FROM HOOVES TO FLIPPERS: NEW INSIGHTS INTO THE ECOLOGY OF EOCENE MARINE MAMMALS FROM STABLE ISOTOPES

The evolution of cetaceans and sirenians is marked by similar morphological changes, such as loss of hindlimbs and development of tail flukes, which are associated with adaptation to a fully aquatic existence. Yet, how these changes correlate with ecological transitions within these lineages remains unclear. Coupled carbon and oxygen isotope analysis is one source of ecological information for early taxa within these groups. Enamel d¹³C values are controlled by diet and are labeled by the d¹³C composition of primary producers at the base of the food web. Enamel d¹³C values are low for freshwater foragers, intermediary for C3 terrestrial consumers, and high for marine foragers, particularly consumers of marine macrophytes. Mean enamel d¹⁸O values can reflect the d¹⁸O composition of environmental waters, and thereby distinguish marine and terrestrial taxa. However, physiological and dietary differences also influence enamel d¹⁸O values, which complicates interpretation of mean values. An alternative is to look at differences in d¹⁸O variance among populations, since terrestrial taxa typically exhibit higher variance than aquatic taxa.

Using a coupled isotope approach, we assessed the habitat and dietary preferences of early cetaceans (Pakicetus, Basilosaurus, and Dorudon) and sirenians (Protosiren, Eosiren, and Eotherium) through comparison with co-occurring terrestrial mammals. Pakicetus and Protosiren represent basal members of these two lineages that morphologically were still capable of terrestrial locomotion, while the other taxa were most likely fully aquatic. Among cetaceans, the mean d¹³C value for Pakicetus was significantly lower than terrestrial taxa, as was the d¹⁸O variance, suggesting that Pakicetus was principally aquatic and foraging within freshwater ecosystems. More derived cetaceans, Basilosaurus and Dorudon, also showed low d¹⁸O variance, but mean d¹³C values were higher than terrestrial taxa, indicating these cetaceans were marine foragers. Likewise, sirenians exhibited low d¹⁸O variance indicative of aquatic habitat use, but d¹³C values for all sirenians, including Protosiren, were significantly higher than values for all other taxa, and represent the earliest clear evidence of feeding on seagrass by any lineage of mammals.