Abstract:

All 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.

Key words: Jiulongkou, Cixian, Hebei, Middle Miocene, Lartetotherium cixianensis, Rhinocerotini

摘要：

现代犀牛都属于犀科中的真犀族，目前中国最早的真犀族动物化石记录是来自河北磁县九龙口中中新世的磁县“双角犀”, 化石材料为幼年头骨和下颌。分析研究认为：这个物种与现生的双角犀Dicerorhinus的确存在相似之处，但也存在一些头骨结构上的差异，不应该被归入现生属。九龙口的真犀其实更加接近欧洲中中新世的拉尔泰犀Lartetotherium, 尤其是西班牙La Retama中中新世的材料，并应该归入该属。这一发现指示此时在欧亚大陆两端存在生物迁徙交流。九龙口动物群是中国中中新世目前唯一记录有真犀族动物的化石点，说明其环境相对湿润封闭，和当时中国更靠西北的化石点存在差异。研究发现鼻切迹位置是真犀族动物演化中具有较好演化水平指示意义的特征。现生苏门犀鼻切迹相对靠前以及其整体与磁县拉尔泰犀的相似性，指示这个属在与史蒂芬犀、披毛犀等更特化的双角犀分化后超过1000万年的演化历史中，只经历了比较小的形态学变化，这可能和南亚/东南亚地区环境相对稳定，选择压力较低有关。

关键词: 河北磁县九龙口, 中中新世, 磁县拉尔泰犀, 真犀族