MEDITERRANEAN VS CARIBBEAN OLIGOCENE REEF CORALS: A NEW PALEOBIOGEOGRAPHIC SCENARIO UNRAVELED BY PROGRESS IN CORAL SYSTEMATICS

Molecular analyses have transformed our understanding of the evolution of scleractinian corals and challenged traditional morphology-based systematics that shows close evolutionary relationships between Caribbean and Indo-Pacific “faviid” and “mussid” reef corals. Instead, they show that much of the modern Caribbean fauna appears to be only distantly related to the Indo-Pacific fauna and recognize three distinct family-level clades, which diverged by middle Eocene time: (1) Caribbean faviids + mussids; (2) Indo-Pacific faviids; (3) Indo-Pacific mussids. During the early Cenozoic, members of these clades also occurred in a third geographic region, the Mediterranean, which was part of the westward flowing, pantropical Tethyan Seaway and traditionally revealed a strong similarity with the Caribbean at both the generic and species level.

As a first step in reconstructing the pattern of divergence between the three regions, we focused on a subset of the Paleogene Mediterranean reef coral fauna consisting of Oligocene meandroid reef corals that belong to the families Merulinidae, Lobophylliidae, and Mussidae. We performed morphological phylogenetic analyses including Caribbean, Indo-Pacific, and Mediterranean fossil and Recent taxa, using a dataset consisting of 76 taxa (63 Recent, 13 fossil) and 50 characters (including newly discovered micromorphological and microstructural features observed in transverse thin section). At a larger taxonomic scale, we have also updated available compilations of Oligocene coral genera of both Caribbean and Mediterranean regions following recent advances in scleractinian systematics.

The results show that, contrary to traditional systematics and previous paleogeographic interpretations, the Oligocene Mediterranean reef corals are more closely related to modern Indo-Pacific corals than they are to modern Caribbean corals. Mediterranean corals may have been part of a more cosmopolitan Tethyan fauna, whereas Caribbean corals were more isolated, resulting in the development of the genetically unique fauna that occupies the region today.