In 1990, when Peter Larson dug a massive Tyrannosaurus rex skeleton out of the ground in South Dakota, he named the skeleton "Sue" in honor of the woman who first detected the fossil. But he soon began to wonder if "Sue" was really a "Bob."

Boy Or Girl?

Larson set out to determine the sex of his fossil. First, he dissected a series of modern crocodiles. Male crocodiles, like many reptiles, have a penis-like sex organ that can be extended or retracted into the body. The muscles that control this movement are attached to the first of several bony protuberances, called chevrons, that extend from the underside of the tail vertebrae.

In the male crocodiles that Larson examined, the chevrons were all of equal length. In the female crocodiles, however, the first chevronwhere the penis muscles normally attachwas noticeably shorter than the rest.

Looking For Patterns

Next, Larson examined fourteen T. rex skeletons. The dinosaur skeletons showed a pattern similar to that of the modern crocodiles. Some had a shortened first chevron, located farther down the tail, while some had chevrons of equal length. Larson concluded that the dinosaurs with shorter chevrons were probably females.

In addition, the skeletons with smaller chevrons were more massive, an overall size pattern that coincides with earlier observations that female dinosaurs were bigger than males, like many modern reptiles and birds of prey.

A Boy Named "Sue"

So Sue's big bones and her smaller first chevron appear to be feminine traits, making it seem that she was aptly named. If Sue had turned out to be male, would there have been a name change? Not according to Larson. "Then," he says, "we simply would have had a boy named Sue."