SUE the T. Rex has arm removed for research

CHICAGO - SUE, the largest complete T. Rex skeleton found and the star of Chicago's Field Museum, is a little less complete Monday night. 

Scientists are trying to figure out why the Tyrannosaurus Rex had such small arms. 

It's a biomechanical mystery. 

On Monday, a special team from the Field Museum carefully removed SUE's right arm. 

The 67-million-year-old fossils will be CT scanned and photographed with the hope that scientists can figure out how it was used. 

SUBSCRIBE TO FOX 32 ON YOUTUBE

One theory is that they were used to rip apart prey because of the sharp talons. 

A field museum scientist has another theory.

"There's a suggestion that the arms are sexual claspers, like so during mating they're like grabbing the female T. Rex and kind of holding her in place similar to the way that male lions will bite the back of the neck of a female lion," said Jingmae O'Connor, Associate Curator at the Field Museum.

The arm will be scanned at the University of Chicago Radiology Lab. 

Results are expected in February.