Polished serial cross sections and epiphyseal longitudinal sections of Nothosaurus mirabilis long bones in comparison. A, left femur (SIPB R 54/1) in dorsal view, anterior is to the right, proximal is to the top. D, left humerus (SIPB R 54/2) in dorsal view, anterior is to the right, proximal is to the top. Arrows in A and D assign the corresponding polished section to each section plane (marked by a dotted line). Serial transverse sections of femur (B 1–8) and of humerus (E 1–9) are numbered and described in the text from mid-diaphysis to proximal end and distal end, respectively. In all serial sections, anterior is to the right and dorsal side is up. Of B 1 and E 1 only mid-diaphyseal thin sections were produced which are pictured in Figure 3B and Figure 4B; C, longitudinal thin section of the proximal femur epiphysis. Plane of section is in anteroposterior direction. F, longitudinal thin section of the proximal humerus epiphysis. Plane of section is in dorsoventral direction. Femur sections B 1–8 display the generalized tetrapod long bone microanatomy. The cortex becomes thinner, and the amounts of secondary cancellous bone increase from the middle region of the shaft (B 1) towards both epiphyses (B 4 and B 8). The open medullar space enlarges from mid-diaphysis towards both articular ends and extends to the spongy endochondral and endosteal bone which provides the base for the articular cartilage (C). Humerus sections E 1–9 follow the same pattern as B 1–8, but throughout the entire humerus the cortex is much thinner, and secondary cancellous bone and endochodral and endosteal bone are only sparsely present. The medullary region extends proximally and distally to a very thin bony layer onto which the articular cartilage was placed in the living animal (F). Scale bars equal 10 mm.