HOW TO EXPLAIN A DECAPOD CRUSTACEAN DIVERSITY HOTSPOT IN A MID-CRETACEOUS CORAL REEF

The mid-Cretaceous (late Albian) decapod fauna from the Koskobilo quarry in Spain is the most diverse decapod fauna known from the Cretaceous so far. This may be related to the coral reef environment in which these decapods were found. This diversity hotspot is evaluated by means of a detailed paleoecological study focusing on variation in lithology throughout the quarry using carbonate rock hand samples, thin sections, and acetate peels. A range of decapod-rich sites were recognized within the quarry. The northern and stratigraphically oldest part of the quarry contained mostly wackestones, the middle and southern part contained floatstones containing many sponge and algal remains, and the southernmost and stratigraphically youngest part consisted of coral-rich boundstones. Decapod-rich sites were restricted to the southern part of the quarry, possibly in part because of an increased possibility for shelter and food in a coral-rich area in the southernmost part of the quarry. Systematic collecting was carried out at four decapod-rich sites in the quarry to investigate differences in decapod diversity, faunal composition, and specimen size (width). The decapod-rich site within the coral boundstones appeared to be the most diverse based on several diversity measures. This site contained species that were not found in other parts of the quarry and also consisted of a statistically different fauna and smaller decapods compared to other sites. More specimens of species with a smaller maximum width were found here as well as fewer specimens with a large maximum width. One other site within the floatstone zone contained few decapods, which may be related to the abundance of hard-to-eat corallinacean red algae. The results suggest that different decapod subenvironments existed within this reef, thereby explaining at least in part the high diversity of decapods known from the Koskobilo quarry. The results could apply as well to other fossil decapod faunas in coral reefs. This is one of the first times that a detailed paleoecological study was performed for decapods in a fossilized coral reef. The methods of collecting fossil decapods introduced here can be used to explore decapod paleoecology from other periods in Earth's history as well.