Carbon and oxygen isotype ratios in fossil mammalian teeth from the Fayum Depression, Egypt, compared with similar data from living and extinct mammals. (A) δ18O and δ13C data from late Eocene mammals from Egypt. (B) δ13C versus standard deviation of δ18O for a range of mammalian taxa. Each data point represents average values for one taxon. Taxa falling in the dark gray band (SD δ18O < 0.5‰), are considered aquatic, whereas taxa in the larger light-gray field (SD δ18O > 1.0‰) are likely to be terrestrial. Additional data are from Clementz et al. (25, 26) and Kingston and Harrison (39). All δ18O standard deviations are calculated from values normalized relative to standard mean ocean water (SMOW) (modern value). δ13C error bars are shown at 1σ for Fayum taxa only. PDB, Pee Dee Formation belemnite standard; POM, particulate organic matter.