RECOGNITION OF INTRASPECIFIC VARIATION AMONG FOSSIL CRABS (DECAPODA: BRACHYURA) TO BETTER ESTIMATE BIODIVERSITY AND EVOLUTIONARY TRENDS OVER GEOLOGIC TIME

Carapace sexual dimorphism and dimorphism between juvenile and adult brachyurans is poorly studied among extinct brachyurans (crabs). Although sexual dimorphism in the pleon and the chelae are well-documented in extant crabs, carapace dimorphism receives much less attention. Examination of extant specimens of crabs indicates that more variability exists among and between males and females, as well as juveniles and adults, of modern brachyurans than is currently recognized in extinct species. Extant dromiid (sponge) crabs, which are well known from the Cretaceous and Paleogene, may exhibit distinctive differences between males and females in the shape of the carapace and development of lateral spines and dorsal carapace ornamentation. Frog crabs of Raninidae differ in the development of lateral spines between males and females, with males having larger and more complex spines. Raninid crabs can even differ in spine development from right to left lateral margins on the same individual. Anecdotal references describe changes in ratios of carapace attributes as crabs grow. Smaller, younger individuals of the eubrachyuran Aethridae display distinct dorsal keels, which disappear in larger, older individuals. Members of podotrematous (more basal) and eubrachyuran (derived) crab superfamilies often have a fronto-orbital width (width between the outer edges of the orbits) that becomes narrower with respect to the width of the entire crab carapace as the carapace becomes larger. Why are these observations important? Development and type of carapace ornamentation, carapace shape, and morphometric ratios are all routinely used among decapod paleontologists in distinguishing brachyuran species and genera. We hypothesize that some species of extinct crabs are synonyms, representing males and females and/or juveniles and adults, especially when described from the same locality or from localities of similar age in the same general area. We suggest that more attention be directed to recognition of intraspecific variability among extinct brachyurans in order to achieve a more realistic estimate of the biodiversity of the group through time and to understand how intraspecific variability may have contributed to their evolutionary success or their eventual extinction.