collection of additional materials. The middle dorsal neural spines of Troodon (MOR 553S-8.20.92.305, 8.28.92.270) are subrectangular, yet do exhibit a slight caudodorsal slope. The dorsal margin of the neural spine of Talos is transversely expanded on the left side and flush on the right. This discrepancy is likely attributable to weathering and crushing of the spine and it is therefore unclear if Talos possesses the spine-table of Troodon. In contrast to Saurornithoides, the dorsal margin of the parapophyses are projected from the neural arch. A similar condition is evident on Mei. Projection of the dorsal periphery of the parapophyses may reflect incipient development of the stalk- like condition in dromaeosaurids [13,51] (Figure 5B). The neural canal is enlarged as in other paravians [34] (Figure 5B). Postzygapophyseal facets are flat, subcircular, and angled approximately 45u from the sagittal axis. The unfused sacral centrum is platycoelous, possessing one slightly concave kidney-shaped articular facet and one subcircular flat facet and either represents the first (S1) or penultimate (S5) sacral vertebra (Figure 5C). The ventral margin is broadly convex, lacking a ventral groove. The neural arch was not yet fused to the centrum and is not preserved. A largely damaged fragment is the sole representative of the caudal series. The fragment indicates that the distal caudal centrum was elongate, subrectangular, and transversely narrower ventrally than dorsally. There is some indication that the ventral aspect of the centrum may have possessed a groove. The neural arch is excised by a longitudinal sulcus in lieu of a neural spine—a synapomorphy of Troodontidae [26]. Chevron fragments display a cylindrical ventral shaft (as in Saurornithoides and Troodon (NMC 12340))[26]. Forelimb. The ulna is relatively straight shafted, bowing slightly at its proximal extent. The proximal aspect is subtriangular and, although damaged, appears to have possessed a weak olecranon process as in other troodontids (e.g., Sinornithoides, Troodon; MOR 553S-8.12.92.219) (Figure 6). Only the distalmost aspect of the proximal articular facet is preserved so it is unclear if a ridge would have delineated the facet as in other paravians. The PLoS ONE | www.plosone.org 4 September 2011 | Volume 6 | Issue 9 | e24487