THE EVOLUTION OF SEXUAL DIMORPHISM IN TELEOCERAS, A NORTH AMERICAN RHINOCEROS

Recognition of exaggerated sexually dimorphic traits in the fossil record can provide insight into various selection pressures and possible associated behaviors, inferred via homology and analogy. Teleoceras spp. (Mammalia, Rhinocerotidae) had dimorphic lower second incisors (I₂s), where males have larger I₂s than females, similar to the modern greater one-horned rhinoceros (Rhinoceros unicornis). Like R. unicornis, Teleoceras is thought to have used I₂s for display and during pre-courtship combat. Teleoceras includes ten described species and is abundant at many Miocene (ca. 17.5 Ma to 4.8 Ma, Hemingfordian to Hemphillian North American land mammal ages [NALMAs]) fossil sites. Dimorphism is well studied in Teleoceras but changes through time in I₂ size between sexes and among species have not been studied, nor has a phylogenetic analysis of Teleoceras species been done. We measured and calculated the cross-sectional area of 234 Teleoceras I₂s (isolated or in jaws) from the University of Nebraska State Museum and the American Museum of Natural History. We also compiled a character matrix from published generic matrices, species descriptions, and species measurements, and used TNT 1.1 to generate a phylogeny. Results illuminate how I₂ size changed through time and provide possible phylogenetic relationships among Teleoceras species. We find Teleocerasunderwent gradual parallel male and female I₂ size increases during the Barstovian and Clarendonian NALMAs. In the middle Hemphillian, male I₂ size increased rapidly (unlike females, which remained constant). Male I₂ size then rapidly decreased in the late Hemphillian and females decreased slightly. Phylogenetic results suggest all included Hemphillian species are closely related and may represent an anagenetic lineage. During the Barstovian and Clarendonian, speciation was not associated with I₂ size changes. In the Hemphillian, however, trend divergence between the sexes was marked by species change. Because the Hemphillian species may represent an anagenetic lineage, we hypothesize that rapid I₂ size changes were a response to a changing environment. Reduced primary productivity from cooling or spread of C₄ grasses may have encouraged reduced energetic input into exaggerated traits, conserving limited resources in a changing ecosystem.