REVISITING THE GATUNCILLO FORMATION: EOCENE BRAIN CORALS AND THE ORIGIN OF THE RECENT CARIBBEAN CLADE MUSSIDAE

Molecular phylogenies indicate three distinct family-level clades of Recent brain corals, one exclusively Caribbean (Mussidae) and two predominantly Indo-Pacific (Merulinidae, Lobophylliidae). These three clades disagree with traditional systematics but are supported by newly discovered micromorphological and microstructural features observed using scanning electron microscopy and transverse thin sections. A preliminary review of the Caribbean fossil record using traditional systematics suggests that at least two of the three clades first occur in the region during Eocene time, and that one of the first occurrences (Caribbean Mussidae) is global. However the identifications on which this interpretation is based have yet to be confirmed using modern phylogenetics. The present study explores the evolutionary relationships of Eocene Caribbean brain corals with the three modern clades, focusing on one rich Eocene coral fauna, the Gatuncillo Formation of Panama. Morphological phylogenetic analyses including Caribbean and Indo-Pacific fossil and Recent taxa are performed to reconstruct the pattern of divergence of the modern Caribbean clade. The dataset consists of 74 species (62 Recent, 12 fossil) and 50 characters. In addition to traditional macromorphology, the characters include microstructural features used previously in phylogenetic analyses of modern brain corals. The results show that the modern Caribbean Mussidae form a distinct clade, but that the 12 fossil species do not group closely with that clade. This suggests that the modern Caribbean clade (Mussidae) diverged from a more cosmopolitan pantropical fauna as part of one or more post-Eocene evolutionary events associated with Tethyan breakup and Caribbean isolation, and is evolutionarily unique.