Figure 4.

Isotopic composition of oxygen in bone and tooth phosphate of living and fossil whales. Extant cetaceans living in fresh water have lighter isotopic values in the range of 11–15.5 (parts per thousand δ¹⁸O SMOW), whereas those living in marine waters have heavier isotopic values in the range of 16.5–20 (• and shaded areas). Miocene cetaceans have heavy values in the range of 19–22 (○). Few Eocene cetaceans have been reported, and these have been divided into freshwater and marine forms (○). Values for early-middle Eocene P. inachus range from 15.5 for juveniles (◊) to 20.5 for adults (⧫; A. Goswami and P.D.G., unpublished data). Middle early Eocene H. subathuensis tooth-enamel phosphate has a value of 17 ‰ δ¹⁸O Standard Mean Ocean Water (SMOW), which is heavier than oxygen found in phosphate of extant and Eocene freshwater cetaceans. Pakicetus and Himalayacetus are interpreted here as amphibious, spending time on land but ingesting both fresh and marine waters while feeding in rivers, estuaries, and shallow marine Tethys.