Stable isotope evidence for an amphibious phase in early proboscidean evolution - PubMed (original) (raw)

Stable isotope evidence for an amphibious phase in early proboscidean evolution

Alexander G S C Liu et al. Proc Natl Acad Sci U S A. 2008.

Abstract

The order Proboscidea includes extant elephants and their extinct relatives and is closely related to the aquatic sirenians (manatees and dugongs) and terrestrial hyracoids (hyraxes). Some analyses of embryological, morphological, and paleontological data suggest that proboscideans and sirenians shared an aquatic or semiaquatic common ancestor, but independent tests of this hypothesis have proven elusive. Here we test the hypothesis of an aquatic ancestry for advanced proboscideans by measuring delta(18)O in tooth enamel of two late Eocene proboscidean genera, Barytherium and Moeritherium, which are sister taxa of Oligocene-to-Recent proboscideans. The combination of low delta(18)O values and low delta(18)O standard deviations in Barytherium and Moeritherium matches the isotopic pattern seen in aquatic and semiaquatic mammals, and differs from that of terrestrial mammals. delta(13)C values of these early proboscideans suggest that both genera are likely to have consumed freshwater plants, although a component of C(3) terrestrial vegetation cannot be ruled out. The simplest explanation for the combined evidence from isotopes, dental functional morphology, and depositional environments is that Barytherium and Moeritherium were at least semiaquatic and lived in freshwater swamp or riverine environments, where they grazed on freshwater vegetation. These results lend new support to the hypothesis that Oligocene-to-Recent proboscideans are derived from amphibious ancestors.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

Phylogenetic relationships of Paleogene proboscideans, from Seiffert (4).

Fig. 2.

Fig. 2.

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.

Fig. 3.

Fig. 3.

δ18O standard deviations for a variety of aquatic and terrestrial taxa and late Eocene mammals from Egypt. Note the sharp break at ≈0.5‰, below which are definitively aquatic extant mammals. Additional data are from Clementz et al. (25, 26), Kingston and Harrison (39), and MacFadden et al. (41).

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