Variation of medullary index. A, Scatter plot of medullary index (MI) vs. bone circumference of all sampled sauropterygian long bones, subdivided by genus, species and long bone type. Three clusters can be distinguished, reflecting either very small, intermediate, or large medullary regions. Note the trend of increasing MI with increasing body size in the genus Nothosaurus. Pistosaurus differs from the other sauropterygians in its very small medullary region in the humerus. B, comparison of the MI for each bone type of each species. The highest MI is seen in the long bones of Nothosaurus giganteus. Also note the great differences between the MI of the humeri and the femur of Pistosaurus.