Sexual dimorphism is documented in 35 articulated adult skeletons, 24 females, and 11 males, of the Miocene rhinoceros Teleoceras major from Ashfall Fossil Beds, Nebraska. Morphometric analysis of 51 cranial, mandibular, forelimb, and hindlimb characters reveals larger male mean values in 50 of the 51 measurements, of which 23 are significantly different (p ≤ 0.01). The most clearly dimorphic feature is the i2 diameter. The dimorphism evident in additional mandibular and cranial characters is conservative when compared with the dimorphism present in the fore- and hindlimbs. Non-overlapping male and female ranges are recorded for humerus length, radius length, radius proximal width, and femur length, with corresponding dimorphism ratios (DR = male ÷ female) of 1.11, 1.12, 1.11, and 1.10. Maximum male longbone lengths exceed minimum female lengths by an average of 24% (20–29%). Developmental maturity is apparently asynchronous in T. major, with fusion of longbone epiphyses delayed a minimum of two relative adult age classes in males. Significant sexual dimorphism is evident in the radius (DR = 1.34) and femur (DR = 1.19) cross-sectional areas. Estimates of body mass suggest a DR value between 1.13 and 1.23. The cranial, mandibular, and body-size dimorphism in T. major approaches that seen in the extant rhinoceroses Ceratotherium simum and Rhinoceros unicornis. However, the apparent herd structure and breeding-age sex ratio for the Ashfall herd suggests a behavioral ecology for T. major different from that of extant rhinoceroses. Teleoceras was likely a herding polygynous species ecologically more similar to extant Hippopotamus amphibius of Africa.

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