A morphospace for the Eocene fish assemblage of Bolca, Italy: a window into the diversification and ecological rise to dominance of modern tropical marine fishes

The celebrated Eocene fishes of Bolca, northeastern Italy, provide one of the earliest evidence of a modern tropical shallow marine fish assemblage, in the aftermath of the end-Cretaceous extinction. This fish assemblage has been traditionally interpreted as closely linked to a coral reef system based on a certain similarity in taxonomic composition with modern ecosystems. In this study, we use geometric morphometrics to compare the patterns of morphospace occupation and morphological variation between Eocene and extant tropical shallow water fish assemblages. Morphospace analysis revealed that there are not significant differences in morphospace occupation, and the Eocene fish assemblage shows a greater frequency of deep-bodied morphotypes and a higher morphological disparity compared to the extant tropical marine shallow-water assemblages. Because of the highly reduced reef-building potential of early Eocene coral communities and the extremely scarce evidence of corals in the Bolca area, the broad morphospace occupation and the high morphological richness observed for the Bolca assemblage suggest that an Eocene tropical non-coral reef setting shows higher, or at least similar, morphological diversity than modern coral reef-associated fish assemblages. Therefore, our paleontological evidence suggests that coral reefs may have played a secondary role in shaping the morphological richness of these fossil and extant tropical marine fish assemblages, and are consistent with the hypothesis of the rapid niche-filling and early saturation of the teleost morphospace after the end-Cretaceous extinction.