occurs peripheral to the last band in each of the three samples. The proximal fibula (Figure 14A) differs in possessing additional bands of diffuse coloration and a single line, as well as radially arranged primary osteons. Such differences may reflect greater ontogenetic changes in shape occurring in this area of the fibula. All samples exhibit periods of faster growth represented by fibro-lamellar bone interrupted by slow growth periods as represented by the combination of lamellar bone and LAGs. Presuming the LAGs represent annually deposited features, UMNH VP 19479 was, minimally, in its fourth year of growth. If the circumferential bands within the proximal fibula reflect annual decelerations in growth and not simply remodeling reflecting shape change, the individual could be in its sixth year (Figure 14A). The absence of secondary osteons and an external fundamental system [62], as well as the presence of the peripheral row of incompletely developed osteons, suggest that growth was ongoing in UMNH VP 19479. However, tighter spacing of increasingly peripheral bands of lamellar bone and LAGs indicate that overall growth was slowing. The number of growth bands with LAGs in UMNH VP 19479 contrasts with similarly sized, yet contempo- raneous, Troodon formosus specimens from Montana [63]. For example, a Troodon tibia of similar length (320 mm; MOR 553S- 7.20.91.132) exhibits only two growth lines. This suggests that Talos sampsoni would likely have plateaued at a significantly smaller size than Troodon formosus. Observed fusion within this specimen is also consistent with a maturing animal, yet one that has not yet approached maximum size. Predominantly fused neurocentral sutures are present in some, yet not all dorsal vertebrae. Neurocentral sutures are unfused on at least one sacral vertebra and the sacrum has yet to form a cohesive unit. The calcanei and astragali also remain free. Sacral and tarsal fusion occurs in adult Troodon formosus individuals that express an external fundamental system [63]. Kaiparowits troodontid materials Previously, troodontid materials recovered from the Kaipar- owits Formation were referred to Troodon based on the morphology of isolated teeth collected during microvertebrate surveys [64–65]. Over the past eight years, macrovertebrate surveys conducted by the Utah Museum of Natural History (UMNH), the Raymond Alf Museum of Paleontology (ALF), and the University of California at Berkeley Museum of Paleontology (UCMP) have produced additional troodontid remains from the Kaiparowits Formation including an isolated, left frontal (UMNH VP 16303), an isolated caudal vertebra (UMNH VP uncatalogued), a fragmentary partial skeleton (UCMP 149171), and several isolated teeth (UMMH VP 11806, UMNH VP 12507). The troodontid frontal UMNH 16303 compares closely with those comprising the Troodon formosus hypodigm (e.g., AMNH 6174; TMP 79.8.1; MOR 553S-8.4.92.150, 7.15.0.41) in possess- ing an elongate, triangular morphology, an extensive orbital rim, a prominent ridge defining the rostral limit of the supratemporal fenestra, and a large, laterally extensive postorbital process [26]. However, differences can be observed among various specimens referred to T. formosus (AMNH 6174; TMP 79.8.1; MOR 553S- 8.4.92.150, 7.15.0.41), as well as between UMNH VP 16303 and some of the specimens in the T. formosus sample. Variable features include the morphology of the orbital margin, supratemporal ridge, and orbital bulbs. These features may reflect ontogenetic and/or individual variation or taxonomically significant morphol- ogy; however, full evaluation of these potentially diagnostic features awaits the description of multiple frontals collected from the Two Medicine Formation. New Troodontid Theropod from North America A partial skeleton of a paravian theropod (UCMP 149171) was recovered from the Kaiparowits Formation by Howard Hutchison in 1994. UCMP 149171 consists of a proximal tibia, fragmentary metatarsals, pedal phalanges, and pedal unguals, in addition to fragmentary cranial remains, including the basioccipital, fused parietals, and portions of both squamosals. Although initially considered a dromaeosaurid, several aspects of the skeleton suggest the specimen can be referred to Troodontidae, in particular the presence of a ventrally sloping cnemial crest on the tibia. Unfortunately, the specimen is extremely damaged and does not preserve enough overlapping elements to determine its relation- ship to Talos with confidence. Finally, an isolated troodontid distal caudal vertebra and several isolated troodontid teeth are represented in the Kaiparowits materials reposited at UMNH and ALF. The distal caudal vertebra (UMNH VP uncatalogued) preserves a distinctive dorsal sulcus as in other troodontids [26]. Representative maxillary (UMNH VP 12507) and dentary (UMMH VP 11806) teeth preserve enlarged denticles. Both mesial and distal carinae of maxillary teeth are denticulate. However, mesial carinae of the maxillary teeth exhibit subtrapezoidal denticles, whereas, those on distal carinae are subtriangular. All maxillary denticles are tightly appressed and apically oriented. In contrast, denticles are restricted to the distal carinae of at least caudal dentary teeth, are pectinate in morphology (i.e., elongate and subtriangular), widely spaced, and oriented perpendicular to the long axis of the tooth. Although the caudal vertebra, assortment of teeth (e.g., UMNH VP 12507 & UMMH VP 11806), isolated frontal (UMNH VP 16303), and associated skeleton (UCMP 149171) may ultimately prove referable to Talos, at present these materials do not overlap significantly with the holotype specimen. As such, there is no justification other than biogeographic and chronostratigraphic proximity for referring these materials to Talos and we refrain from doing so here. Paleobiogeographic Implications To date, Troodon formosus has been reported from North American formations extending over 4000 kilometers and encompassing a temporal range of approximately 20 million years. These include the Ferris, Lance, Horseshoe Canyon, Judith River, Dinosaur Park, Hell Creek, Scollard, Two Medicine, Prince Creek, and ‘‘El Gallo’’ formations [16,66–67]. The genus Troodon has also been reported from the Upper Cretaceous Kaiparowits, Wahweap, and Dakota formations of Utah and the Wapiti, St. Mary River, and Oldman formations of Alberta, Canada [64,66,68]. In sum, T. formosus (or at minimum Troodon sp.) is reported to have inhabited most of western North America throughout the Campanian and Maastrichtian epochs. We find the likelihood that these specimens represent a single species of troodontid theropod to be low. First, many of these identifications were based solely on isolated teeth, which are not known to be autapomorphic to species level within other theropod clades [69–71]. Moreover, the recovery of Talos sampsoni indicates that not all troodontid materials from the upper Campanian of the Western Interior Basin (WIB) are referable to Troodon formosus. Finally, recent paleobiogeographic studies have demonstrated that even within a short window of the Campanian (e.g., 74–76 million years) coeval formations of the WIB preserve predominantly unique dinosaurian faunas [72–85]. These patterns are most easily recognized among megaherbivorous clades such as ceratopsids [73,82] and hadrosaurids [80–81,83], which are not only more abundantly preserved across the WIB, but also possess extensive PLoS ONE | www.plosone.org 16 September 2011 | Volume 6 | Issue 9 | e24487