What makes a crab a crab? As it turns out, it's not an easy question to answer: A strange new species of fossil crab and a massive new genetic study shed new light on how the animals evolved.

In a new study in Science Advances, researchers unveil a newfound fossil site in Colombia that was formed some 90 million years ago. The site's astounding level of preservation—tiny shrimps' bodies down to the millimeter—is extremely rare, and only a few such sites have ever been found in the tropics.

The bounty includes dozens of specimens of a bizarre crab unlike anything ever found. Named Callichimaera perplexa—the “puzzling, beautiful chimera”—the crab seems to contain a mashup of traits seen in today's larval and