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Identifying Aquatic Habits Of Herbivorous Mammals Through Stable Isotope Analysis

¹ University of Wyoming, Department of Geology and Geophysics, 1000 E. University Avenue, Laramie, Wyoming 82071, USA
² University of California, Berkeley, Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, California 94720, USA
³ University of California, Santa Cruz Earth and Planetary Sciences Department, 1156 High St., Santa Cruz, California 95064, USA mclemen1{at}uwyo.edu

Large-bodied, semiaquatic herbivorous mammals have been a recurring component of most continental ecosystems throughout the Cenozoic. Identification of these species in the fossil record has largely been based on the morphological similarities with present-day hippopotamids, leading to the designation of this pairing of body type and ecological niche as the hippo ecomorph. These morphological characters, however, may not always be diagnostic of aquatic habits. Here, enamel ¹³C and ¹⁸O values from living hippopotamuses were examined to define an isotopic signature unique to the hippo ecomorph. Although ¹³C values do not support unique foraging habits for this ecomorph, living and fossil hippopotamids typically have low mean ¹⁸O values relative to associated ungulates that fit a linear regression (¹⁸O_{hippopotamids} = 0.96 ± 0.09·¹⁸O_fauna – 1.67 ± 2.97; r² = 0.886, p < 0.001). Modeling of oxygen fluxes in large mammals suggests that high water-turnover rates or increased water loss through feces and urine may explain this relationship. This relationship was then used to assess the aquatic adaptation of four purported hippo ecomorphs from the fossil record: Coryphodon (early Eocene), Moeritherium and Bothriogenys (early Oligocene), and Teleoceras (middle–late Miocene). Only fossil specimens of Moeritherium, Bothriogenys, and large species of Coryphodon had ¹⁸O values expected for hippo ecomorphs; ¹⁸O values for Teleoceras and a small species of Coryphodon were not significantly different from those of the associated fauna. These results show that the mean ¹⁸O value of fossil specimens is an effective tool for assessing the aquatic habits of extinct species.

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