CHANGES IN MORPHOLOGY AND ENVIRONMENT IN THE CRAB CHACEON (DECAPODA, BRACHYURA, GERYONIDAE) FROM THE OLIGOCENE TO HOLOCENE

Chaceon is a genus of crab (Brachyura) whose earliest record is in the Oligocene. There are over 40 modern and fossil species, of which the majority are modern. Over its time span, morphological and ecological changes have occurred. Chaceon has progressively moved into cooler and deeper water. The fossil species lived mostly in shallow marine waters, but the modern species have been observed to be up to 2243 m, with an average depth of 926 m. The estimated depth of fossil species is inferred from sediment types and paleogeographic position. The changes in bathymetric and thermal settings were accompanied by morphological variations, particularly related to feeding structures. The exopod of the third maxilliped has become more rectilinear over time. This structure functions in filtering, cleaning, and manipulation food. The mandibles of the extant Chaceon quinquedens from the Gulf of Mexico are relatively large and bear a single triangular projection. The mandibles of the Oligocene C. peruvianus from the Oligocene of southern Argentina appear to be somewhat smaller, but because they cannot be fully excavated from the sediment, that interpretation is in question. The overall size of the buccal cavity, which houses the mouth parts, is longer in modern species, appendages of the extant species are longer, and the chelae are more slender in extant forms than those of C. peruvianus. Overall, modern Chaceon spp. are larger and longer than the fossil species. Because Chaceon spp. have moved into cooler and deeper water over geologic time, changes in morphology may be a result of changes in food availability. The overall driver of the change in habitat of Chaceon spp. may be climate change. As Chaceon spp. comprise fisheries in some areas, further changes in habitat or population might be predicted if overfishing occurs.