CRETACEOUS RANINOID CRABS (CRUSTACEA: DECAPODA: BRACHYURA) FROM THE NEOTROPICS: CALLING FOR SPATIO-TEMPORAL REEVALUATIONS

Raninoid crabs, also called “frog crabs”, are perhaps one of the most intriguing groups within the Brachyura (true crabs) principally due to their cryptic lifestyle and particular morphology which include a fusiform carapace, unfolded abdomen, narrow sternites, elongated buccal frame, modified pereiopods and reduced eyes. Extant raninoids inhabit tropical and temperate marine waters, and are widespread along a wide bathymetric range (from ~5 to 1000 m depth). Although their appearance seems primitive and aberrant, some authors consider their morphology to be an advanced (derived) adaptation to their burrowing lifestyle. This combination of primitive and derived traits makes them a phylogenetically controversial clade. Thus, knowledge of ancient raninoids is important for understanding taxonomic relationships among the principal crab groups: Dromiacea (sponge crabs) and Eubrachyura (‘higher’ crabs), since frog crabs occupy an intermediate position. The oldest known frog crabs appeared during Albian times (~100 M.y) and experienced a rapid diversification during the Cretaceous (~100 to 65 M.y.). While Cretaceous raninoid taxa from higher latitudes are well known, records from tropical South America are scarce. The general lack of knowledge of crustacean paleontology from the Neotropics may result in considerable biases when attempting to address major evolutionary, biogeographic and phylogenetic questions. Recently, seven species (five new) of raninoid and raninoid-like faunas, arrayed in five genera, four families, and two superfamilies, have been collected in Cretaceous (Albian-Maastrichtian) rocks from Colombia, South America. These new findings represent some of the oldest (or even the oldest) records for their lineages, calling for re-evaluations of their spatio-temporal patterns, in order to gain a more accurate understanding of the role of the Neotropics on the diversity, origins and macroevolutionary patterns of primitive brachyuran crabs throughout time.