A 50 MILLION YEAR RECORD OF ECOLOGICAL AND TAXONOMIC DIVERSITY FOR SIRENIANS FROM THE TETHYS-MEDITERRANEAN AND WEST ATLANTIC-CARIBBEAN REGIONS

Sirenians (sea cows; manatees and dugongs) have been primary consumers in tropical and subtropical shallow water marine ecosystems for over 50 Ma. The four living species are only a small remnant of their past diversity and geographic distribution. Two locations where there is an extensive sirenian fossil record are the West Atlantic-Caribbean (WAC) and Tethys-Mediterranean (TM) regions. Both areas experienced significant climate change over the course of the Cenozoic, which undoubtedly impacted the diversity and ecology of sirenians and other marine mammals living in these areas. Application of stable isotope analysis can provide new insight into how diet and habitat preferences were impacted by climate change and, in turn, how these ecological changes correlate with diversity patterns for these regions. Enamel carbon isotope values are capable of differentiating among freshwater, marine algae, and seagrass diets, whereas oxygen isotopes are strongly correlated with salinity preferences of aquatic animals. In combination, carbon and oxygen isotope values provide a powerful tool for gathering ecological information from fossil sirenians that is independent of morphology. Enamel from 120 specimens of sirenians from WAC and 60 specimens from TM were sampled for stable isotope analyses. In the Paleogene, sirenian diversity and morphological disparity were relatively high with several sympatric species found in each region. High and consistent enamel δ¹³C and δ¹⁸O values for Paleogene specimens indicate that sirenian diets, however, were not as diverse and were restricted to marine seagrasses for all species. For much of the Neogene, sirenian species diversity remained high in WAC but declined in TM, while dietary diversity expanded to include consumption of freshwater plants and possibly marine algae by some species. The high taxonomic and morphologic diversity in the face of low dietary diversity for sirenians from these regions is challenging to explain, and may indicate these animals were partitioning their use of "seagrasses" more finely than our isotopic techniques can yet perceive. Declines in sirenian taxonomic diversity were likely due to the rapid climate and oceanographic changes during the latter half of the Cenozoic and their impacts on available dietary resources.