apatite crystals as in lamellar bone. However, the observed alterations to the bone may account for the absence of parallel extinction. The consistent deviation around potentially developing osteons as well as the parallel orientation of associated osteocyte lacunae suggests that these bands represent biologically significant signals. Pathology Phalanx II-1 of the left foot exhibits extensive pathology consisting of both external and internal bony modifications (Figure 13). External pathologies include the presence of a surface opening (cloaca) at the distolateral margin of the shaft (Figure 13A), a circumferential bony collar developed along the length of the shaft (Figure 13B), and modification of the distal condyles. Although both condyles exhibit atypical (i.e., angular) peripheral margins, the dorsodistal margin of the lateral condyle appears distorted to the greatest extent. The bony collar is formed around the entire circumference of the phalanx, yet is preferentially developed along the lateral and flexor margins of the shaft, contributing to its distinctly straight appearance (Figures 12A and 12H– I). MicroCT analysis reveals both the extent of the original cortical surface (Figure 13C and 13G) and the nature of internal remodeling within the element (Figures 13B, 13E, and 13H). It is apparent from the microCT results that the bony collar is comprised of disorganized bone around most of the peripheral portions of the outermost cortex and is morphologically consistent with bony callus formation (Figure 13B). Drainage and/or nutrient canals are visible passing from the medullary cavity through the cortical and exostotic bone to open on the external surface. Phalanx II-2 also exhibits subtle modifications of the articular surfaces (described above), but does not appear to possess any cortical or sub-cortical modifications. Given that such trauma may have impacted function of the entire left hind limb, with the potential to induce morphological modifications in other elements in the series, we scanned additional phalanges, metatarsals, and tarsal elements of UMNH VP 19479. No evidence of abnormal cortical remodeling was identified outside the second pedal digit. Phylogenetic Analysis Phylogenetic analysis recovers six MPTs varying in relationship between the basal troodontids Mei long and Sinovenator changi, and in the position of Talos. Talos is recovered in three equally parsimonious positions: as sister-taxon to the derived subclade containing Byronosaurus, Saurornithoides, Zanabazar, and Troodon; sister-taxon to Saurornithoides, Zanabazar, and Troodon only; and sister-taxon to Saurornithoides mongoliensis (Figure 15). Nonetheless, in all topologies Talos is substantiated as a member of a derived, latest Cretaceous subclade of Troodontidae containing at least the taxa Troodon, Saurornithoides, and Zanabazar. This position is supported by the following traits: obturator process located at ischial midshaft, caudolateral ridge on ischial shaft, hypertrophied groove delineating the base of the ascending process of the astragalus, a kinked fourth metatarsal, and a notch in the dorsolateral aspect of the distal condyle the fourth metatarsal. However, none of these characters can be coded for Byronosaurus, therefore they may ultimately be found to character- ize a more inclusive clade. Only a single feature supports a monophyletic relationship among these taxa to the exclusion of Byronosaurus: the potential for Talos to possess a medially recurved dentary symphysis (which is absent in Byronosaurus). However, missing data are hardly evidence for a particular topological position. In addition, the hypothesized New Troodontid Theropod from North America Figure 15. Evolutionary relationships of Talos sampsoni (in red). Tree illustrates combined topology of six MPTs (Tree Length = 48). Dashed lines indicate equally parsimonious positions for T. sampsoni (three alternates) and Sinovenator changi (two alternates). Node numbers represent support values (black = Bremer; blue = standard bootstrapping, GC values; green = symmetrical resampling, GC values). doi:10.1371/journal.pone.0024487.g015 sister-taxon relationship between Saurornithoides and Talos recov- ered in two of the six MPTs is also supported by only a single feature—the presence of a caudodorsal ridge on the ischium that originates at the proximodorsal tuberosity (char. 40.1). This feature is unknown in several closely related forms (e.g., Byronosaurus, Zanabazar) and may prove to have a wider distribution within the clade. Moreover, the states of this trait may reflect individual or ontogenetic variation and may prove indistinguish- able with the collection of additional specimens. Therefore there is little current phylogenetic support for any particular MPT in this study. Not surprisingly, standard bootstrapping, symmetrical resam- pling, and Bremer values are low across the tree, further underscoring the general lack of support for the hypothesized relationships among derived troodontids (Figure 15). Lability of Talos primarily results from a lack of overlapping elements with other troodontids (specifically Byronosaurus and Zanabazar) and character conflict between Talos and Troodon. A comprehensive reevaluation of North American troodontid materials is required to resolve relationships among derived, large-bodied, Late Cretaceous troodontids and will likely add significant information toward the resolution of these issues, yet is clearly beyond the scope of this manuscript. Discussion Age assessment of Talos sampsoni We used samples derived from the femur and fibula to conduct an age assessment of UMNH VP 19479 (Figure 14). All three samples display consistent histological features; each is comprised primarily of fibro-lamellar bone separated by at least three lines of arrested growth (LAGs). Interlag tissue exhibits several interwoven lines, as is common for non-avian dinosaurs [61], and few if any osteons. Where preserved, LAGs also display parallel extinction patterns consistent with lamellar bone. Midshaft sections of the femur and fibula exhibit similar spacing between bands. A single row of incompletely developed osteons PLoS ONE | www.plosone.org 15 September 2011 | Volume 6 | Issue 9 | e24487