"It's a special tissue that is built up as easily mobilized calcium storage just before egg laying," she said. "The outcome is that birds do not have to pull calcium from the main part of their bones in order to shell eggs, weakening their bones the way crocodiles do."
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Crocodiles, she said, are the closest living relatives of dinosaurs.
"Medullary bone is thus present just before and during egg laying, but is entirely gone after the female has finished laying eggs," she said.
Early on, Mary Schweitzer suspected that medullary bone was present in the tyrannosaur remains, and was able to confirm her suspicions after she, Zanno and their team conducted a chemical analysis of the T. rex's femur.
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The material, found to be consistent with known medullary tissues from ostriches and chickens, contained karatan sulfate, a substance not present in any other bone types.
"This analysis allows us to determine the gender of this fossil, and gives us a window into the evolution of egg laying in modern birds," Schweitzer said.