Figure 14. Proximal (A) and midshaft (B) histologic cross- sections of the fibula of Talos sampsoni (UMNH VP 19479). Medullary spaces are towards the bottom. Bands representing slowed growth including lamellar bone and lines of arrested growth are marked with ‘‘,’’. The proximal section preserves five or six bands, whereas the midshaft has three. The third band of the midshaft section is not represented in this view due to taphonomic loss of the bone periphery. Between these bands, bone consists primarily of fibrolamel- lar bone with longitudinally oriented osteons. Episodes of endosteal deposition and erosion occur in the medullary regions of both sections. Abbreviations: do, developing osteons; el, endosteal deposition of lamellar bone; fl, fibrolamellar bone; mc, medullary cavity. Scale bars equal 0.3 mm. doi:10.1371/journal.pone.0024487.g014 proximal fibula underwent greater shape changes during ontogeny. The endosteal region of the proximal fibula preserves portions of at least six major growth intervals delineated by avascular lamellar bone. Also present are two large circular spaces lacking any distinct endosteal linings. These features exhibit a variety of crosscutting relationships relative to one another. The majority of the 3.5 mm-thick cortex consists of fibro- lamellar bone with osteons exhibiting both longitudinal and radial organization. Five circumferential bands of various forms divide the cortex. Moving from the endosteal region outward, the innermost definitive band appears as a fuzzy gray zone approximately 50 mm thick. The second band (approximately 50–130 mm in thickness) consists of three lines that converge at either end of the sample. Osteons occur between the lines where separation exists. A single line, less than 25 mm thick, represents a third band and osteons are abundant beyond its periphery. The density of the spacing between osteons decreases towards the fourth band (approximately 50 mm thick), which consists of closely spaced lines, occasionally separated sufficiently to include an osteon. A fifth band (approximately 80 mm-thick) is marked by few New Troodontid Theropod from North America osteons. Towards one end of the sample it continues as a series of closely spaced lines. The four interspaces between these five definitive bands vary across the sample: 300–560 mm, 80–320 mm, 370–800 mm, and 240–500 mm, respectively. In general, the bands show parallel extinction consistent with the presence of lamellar bone. Overall, some portions of the cortex display higher osteonal density, suggesting more rapid growth in this direction across multiple bands. The periphery of the sample is marked by abundant osteons with wide vessel canals, suggesting the osteons were in an early stage of development. Fibula midshaft. Due to post-fossilization bone loss, the cross-section of the fibular midshaft (Figure 14B) is incomplete and missing regions of the outermost cortex. The section is roughly oval in outline with long and short axes of 3.3 and 2.7 mm, respectively. The medullary cavity is similarly proportioned with its long axis parallel to that of the overall cross-section. As with the femur sample, considerable alteration, recrystallization, and replacement have occurred, particularly within the inner cortex. Additional smaller patches of altered bone are present in the middle and peripheral regions of the cross section. Adjacent to the medullary cavity, the cortex is comprised of two or three generations of crosscutting lamellar bone along the endosteal surface. The cortex consists primarily of fibro-lamellar bone with longitudinally arranged osteons. This tissue differs from that of the femur, in that the osteons are positioned more randomly such that circumferential rows are not clearly developed. Those osteons close to the medullary cavity possess larger vessel spaces. As with the femur, the fibula midshaft cross-section does not exhibit an extinction pattern following the osteons, a typical feature of unaltered bone. Instead, the bulk of the cortex shows parallel extinction except where interrupted by two sets of lineations arranged at 110u–120u to each other. These lineations show parallel extinction and are significantly more prominent under cross-polarized light. Further, under high magnification (400x), the bone cortex exhibits a grainy, mosaic pattern suggesting significant recrystallization. This texture is absent from the endosteal bone, which exhibits a normal, parallel extinction pattern typical of lamellar bone. Several, darkly stained circumferential lines or bands occur within the cortex. A pair of closely spaced lines lie about midway through the cortex. For most of the circumference these lines are approximately 70 mm apart; however, on both the lateral and medial aspects of the cortex, they converge to form a single line. Whereas elsewhere the coalesced or paired lines are outwardly convex, on one side they bend forming a concave stretch. Scattered osteons, circular or asymmetric, occur between the paired lines where separated. Both lines show small inward bends associated with more externally located osteons. A second dark, 30 mm-thick band occurs more peripherally to the first set of paired lines. Measured at the point of greatest cortical thickness, the two are separated by approximately 510 mm. This band consists of two to three closely spaced lines, bends inwards to accommodate more external primary osteons, and is present throughout the cortex except where the sample is incomplete. A third band also appears to be present, yet occurs only on a small portion of the cortex. Its absence elsewhere may be due to bone loss from the outer cortex. In the area where preserved, approximately 110 mm separate both the inner and middle bands and the middle and outer ones. These three bands suggest zones of slowed growth. The bands lack parallel extinction typical of circumferentially arranged PLoS ONE | www.plosone.org 14 September 2011 | Volume 6 | Issue 9 | e24487