Unravelling the distribution of decapod crustaceans in the Lower Eocene coral reef mounds of NE Spain (Tremp-Graus Basin, southern Pyrenees)

Abstract

Abstract Modern reefs are considered important hot spots of biodiversity, but the analysis of the distribution of the invertebrate fauna across different reefal domains in ancient ecosystems can be challenging, because the fossil record is usually affected by strong taphonomic biases. The lower Eocene coral reef in the well-exposed outcrops of Ramals (Tremp-Graus Basin, southern Pyrenees, northeast Spain), preserve a high diversity of invertebrate groups, including decapod crustaceans. In Ramals the reefal facies belt is formed by a 100–200\xa0m width E–W trending facies belt, including a set of closely spaced reef mounds up to five meters high, surrounded by the skeletal-rich (packstones, rudstones) inter-reef facies. These outcrops also allow the analysis of the fossil-association present in the inner and outer fore-reef facies, which are dominated by skeletal packstones with molluscs, foraminifera, corals, bryozoans, decapod crustaceans, echinoderms and vertebrate fragments (fishes and crocodiles). The reef framework consists of framestones with bioclastic wackestone to packstone matrix, including abundant colonial corals, as well as crustose red algae, encrusting foraminifera (Solenomeris), solitary corals and bryozoans. These reef mounds developed within the mesophotic zone, disturbed by the episodic activity of storm-induced waves. The distribution of decapod crustaceans across the different reefal domains was subjected to extensive paleontological and statistical analyses. The 911 specimens of decapod crustaceans include 41 species belonging to 21 families. Most crustaceans were concentrated in the periphery of the mound reefs and suggest that the core of the reef hosted the highest diversity and abundance of decapod crustaceans. Carpilioids were the most abundant group within the reefal facies belt, Ctenocheles sp. dominated the inner fore-reef areas, and Litoricola macrodactylus pyrenaicus showed preferences for outer fore-reef environments. Decapod crustaceans and associated faunas lived in close association with coral reefs but disappeared from the area after the demise of the reefs due to the increase of the depositional depth and fine terrigenous sedimentary input, illustrating how diversity changes at local scale due to extrinsic factors.