Fig. 4

Histology of humeri of the thalattosauroid from Oregon. A, Humerus (MNCH F64270), longitudinal section, core sample. Note that the core was cut somewhat off-center, the neutral area is to the left, distal to the right. Image in normal light. B, Humerus (MNCH F64316), transverse section in normal light. C, Interpretative sketch of the tissue distribution in MNCH F64316. D, Detail of periosteal cortex in MNCH F64316 in normal light. Note the radial trabecular structure of the primary periosteal cortex. The middle layer is seen in the lower left corner, but most of the field of view is covered by the outer layer. The outermost layer is in the upper right corner. E, Detail of primary periosteal cortex in MNCH F64316 in normal and F, polarized light showing the transition zone between the middle (right) and outer layers (left). Note the high amount of plump osteocyte lacunae. Simple vascular and incipient primary osteons canals are mainly oriented longitudinally. G, Enlargement of the transition zone between the middle and outer layers in the periosteal cortex of MNCH F64316 in normal and H, polarized light. Note the low amount of osteocyte lacunae in a relatively high organized parallel-fibred tissue. Vascularity is high, consisting of large longitudinal to radial simple vascular canals with limited osteonal lining. I, Enlargement of parallel-fibred tissue and simple vascular canals in MNCH F64316 in polarized light. J, Detail of the middle layer of the periosteal cortex transitioning to the inner layer (upper right) in MNCH F64316 in normal and K, polarized light. The primary parallel-fibred tissue of the middle layer is here less organized between the vascular canals but partially starts to line vascular canals in higher organization. L, Detail of secondary endosteal trabeculae (top) and primary periosteal ones (bottom) at the transition from the middle to the inner layer of the periosteal cortex in MNCH F64316 in normal and M, polarized light