One of the frustrating aspects that paleontologists face when studying dinosaur fossils is the odd lack of sexual dimorphism in the ancient creatures — being able to tell the difference between male and female individuals based on their physical features. However, one method of telling whether an individual specimen is a female or not has been uncovered, with the confirmation of a Tyrannosaurus rex fossil that came from a pregnant female.

"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," says the study’s lead researcher, North Carolina State University evolutionary biologist Mary Schweitzer.
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