We will examine three examples of deep-time isotopic paleoecology. The first is an exploration of the habitat and feeding preferences of desmostyilans. The Desmostylia are an extinct order of mammals related to sirenians and proboscideans (Domning 2002a). They are recovered from nearshore and, sometimes, offshore deposits along the Pacific coast of Asia and North America that range in age from 30 mya to 10 mya. The posture of these hippopotamus-sized animals, which have four weight-bearing limbs, is controversial, leading

to debate about how much time they spent out of the water. Their dentition is also unusual, with thick enamel and pillar-like cusps on high-crowned teeth, and procumbent tusk-like incisors and canines. Most researchers think they were herbivores, though some suggest a diet Erismodegib rich in mollusks or other hard-shelled invertebrates. Clementz et al. (2003) analyzed the isotopic composition of tooth enamel from the genus Desmostylus and co-occurring terrestrial and marine species to address the debate surrounding its ecology. Desmostylus had much higher δ13C values than coeval terrestrial or

marine mammals, suggesting a diet that consisted of submerged aquatic vegetation (sea grass or kelp). Fossil marine mammals and Desmostylus had low δ18O variability, indicating that Desmostylus spent as much time in water as a seal. Finally, the strontium isotope composition of marine organisms reflects that of the ocean and is relatively invariant when compared with values from land animals. The mean and variation in strontium isotope old values for Desmostylus were GSK1120212 cost similar to those for terrestrial, not marine, mammals. Clementz et al. (2003) concluded that Desmostylus spent time in estuarine or freshwater environments. Overall, isotopic data suggest that Desmostylus was an aquatic herbivore that spent a considerable portion of its life foraging in estuarine or freshwater

ecosystems. The paleoecology of other desmostylians, including those found more commonly in offshore deposits, has not been examined isotopically and may differ from that of Desmostylus. Isotopic methods have also been used to illuminate sirenian origins and evolution. At present, there are no isotopic data for the least derived sirenians, the Prorastomidae, which include taxa with four weight-bearing limbs such as Pezosiren (Domning 2001, 2002b). However, relatively high δ13C and δ18O values from another extinct clade, the Eocene-aged Protosirenidae, indicate that these fully aquatic mammals inhabited marine ecosystems, where they foraged in sea grass beds (MacFadden et al. 2004, Clementz et al. 2006) (Fig. 6A). Isotopic data reveal that Eocene-aged members of the Dugongidae (e.g., Eosiren, Eotheroides, Halitherium), which include extant dugongs and Steller’s sea cow, were also marine animals foraging on sea grass.