CORALS ON SOFT SUBSTRATES IN THE CAMPANIAN OF JAMAICA

Coral faunas inhabiting soft or mobile substrates are dominated by weakly attached or free-living growth forms. In the Caribbean, these corals are well known from the Modern and Quaternary. In the Neogene, free living corals made up a well-documented, significant fraction of the overall Caribbean coral fauna. Older free living coral faunas from the Caribbean are, however, not well known. The present research focuses on a diverse coral association collected from the Campanian Green Island Inlier of western Jamaica. A shallow water mudrock unit from this inlier yields abundant well-preserved molluscs and likely the oldest free-living coral community in the Caribbean. Comparisons of coral colony form and size to that of modern analogs provides critical information concerning their life habit and paleoenvironment.

The coral association includes a thin branched octocoral and representatives of at least 11 scleractinian genera. Two small tympanoid forms are found cemented to rudist or other bivalve fragments while the rest of the species show little or no attachment surfaces. There are meandroid, plocoid and cerioid circumrotary corals that are similar in size and shape to spherical colonies found in modern Caribbean grass flat habitats. The fauna also includes 3 large turbinate and flabellate species. The most common coral forms 5-7cm diameter, flat-based, domal colonies. This coral displays a distinct pattern of astogenetic development adapted to its free-living habit. It grows first as a trochoid, attached solitary coral up to ~2.5 cm high, the growth direction then shifts 90° and expands through hydnophoroid budding, doming upward and partially enveloping the trochoid portion forming the new flat, unattached base of the colony.

Many of the growth forms in this Campanian coral association are remarkably similar to those of modern Caribbean analogs found in soft substrate habitats. There are as well some species with unique adaptations to a free-living life habit. The taxonomic richness of the free-living fauna is higher than what is generally seen in the modern Caribbean. This could reflect the fact that shallow, hard substrate habitats throughout the Late Cretaceous were dominated by rudist bivalves not corals, leaving scleractinians to diversify in soft substrates as unattached, free living forms.