A restudy of Rhinocerotini fossils from the Miocene Jiulongkou Fauna of China

ll the extant rhino species belong to Rhinocerotini and either have one horn (a nasal horn) or two horns (a nasal horn and frontal horn). So far, the earliest Rhinocerotini to have been identified in China is the “Dicerorhinus” cixianensis, which was based on a juvenile skull with an associated mandible from the Middle Miocene locality of Jiulongkou in Cixian County, Hebei Province of northern China. Our analyses suggest that there are similarities between this specimen and the modern genus, Dicerorhinus, but it differs in several cranial traits and therefore cannot be assigned to the modern genus. Instead, it is closer to the Middle Miocene Lartetotherium from Europe, especially the specimen from La Retama in Spain and should be assigned to that genus, indicating the presence of intracontinental dispersal at this time. The Jiulongkou fauna is the only Middle Miocene fauna with Rhinocerotini in China, and, together with the faunal composition, this implies a more humid and closed environment, in contrast to those found in western China. We suggest that the position of the posterior border of the nasal notch is a good indication of the specimen’s evolutionary level in Rhinocerotini. The anterior position of the nasal notch as seen in modern Dicerorhinus, together with its certain similarities to L. cixianensis as well as its differences with more specialized species of the Dihoplus-Pliorhinus-Stephanorhinus -Coelodonta lineage, supports the conclusion that Dicerorhinus experienced little change during a nearly 10 Myr evolutionary history, possibly due to the low selection pressure seen in the tropical/subtropical forests in southeastern Asia.