Bone histology of Pistosaurus longaevus and Plesiosaurus dolichodeirus . A and B, Pistosaurus longaevus femur (SIPB R 74), seen in normal-transmitted (A) and polarized light (B). C and D, Plesiosaurus dolichodeirus femur (SIPB R 90) seen in normal-transmitted light (C) and with gypsum plate (D). A and B, detail of the lower right sector in cross section in Figure 5C. Unlike the humeri, the cortex of the femur consists of LZB except for the deepest cortical regions. The detail shows a growth cycle that locally consists of radial FLB following one of LZB. C, cortical bone composed of FLB of Plesiosaurus dolichodeirus (SIPB R 90). The primary cortex was subject to Haversian remodeling, partially obscuring it. An EFS is observable. Note the relatively small primary vascular canals and the large secondary osteons. D, detail of SIPB R 90, showing the radial fabric of primary bone, which appears to have been inducing resorption activity that led to the formation of secondary osteons. Abbreviations: pc, primary vascular canal, rflb, radial fibrolamellar bone, so, secondary osteon.