Pterosaurs have among the thinnest bone walls of any vertebrates. Integrative studies of their bone histology, coupled with mechanical engineering considerations and growth trajectories of the skeleton, indicate that pterosaurs reduced skeletal mass but actually increased the strength of their bones in flight by increasing the diameter of their long bones and by building struts in the marrow cavity that may have assisted in bracing the external structure of the bones. Indeed, pterosaur bones are over 80% stronger than mammal bones of the same size in resisting the tensile stresses of flight. Additional mechanisms increased tensile strength as the bones were remodeled during ontogeny.

Pterosaur bone is primarily fibro-lamellar with extensive vascularization, indicating rapid growth and probably sustained high metabolic levels. Although rest lines are not completely absent, there are never growth annuli that would suggest extensive periods of non-deposition. Bone is eroded from the internal cortex nearly as quickly as it is deposited periosteally. In mature individuals extensive Haversian reconstruction may be seen, and a relatively avascular layer is deposited on the outside of the bone, suggesting determinate growth. As the bone is eroded and remodeled internally, endosteal bone is often deposited outside bases of what later become the internal struts. As the bone grows further, the struts themselves are modified and often destroyed, leaving a ridge similar to earthquake bracing along the inner cortex of the bone. We suggest that this might represent a further aid to resisting tensile stresses in flight.

75/125 YEARS