Open Access Kent State (OAKS)

The mid-Cretaceous (late Albian) decapod crustacean fauna from the Koskobilo quarry in Spain is the most diverse decapod fauna known thus far from the Cretaceous. This may be related to the coral reef environment in which these decapods were found within the Aldoirar patch reef. This diversity hotspot was further investigated by a detailed paleoecological study focusing on variation in lithology throughout the quarry using carbonate rock hand samples, thin sections, and acetate peels; and by studying decapod-rich sites within the quarry. The northern and stratigraphically oldest part of the Koskobilo quarry contained mostly wackestones and biomicrites, the middle to southern part contained floatstones and biosparites or biomicrites with many sponge remains, whereas the southernmost and stratigraphically youngest part consisted of coral-rich boundstones and biolithites. Fossils were more abundant and generally larger in the southern part of the quarry. 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. Water depth was estimated to be between 20 and 80 m for at least the southern part, with energy levels apparently increasing from the northern to the southern part of the quarry. Systematic collecting was performed at four decapod-rich sites in the quarry to investigate differences in decapod diversity, composition, and size (width). The decapod fauna from the site within the coral-rich boundstones and biolithites appears to be the most diverse based on several diversity measures, has a statistically different faunal composition, contains species that were not found in other parts of the quarry, and consists of 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. Smaller, presumed juvenile specimens of the crab Goniodromites laevis were also excavated here. This shows that decapod size can vary within a reef. Paguroids (hermit crabs) were mostly restricted to this coral-rich site. Some of the many decapods at this site may have been obligatory associates with the mostly branching corals. One site within the floatstone and biosparite zone contained hardly any decapods, which may be related to the abundance of hard-to-eat corallinacean red algae. A site in the Olazagutía quarry in the same patch reef consisting of massive colonial corals did not yield any decapods, presumably because of the inaccessible nature of the coral framestone leaving few places as shelter for decapods. The results suggest that different decapod sub- and microenvironments existed within this patch reef, thereby explaining at least in part the high diversity of decapods known from the Koskobilo quarry. This is one of the first times that a detailed paleoecological study has been performed for decapods in a fossilized coral reef. These results concur with modern reefs in that decapod communities also differ among subenvironments. The methodologies introduced herein for studying fossil decapods in reef environments can be used as well to study the paleoecology of decapods as well as other invertebrates from other periods in Earth's history.