and often bizarre species-recognition signals [86] that aid in the identification of unique taxa from fragmentary skeletal remains. In contrast, coelurosaurian theropod specimens are compara- tively conservative (i.e., they generally lack obvious species signaling structures such as horns and crests), a factor that may reflect actual lower taxonomic diversity or may act as a bias against the recognition of unique species. Conservative skeletal morphology coupled with the exceedingly fragmentary fossil record of small WIB theropods offers a potential explanation for the contemporary view that T. formosus exploited an anomalously large geographic and temporal range when compared to other, more thoroughly documented dinosaur clades of the WIB. The hypothesis that small theropod clades of the WIB exhibited higher taxonomic diversity than currently recognized is supported by multiple lines of evidence, including the constrained paleobio- geographic ranges observed in other WIB terrestrial vertebrate clades during the late Campanian [84] and the correlation between small body-size and restricted geographic range docu- mented in terrestrial vertebrates more generally, e.g., [87]. Although many extant carnivores, for example the mountain lion, Felis concolor, and coyote, Canis latrans, exhibit historical ranges across nearly the entirety of the Western Hemisphere and North America, respectively [88–89], carnivorous theropod species of the late Campanian WIB do not appear to follow the same pattern [79,90–91], even among taxa of large body size (e.g., tyrannosaur- ids). Moreover, those theropod specimens from the Upper Cretaceous Kaiparowits Formation that have proven diagnostic all represent new species (i.e., Hagryphus giganteus, Teratophoneus curriei, and Talos sampsoni) and clearly demonstrate greater diversity among small theropods of the WIB than previously appreciated. In short, we hypothesize that the highly fragmentary fossil record of small theropods across the WIB together with a lack of pronounced osteological species recognition signals has led to over-synonymization of theropod taxa and that the taxonomic diversity of Late Cretaceous coelurosaurians including troodontids in North America is presently underestimated. It is unfortunate that the holotype of Talos (UMNH VP 19479)—the first troodontid skeleton from the late Campanian WIB to be recognized as distinct from skeletal materials currently referred to Troodon—does not preserve craniodental materials that can be compared to the type specimen of T. formosus. Ultimately, a comprehensive reevaluation of troodontid materials recovered from the Late Cretaceous WIB may shed additional light on this topic; however, such an endeavor is beyond the scope of the present study. Hind Limb Pathology in Talos sampsoni Trackway evidence indicates that the second digit of deino- nychosaurians did not function as a primary weight-bearing digit during locomotion [59]. Rather, the enlarged ungual of the second digit likely functioned in prey capture as a grappling, puncturing, and/or climbing instrument [92–95], an intraspecific weapon analogous to the spurs of galliforms [96], or a defensive weapon as in the enlarged second pedal ungual of cassowaries [97]. It is particularly noteworthy then, that the second pedal digit on the left hind limb of Talos exhibits extreme pathological modification of the first phalanx (II-1). Not only is there evidence of a surface lesion on the distolateral margin of the element (Figure 13A), microCT reveals that the internal structure of the phalanx is extensively reorganized. Constraining the etiology of this type of pathology is challenging, as the generalized nature of the surface and internal bony modifications could be related to a myriad of causes, including physical trauma (e.g., fracture/puncture injury), a variety of local or systemic infectious processes, or perhaps the New Troodontid Theropod from North America former followed by some variant of the latter. Moreover, any scenario for how such morphology was formed from the information at hand would be speculative at best. However, it is clear from the degree of remodeling that, at minimum, weeks to months would have transpired during which time the internal structure of the injured phalanx was reorganized and the bony collar was formed around its peripheral margin. MicroCT data allow the reconstruction of the original and seemingly pathology-free external contour of the cortical bone. From this it is apparent that the majority of the callus is lateralized to the left side of the element, coincident with the same side as the external opening. Such a focal lesion could reflect: (1) the site at which a systemically-mediated infectious process (e.g., a non- specific hematogenous osteomyelitis) exited from the interior of the bone (i.e., it represents the cloaca associated with a suppurative osteomyelitis); (2) the site of localized bony trauma (e.g., fracture or puncture wound); or (3) the site of localized non-bony trauma (e.g., a disease process affecting the integument) that progressed to involve the phalanx. In all three cases, the bony collar or callus formation then represents secondary (i.e., post-traumatic/post- infectious) bone proliferation. Notably, such callus formation is typically associated with the reparative phase of bone healing following a fracture [98–100]. It also may be the case that callus formation in this specimen results not from a major (i.e., macroscopically observable) fracture, but rather, from (a) stress fracture(s) not observable in the microCT dataset. Regardless of the primary cause for the callus formation, morphology of the cloaca is consistent with suppurative osteomyelitis. As the skeletal pathology is restricted to this single element, with perhaps concomitant minor changes in next distal element (i.e., left pedal phalanx II-2), the conservative interpretation is that this represents a localized phenomenon resulting from acute injury followed by infection and remodeling, rather than from any systemic etiology. As such, this is not entirely unexpected in a deinonychosaurian given the hypothesized use of the second pedal digit in prey acquisition/manipulation and/or conspecific aggres- sion. Whether this morphology pertains to post-traumatic bone proliferation following a bite/scratch injury, fracture callus formation, or perhaps a combination of the two, remains a topic for future research. Injury to the second pedal digit in T. sampsoni supports the interpretation that this digit was not only subjected to instances of potentially high loading during use, but that it was, not surprisingly, susceptible to trauma resulting directly from defensive strategies (e.g., biting, slashing) employed by prey or conspecifics. Interestingly, the proximal and distal interphalangeal joints of this digit appear to have retained at least some mobility based on the maintenance of congruent articular surfaces. It is unclear how such bone pathology may have affected performance of this digit or the entire limb with regard to both locomotor and inferred functional abilities; however, the lack of extensive remodeling across the remainder of the hind limb is consistent with the hypothesis that the hypertrophied digit II of many paravian species was not extensively employed in locomotion. Supporting Information Figure S1 Proximal histologic cross-sections of the femur of Talos sampsoni (UMNH VP 19479). Medullary space is towards the bottom. Bands representing slowed growth including lamellar bone and lines of arrested growth are marked with an arrow. The proximal aspect of the section is obscured by diagenetic alteration. (TIF) PLoS ONE | www.plosone.org 17 September 2011 | Volume 6 | Issue 9 | e24487