Cornell University Adjunct Professor Greg Dietl was waiting for colleagues at a small nature history museum in the state of Chiapas, Mexico, when he recognized a 69 million-year-old crab fossil from the late Cretaceous period that had an oversized right claw -- a feature previously thought to have appeared more than 20 million years later in the early Cenozoic era.

The crab also had a curved tooth on the movable finger of the larger claw -- another specialized feature that scientists had thought developed millions of years later.

"I was really excited when I found it," said Dietl. "The fossil reopens the question of the role crabs played in the well-documented restructuring of marine communities that occurred during the Mesozoic era."

The discovery, co-authored by Francisco Vega, a geologist at the National Autonomous University of Mexico, appeared in the March 10 online version of the Royal Society journal Biology Letters.