Medullary bone is a temporary bone tissue found within the main bone cavities of certain animals. This porous, spongy material forms inside the hollow shafts of long bones. Its presence is linked to specific biological states, making it a transient component of the internal bone structure.
The Primary Function of Medullary Bone
The primary function of medullary bone is to serve as a readily mobilized calcium reservoir. This calcium is used for eggshell formation, which requires large amounts of the mineral in a short period. By utilizing medullary bone, the animal can produce strong eggshells without depleting the structural, cortical bone that provides skeletal support.
Hormonal changes trigger the formation of this bone tissue. An increase in estrogen levels stimulates the rapid deposition of medullary bone before the egg-laying cycle begins, ensuring a sufficient calcium supply for eggshell calcification.
Once eggshell formation is underway, the medullary bone is broken down, or resorbed, as needed. This cyclical process allows for efficient calcium transfer from the skeleton to the developing eggs. The rapid turnover of this tissue highlights its role in reproductive biology.
Animals with Medullary Bone
Medullary bone is predominantly found in reproductively mature female birds. It appears only during the egg-laying season and is absent in males and non-reproductive females. This tissue forms in various long bones, including the femur, tibia, and humerus, to support eggshell production.
Beyond modern birds, evidence suggests the presence of medullary bone in some extinct, non-avian dinosaurs. This discovery reinforces the evolutionary link between dinosaurs and birds, indicating this calcium storage mechanism has ancient origins. Medullary bone has also been observed in other archosaurs, a group that includes crocodiles, broadening its known distribution among egg-laying species.
Paleontological Significance
The discovery of medullary bone in fossil remains holds paleontological significance. Its presence serves as a definitive marker for identifying a fossilized individual as a reproductively mature female. This provides a rare glimpse into the life history and reproductive biology of extinct animals, allowing scientists to understand their breeding cycles.
A famous example is the Tyrannosaurus rex specimen MOR 1125, also known as “B-rex.” The identification of medullary tissue within its femur confirmed this specific T. rex was a female of egg-laying age. This finding helped answer long-standing questions about the sex and reproductive status of individual dinosaurs, which are typically difficult to determine from skeletal remains alone.