The Eocene–Oligocene transition (ca. 33 Ma) is associated with one of the most pronounced climate changes of the Cenozoic, with continental mean annual temperature dropping ∼8 °C over a span of ∼400,000 years. Leptomeryx is a small, ruminant artiodactyl that spans the transition, known from the late Eocene (Chadronian North American Land Mammal Age, or NALMA) through the early Oligocene (Orellan NALMA). The hypothesis that early Oligocene Leptomeryx had more complex enamel surface area than those found in the late Eocene has been demonstrated qualitatively, but the potential change in the amount of enamel has never been quantified. Here we calculate the area of the occlusal surface enamel (OSE) of Leptomeryx specimens from both the Chadronian (n  =  29) and Orellan (n  =  35) of northwestern Nebraska. Areas of the OSE were calculated by isolating the enamel into polygons on digital photographs of each specimen. The mean areas confirm that the OSE significantly increased by approximately 27% from the Chadronian sample to the Orellan sample.

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