A new species of Tsaidamotherium(Bovidae,Artiodactyla),T.brevirostrum,described here represents only the second discovery of the genus.Tsaidamotherium is a mid-sized late Miocene bovid with an odd-looking plate-like horn-core apparatus so far known only from northwestern China.The holotype of T.brevirostrum is a cranial part of skull from the Liushu Formation(late Miocene)of the Linxia Basin.The new species differs from the type species,T.bohlini,mainly in horn-core morphology,having the frontal horn-cores proper much smaller and less distinctly separated from the plate-shaped posterior portion.Furthermore,the new species has smaller frontal sinuses,but a thicker layer of compact bone beneath the horn-core apparatus as seen in frontal section.A complete skull with a smaller horn-core apparatus is considered to represent a female individual of T.brevirostrum.The facial part of the skull,which is first known for this genus,is very short and high,with a high nasal cavity,strongly retracted nasals,and shortened premaxillae and premolar rows.All this probably suggests that the animal with an enlarged nasal cavity,like the modern Saiga and Budorcas,may have had a special adaptation to the high plateau environment.
Although the modern Indian and Javan rhinos have a single horn on their noses,the extinct one-horned rhino Elasmotherium was a source for the legendary unicorn,because the latter had a very long horn on its forehead and lived with the prehistoric human beings who drew its images on cave paintings.Elasmothere rhinos first appeared in South Asia in the Early Miocene,but the origin of Elasmotherium has been unclear.All other elasmotheres have a weak or strong nasal horn,whereas Elasmotherium seems to lose the nasal horn of its ancestors and to get a huge frontal horn apparently abruptly.Here we report the first discovered skull of Sinotherium lagrelii from the Late Miocene red clays in the Linxia Basin,northwestern China.This skull has an enormous nasofrontal horn boss shifted posteriorly and a smaller frontal horn boss,which are connected to each other,indicating an intermediate stage for the single frontal horn of Elasmotherium.Morphological and phylogenetic analyses confirm that Sinotherium is a transitional taxon between Elasmotherium and other elasmotheres,positioned near the root of the giant unicorn clade and originated in a subarid steppe.The posteriorly shifted nasal horn has a more substantial support and the arched structure of the nasofrontal area is an adaptation for a huge horn.
