THE ROLE OF REEFS IN MESOZOIC DECAPOD EVOLUTION

Quantitative patterns of decapod crustacean biodiversity through time have not been thoroughly studied. In addition, a lack of quantitative knowledge exists about sedimentary environments in which decapods lived. Here, we focus on the Mesozoic Era in which lobsters and shrimp became abundant and the Brachyura (true crabs) and Anomura first appeared and radiated. To gain more insight into Mesozoic decapod evolution, diversity curves on the species and genus level were made for all decapods, and for subsets of decapods such as lobsters and shrimp, Anomura, and Brachyura. The variable duration of stages within the Mesozoic and variations in rock outcrop area per stage may affect the decapod diversity curves. These factors were taken into account when creating the curves. The peak in decapod species diversity in the Late Jurassic is in part caused by the radiation of Anomura and Brachyura in reefal environments, and is coincident with a relatively high eustatic sea level. Spearman's Rank Correlation and Kendall's tau Coefficients suggest that the correlation between Jurassic and Cretaceous decapod curves on the species and genus level and the reef curve over the same time is statistically significant at the 1% level for most decapod curves. This suggests that the number of reefs on Earth has influenced the number of decapod taxa during the Late Jurassic. A somewhat weaker correlation was found between decapod curves and the sea level curve as the correlation was mostly significant at the 5% level. Brachyuran and anomuran diversity was generally low in the Early Cretaceous on the species level after eustatic sea level dropped in the earliest Cretaceous. More decapods have been found in the mid-Cretaceous and Late Cretaceous, possibly due to a radiation of Brachyura, which is coincident with a relatively high eustatic sea level. The fact that the biodiversity of reef-associated decapods such as brachyurans and anomuran species dropped dramatically near the Jurassic/Cretaceous boundary may have implications for decapods living in today's reefs, notably brachyurans, anomurans, and Caridea. Anthropogenic pressure may cause many corals to go extinct by the end of this century. This pressure is also a threat for the diverse group of reef-associated decapods.