The three images are cross sections of femurs from a marsupial (the Virginia opossum, left), a placental (the eastern chipmunk, center) and a 66-million-year-old multituberculate fossil (right).

Akiyoshi Matsumura, Yutaka Takahashi, Morihiko Okada, Cross-sectional geometric properties along the diaphysis of femur and humerus in chimpanzees and humans, Zeitschrift für Morphologie und ...

Cross section of the femur bone viewed under polarized light showing the thick layer of medullary bone within the medullary cavity. Disclaimer: AAAS and EurekAlert! are not responsible for the ...