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).

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 ...