Although crabs first appeared during the Jurassic Period, many species emerged in the Cretaceous, including an oversized behemoth that had a curved tooth on a movable finger on the right claw. Known as Megaxantho zogue, the crab's large right claw was able to break shells, while the smaller left claw allowed the crab to funnel its prey into the "crusher" claw.
M. zogue did not survive the great extinction, but its features lived on in other species of crabs during the Cenozoic Era, which came right after the Cretaceous, and those species evolved into modern crabs [source: Ramanujan].
Interestingly, some Cretaceous crabs had both male and female characteristics. There weren't many of these so-called intersex crabs, but a few lived in what is now present-day South Dakota, where shallow seas provided a home for strange marine creatures. A subset of the Dakoticancer crabs, as scientists call them, were not strictly male, nor were they strictly female. Scientists speculate the crabs' habitat was contaminated, which caused the birth defects. Others say a prehistoric barnacle latched on to young crabs and wreaked havoc with their male hormones. This created a crab shaped like an adult female but that was, in reality, male [source: Pappas].