SIZE VARIATION IN THE PORTUNOID CRAB OPHTHALMOPLAX FROM THE MAASTRICHTIAN OF THE AMERICAS: SYSTEMATIC DIFFERENCES OR INTRASPECIFIC VARIATION?

The oldest record for the crab Ophthalmoplax Rathbun, 1935, is from the Campanian of Morocco, where it attained a small to medium size. American representatives of this genus are recorded from the Maastrichtian throughout the Americas. During the early Maastrichtian, O. stephensoni Rathbun, 1935, attained a medium size, but by the late Maastrichtian, large decapods are found in Brazil, Venezuela, Colombia, Mexico and North Carolina, where crabs are abundant in discrete beds. South American Ophthalmoplax have been previously described as two different species: Ophthalmoplax brasiliana (Maury, 1930) and O. triambonatus Feldmann and Villamil, 2002, from the Gramame and Puerto Romero formations, respectively. Type specimens for these species are very similar in morphology and size, and therefore may correspond to the same species. Undescribed specimens from the upper Maastrichtian lithostratigraphic units of Venezuela and Colombia (Mito Juan and Puerto Romero formations, respectively) are also very similar to the previously reported specimens. In northeastern Mexico, large Ophthalmoplax occur abundantly in the upper Maastrichtian Cerro Grande/Las Encinas Formation. Although the general preservation of the specimens is relatively good, most of the anatomy, including pereiopods and chelae are not observed in any of the South American representatives. The upper Maastrichtian Island Creek Member of the Peedee Formation from North Carolina includes an interval with numerous concretions containing large Ophthalmoplax, very similar to those from Mexico and South America. Ophthalmoplax stephensoni was reported from the lower Maastrichtian of Texas. There are no diagnostic differences between O. stephensoni and the rest of the American material. Furthermore, medium sized specimens are found along with large specimens in early Maastrichtian deposits from Brazil, Mexico, and North Carolina. It is thus suggested that Ophthalmoplax stephensoni had a wide paleobiogeographic distribution and that increase in size towards the end of Cretaceous may have been triggered by environmental stress and/or biological interactions. Incidentally, these large crabs may be used as a tool for correlation among lithostratigraphic units in the Americas. The genus became extinct by the end of Cretaceous.