Medullary bone is a layer of material found inside a regular bone in birds and dinosaurs. This porous and spongy layer forms inside the bones of females when they are going to lay eggs. Medullary bone is full of extra calcium. The females use that calcium to form the hard shells of their eggs.
The name medullary bone comes from the word “medulla” (meh-DUH-lah). Medulla means “middle,” and refers to the center of the bone structure. In birds, the medulla of the bone is full of air pockets, which help make birds light enough to fly. Medullary bone fills in those air pockets when the bird is going to lay eggs. But medullary bone is spongy and light enough that expectant bird mothers can still take off.
In a sentence
In the fossils of a Tyrannosaurus rex, medullary bone can serve as a pregnancy test.