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How did paleontologists determine that some Tyrannosaurus rex fossils were female?
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tyrannosaur
The fossilized remains of Sue, the most complete Tyrannosaurus rex skeleton in the world, on display at the Field Museum of Natural History, Chicago.
Paleontologists determine the sex of Tyrannosaurus rex specimens by identifying the presence of medullary bone within their skeletal remains.12
This specialized layer of bone is deposited on the inner walls of long bones in female birds during their reproductive cycles to provide a calcium source for eggshell production. By applying this knowledge of modern avian biology to the fossil record, scientists have successfully identified female T. rex specimens, such as the "B-rex" fossil, which preserved both this unique bone tissue and soft tissues.12
While earlier theories suggested that the shape of tail vertebrae near the pelvis could distinguish between males and females, this method was ultimately proven to be non-diagnostic.1