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How to tell if a dinosaur fossil is from a male or female

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Scientists studying a Tyrannosaurus rex fossil have figured out that the dinosaur was a female. How could they tell just by looking at preserved bones?

The researchers got their first clue from living birds. Many scientists believe that dinosaurs and birds both evolved from a common ancestor.

Female birds have a special type of bone, called "medullary bone," that grows in their limbs when they are laying eggs. Medullary bone contains many small blood vessels and is rich in calcium, which is used for making eggshells. Medullary bone forms a layer inside the hard outer bone layers.

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Mary Higby Schweitzer of North Carolina State University and the North Carolina State Museum of Natural Sciences in Raleigh, N.C. and Montana State University in Bozeman, Mont. and her colleagues found a bone layer in their T. rex fossil that looked a lot like the medullary bone of modern birds.

The fossils were discovered in Montana. This T. rex was probably about 18 years old when she died.

The medullary bone in the dinosaur fossil is most similar to medullary bone from flightless birds like ostriches and emus, the researchers report. They describe their discovery in the 3 June issue of the journal Science.

This study suggests that birds and dinosaurs had similar ways of making eggs. It therefore adds support to the idea the birds and dinosaurs have a common ancestor.

To read more about Dr. Schweitzer's T. rex discoveries, see the Science For Kids story from March 25th.