Stan, one of the largest and most complete Tyrannosaurus rex fossil discovered, is on display, Tuesday, Sept. 15, 2020, at Christie's in New York. The teeth on T. rex and other big theropods were likely covered by scaly lips, concludes a study published Thursday, March 30, 2023, in the journal Science. The dinosaur's teeth didn't stick out when its mouth was closed, and even in a wide open bite, you might just see the tips, the scientists found. (AP Photo/Mary Altaffer, File)