There has been considerable debate surrounding the origins of birds. Many believe that birds are descendants of dinosaurs, who were nearly wiped out about 65 million years ago (See my old post on periodic mass extinctions). But now scientists might have finally solved the puzzle once and for all:). A type of bone that female birds use as a calcium reserve for making eggshell has been found inside the fossilised thigh bone of a 68 million years old Tyrannosaurus Rex. The bone, called medullary bone, is densely rich in minerals and blood vessels, and is unique to ovulating female birds, who need the calcium to make the egg shells.

T. Rex tissue and bone (Courtesy: Pangea Institute)The calcium reserves are critical for female birds because they have strong but lightly-structured bones. Without medullary bone deposited in their marrow cavities, the calcium used for egg shells would come from their bones, giving them avian osteoporosis. Forming the shells depletes medullary bone, which in living birds remains depleted during brooding and until their next ovulation. The formation of the bone is triggered by an increase in estrogen levels.
The same distinctive structure of medullary bone were found in the T. Rex tissues. The T. Rex medullary bone is virtually identical in structure, orientation and even color, to that found in living emus and ostriches - large animals which are close to the evolutionary roots of modern birds. Living crocodiles do not have medullary bone, perhaps putting them farther in the evolutionary tree from dinosaurs than the birds:D:D.