However, it can be sometimes difficult to imagine an animal like Tyrannosaurus rex being related to the small songbird visiting your backyard bird feeder.
Researchers from the Museum of the Rockies and North Carolina State University have illustrated that Tyrannosaurs did indeed share a very important characteristic with birds.
As female birds produce eggs their bodies borrow calcium from their skeletons to produce the protective eggshells. As eggs form, skeletal calcium is removed from the inner bone tissue that lines the marrow. This interior woven bone tissue is called medullary bone and is found in female birds primarily as they are the gender that lays eggs.
Dr. Mary Schweitzer of North Carolina State University, and colleagues, have found and identified medullary bone in a Tyrannosaur specimen, which was one of the first discoveries of this kind of bone structure in dinosaurs. It also allowed her to confirm that the gender of the Tyrannosaur specimen was a female animal that was pregnant at the time of its death.
Dr. Schweitzer's research has also revealed that the production of medullary bone was a unique characteristic shared by dinosaurs and birds but not basal archosaurs, like crocodillians. She found that American Alligators, as representatives of more basal archosaurians, do not produce medullary bone before, during, or after egg-laying in females and definitely not in males alligators. This makes sense given that crocodillians and other modern basal archosaurs (turtles) produce soft leathery eggs rather than the hard-shelled eggs of birds and dinosaurs.
Since the discovery of a pregnant female Tyrannosaurus, several other species of dinosaurs, including Tenontosaurus and Allosaurus, have been discovered with similar internal bone structure revealing gender and breeding status.