Cenozoic sediments of Florida contain one of the most highly fossiliferous sequences of extinct sirenians in the world. Sirenians first occur in Florida during the Eocene (ca. 40 Ma), have their peak diversity during the late Oligocene–Miocene (including the widespread dugongid Metaxytherium), and become virtually extinct by the late Miocene (ca. 8 Ma). Thereafter during the Pliocene and Pleistocene, sirenians are represented in Florida by abundant remains of fossil manatees (Trichechus sp.). Stable isotopic analyses were performed on 100 teeth of fossil sirenians and extant Trichechus manatus from Florida in order to reconstruct diets (as determined from δ13C values) and habitat preferences (as determined from δ18O values) and test previous hypotheses based on morphological characters and associated floral and faunal remains. A small sample (n = 6) of extant Dugong dugon from Australia was also analyzed as an extant model to interpret the ecology of fossil dugongs.
A pilot study of captive manatees and their known diet revealed an isotopic enrichment (ε*) in δ13C of 14.0‰, indistinguishable from previously reported ε* for extant medium to large terrestrial mammalian herbivores with known diets. The variation in δ18OV-SMOW reported here is interpreted to indicate habitat preferences, with depleted tooth enamel values (≈25‰) representing freshwater rivers and springs, whereas enriched values (≈30‰) indicate coastal marine environments. Taken together, the Eocene to late Miocene sirenians (Protosirenidae and Dugongidae) differ significantly in both δ13C and δ18O from Pleistocene and Recent manatees (Trichechidae). In general, Protosiren and the fossil dugongs from Florida have carbon isotopic values that are relatively positive (mean δ13C = −0.9‰) ranging from −4.8‰ to 5.6‰, interpreted to represent a specialized diet of predominantly seagrasses. The oxygen isotopic values (mean δ18O = 29.2‰) are likewise relatively positive, indicating a principally marine habitat preference. These interpretations correlate well with previous hypotheses based on morphology (e.g., degree of rostral deflection) and the known ecology of modern Dugong dugon from the Pacific Ocean. In contrast, the fossil and extant Trichechus teeth from Florida have relatively lower carbon isotopic values (mean δ13C = −7.2‰) that range from −18.2‰ to 1.7‰, interpreted as a more generalized diet ranging from C3 plants to seagrasses. The relatively lower oxygen isotopic values (mean δ18O = 28.1‰) are interpreted as a more diverse array of freshwater and marine habitat preferences than that of Protosiren and fossil dugongs. This study of Cenozoic sirenians from Florida further demonstrates that stable isotopes can test hypotheses previously based on morphology and associated floral and faunal remains. All these data sets taken together result in a more insightful approach to reconstructing the paleobiology of this interesting group of ancient aquatic mammalian herbivores.