Morphology and size variation of a portunoid crab from the Maastrichtian of the Americas

The portunoid crab Ophthalmoplax Rathbun, 1935, is known from late Cretaceous deposits of Africa and the Americas. A review of 76 specimens from many localities in North and South America reveals that the genus is represented by only two species — one in Africa (recently described) and the other in the Americas. Ophthalmoplax brasiliana (Maury, 1930) was distributed along the Atlantic and Gulf coasts of the Americas throughout the Maastrichtian — from Brazil to North Carolina. In early Maastrichtian deposits of North America (∼69.0 Ma), the species is represented by local populations of medium-sized individuals, and by the late Maastrichtian (∼67.0 Ma), populations of larger individuals became abundant. This size increase may be related to a decrease in ocean water temperatures. Populations of medium-sized individuals are found again in the latest Maastrichtian (∼66.2 Ma), below strata with ejecta deposits in Coahuila, Mexico, and in the uppermost Owl Creek Formation, Mississippi. This size decrease is possibly linked to an increase in seawater temperature occurring just below the K/P boundary, when Ophthalmoplax became extinct.