EVIDENCE FOR SPECIALIZED SHELL-BREAKING CRAB CLAWS FROM THE LATE CRETACEOUS OF MEXICO

Brachyuran crabs are among the most important shell-breaking predators in modern seas. It is believed that the first crabs with specialized molar-like claws used to break the shells of mollusks date from the Paleocene. Here we report on two species of large crabs from the lower Maastrichtian of Mexico that show a bulb-like structure at the proximal portion of the movable finger of the large, right claw relative to the left one, which is indicative of a highly specialized shell-breaking habit. The structure is similar to the one observed in calappids, which use it as a tool to peel or break open the shells of mollusks. Megaxantho zoque is found from the lower Maastrichtian of Chiapas, southeastern Mexico, and is one of the largest species of fossil crab described for the Cretaceous. The right chela is proportionally very large in this species, and has a bulb-like structure on the base of the movable finger. The left chela is more slender, but its base has not been preserved in the two specimens known of this genus. Ophthalmoplax was a widely distributed genus in southern North America and northern South America during early Maastrichtian times. Large specimens of Ophthalmoplax (probably a new species) recently collected from Northeastern Mexico also preserve a conspicuous bulb-like structure on the large, right chela. These observations suggest that highly specialized shell-breaking crab predators were already present in the Cretaceous in families that became extinct by the end of this period. The presence of these large, widespread predators might help explain the occurrence of jagged repair scars on the shells of some Late Cretaceous mollusk species that are similar in shape to those inflicted by predatory crabs in the Cenozoic.