SEXUAL DIMORPHISM AND ONTOGENY WITHIN THE FOSSIL CRAB PALAEOPINNIXA SP. (BRACHYURA: HEXAPODIDAE)

Modern decapods are well-known for displaying sexually dimorphic and ontogenetic characteristics. Within the fossil record, however, ontogeny and sexual dimorphism are poorly studied, as many fossil species are known from only a few specimens. Recent focused collecting of fossil decapods within new exposures of the mudstones of the lower Miocene Culebra Formation of the Panama Canal expansion zone have resulted in hundreds of carapaces of the crab Palaeopinnixa sp. These crabs are typically 8-20 mm wide, and are often preserved articulated (i.e. legs, claws, and abdomen/sternum intact). The preservation of these crabs presents a rare opportunity to complete a population study, focusing on ontogeny and sexual dimorphism within Palaeopinnixa. Although length/width ratios do not differ between the two sexes, females are 1 mm wider on average. Another distinct difference, though not unexpected, is that the maximum abdominal width is greater in females compared to males. Ontogenetic differences include a statistically significant decrease in the ratio of frontal orbital width to total width as the crab grows. This is true for both males and females. Research supported by NSF grant OISE, EAR, DRL 0966884.