Resource partitioning and paleoecology of Neogene free-living corals as determined from skeletal stable isotope composition

Changes in the Caribbean coral fauna associated with Plio-Pleistocene extinctions are unique in that origination preceded extinction. This pattern is particularly evident in free-living solitary and flabello-meandroid (FSFM) corals. Here we use the stable isotopes of carbon and oxygen to assess the paleoautecology of pre-turnover FSFM corals from the Cibao Basin, northern Dominican Republic. We assess photosymbiosis and resource utilization within FSFM corals and how changing Neogene paleoclimatic and paleoceanographic conditions may have contributed to originations and extinctions within this unique ecological group. Measurements of δ¹³C and δ¹⁸O were compared between two known zooxanthellate coral species, two known azooxanthellate species, and six FSFM species of the Cibao Basin. Skeletal δ¹³C provided the best indicator of resource utilization by FSFM corals. For both modern reef corals and Mio-Pliocene reef corals from the Cibao Basin, values of δ¹³C fall between 0‰ and –2‰ whereas the δ¹³C values of azooxanthellate corals generally fall between –3‰ and –9‰. Both modern and fossil FSFM corals lie intermediate to these end members. FSFM values of δ¹³C fall between –1‰ and –4‰ and suggest reduced photosymbiosis in comparison to typical reef corals. Reduced photosymbiosis of FSFM corals in comparison to reef corals is also reflected in a stronger covariation between δ¹³C and δ¹⁸O. Though FSFM corals are clearly utilizing translocated carbon from associated algal symbionts, the more depleated values of δ¹³C and covariation with δ¹⁸O suggest a greater dependence on heterotrophic feeding. This ecological lifestyle likely has made FSFM corals sensitive to changes in primary productivity that characterize the Caribbean Neogene.