Sexual dimorphism and rare intersex individuals in Cretaceous (Maastrichtian) Dakoticancer Rathbun, 1917 (Decapoda: Brachyura: Dakoticancroida)

Abstract

Abstract Several hundred specimens of the crab Dakoticancer overanusRathbun, 1917 represent a normal size-distributed adult population, with a skewed sex ratio of 21% females to 79% males. Sexual dimorphism is clearly developed in the shape of the sternum and pleon as well as overall carapace size. The Dakoticancer Assemblage displays a variety of intersex conditions, including gonopores (vulvae) on the coxa of the fourth pereiopod and female spermatheca associated with male sternal morphology. Gonopores on the coxae of the fourth pereiopods have been reported for shrimps, clawed lobsters, and palinurid lobsters, but not for brachyurans. Understanding the occurrence of intersex specimens within an extinct species of an extinct section is extremely difficult, as there seem to be no modern analogs concerning gonopores on the fourth pereiopod in Brachyura. The low percentage of affected individuals, about 1%, suggests developmental or genetic conditions were most likely responsible for the intersex individuals. The absence of such individuals in another large collection of a species of the same genus, of more or less the same geologic age, suggests that the factors leading to intersexuality in the Dakoticancer overanus population were endemic to that population.