Fig. 4: The histology of Whatcheeria femora from size class IV.

Under polarized light and a lambda filter, FMNH PR 5023 contains an abundant trabecular network in the medullary cavity and a narrow cortex (a). The cortex is composed primarily of lamellar bone (b, c) with some secondary remodeling (c). The trabeculae in this specimen are well-developed and composed of lamellar tissue (d). These general observations are consistent with an additional specimen, FMNH PR 1958, visualized with µCT imaging (e), including a developed trabecular network in the medullary cavity and a narrow lamellar cortex (f). In a and e the adductor crest (ventral) is oriented towards the bottom and marked with an asterisk. Abbreviations: lfb lamellar-fibered bone, mc medullary cavity, pfb parallel-fibered bone, so secondary osteon, tr trabeculae. Scale bars: a, e = 5 mm; b, f = 250 µm; c, d = 100 µm.