Aphelops
Aphelops is an extinct genus of hornless rhinoceros endemic to North America. It lived from the Middle Miocene to the early Pliocene, during which it was a common component of North American mammalian faunas along with "Teleoceras". Description. On the basis of skull size, the largest species of "Aphelops" is "A. mutilus" (which is the largest North American rhinoceros) and the smallest is the type species "A. megalodus". "A. mutilus" has been estimated to have weighed , and "A. malacorhinus" has been estimated at . "Aphelops" can be distinguished by other members of the Aceratheriinae by two traits: the arched top  of the skull, and the long diastema (gap) between the second incisor (lower tusk) and first premolar. Many other aspects of its anatomy are typical of aceratheriines, including: the absence of a horn on the broad, unfused nasal bones; the reduced premaxilla and lost first incisor; a nasal incision (or notch below the nasal bones) reaching at least the level of the fourth premolar; a triangular-shaped skull when viewed from the rear; narrow zygomatic arches; brachydont or low-crowned teeth without cement; upper molars bearing a fold of enamel known as an anterocrochet; and lower tusks that are subcircular  in cross-section. More derived (specialized) species have larger teeth and longer, more slender nasal bones (also producing a larger incision). The various species also differ in the depth of the nasal incision: in "A. megalodus", it reaches the front of the fourth premolar; in "A. malacorhinus", it reaches the rear of that tooth; and in "A. mutilus", it reaches the first molar. Palaeobiology. The lower tusks of "Aphelops" are informative about the sex and age of individuals, as in "Teleoceras": males have much thicker tusks, while older individuals have more strongly erupted tusks showing more extensive wear patterns. The  stages in the eruption of the molar-like cheek teeth were identical to living rhinos. These factors were used by Matthew Mihlbacher in 2003 to analyze a bone bed of "Aphelops" known as the Love Island bonebed; adults (between 35% and 68% of maximum lifespan) are most common, there appears to be no male bias, and young males are rare. This is unlike "Teleoceras", where young males are disproportionately preserved in bone beds. Such a difference may imply different social behaviour: young male "Aphelops" perhaps died less frequently from fighting, and used different kinds of ritualistic displays. "Aphelops" were probably  not completely monogamous, with the degree of sexual dimorphism being suggestive of polygyny. Palaeoecology. "Aphelops" is thought to have been a browser on C plants, like the modern black rhinoceros, with its longer limbs adapted to traversing open, brushy country. Isotopic evidence suggests that, in the late Miocene, "Teleoceras" had limited intake on newly-emergent C grasses, but "Aphelops" continued to browse. This difference in feeding ecologies explains how they could coexist sympatrically across many environments for 13 million years. Both went extinct at the end of the Hemphillian North American land mammal age, most likely due to rapid climate  cooling, increased seasonality and expansion of C grasses.