Although modern Indian and Javan rhinos have a single horn on their noses, the extinct one-horned rhino Elasmotherium was a source of the unicorn legend because it had a two meter-long horn on its forehead and lived with prehistoric humans that drew its image on cave paintings.
All other elasmotheres had a weak or strong nasal horn, whereas Elasmotherium lost its ancestral nasal horn and instead developed a long frontal horn. Dr. DENG Tao (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences) and his colleagues reported the first discovered skull of Sinotherium lagrelii (Figure 1) from Late Miocene red clays (~7 Ma) of the Linxia Basin, northwestern China (see Chinese Science BulletinVol. 58, No. 15, pages 1811-1817).
The transition from a nasal horn to a frontal horn in elasmotheres has been difficult to explain because a major transformational gap exists between nasal-horned ancestors and frontal-horned descendants. This skull has connected a large posterior nasofrontal horn boss and a smaller frontal horn boss, indicating an intermediate stage to the single frontal horn of Elasmotherium. Morphological and phylogenetic analyses confirm that Sinotherium is a transitional taxon between Elasmotherium and other elasmotheres.
It is positioned near the root of the giant unicorn clade and originated in a subarid steppe. A posteriorly shifted nasal horn provides more substantial support and the arched structure of the nasofrontal area is an adaptation for a huge horn. The S. lagrelii skull provides new information about the origin of the giant unicorn Elasmotherium.
Previously, S. lagrelii was only represented by cranial and mandibular fragments and isolated teeth from Late Miocene deposits in China (Baode County, Shanxi Province), Kazakhstan, and Mongolia, so its horn situation was unknown. S. lagrelii is the closest to the Pleistocene Elasmotherium in dental morphology, but the nature of its frontal horn was undetermined. This skull proves that S. lagrelii has a posteriorly displaced nasofrontal horn, derived from the configuration seen in early elasmotheres. It also has a smaller frontal horn so it differs from both Ningxiatherium (single nasal horn) and Elasmotherium (single frontal horn). It is a morphological intermediate in the nasal-to-frontal horn transition of elasmotheres that connects the evolution and biogeography of the derived elasmotheres (Figure 2).
The nasofrontal area of the skull is strongly elevated, rough, and forms a huge and hollow dome, in sharp contrast to the flat, smooth area in large nasal-horned elasmotheres, such as Iranotherium, Parelasmotherium, and Ningxiatherium. This reduces the weight of the nasal and frontal bones. The nasal horn boss is shifted posterior to reach the frontal bone and connect to the frontal horn boss. This particular horn combination has not appeared in any other extinct or extant rhinoceros. The dorsal surface of the horn bosses has many massive swellings to strengthen the adhesion of a huge nasal horn and a smaller frontal horn. The ventral surface has an ossified sagittal septum and many oblique lateral ribs that form a trussed structure of enhanced support similar to the leaf structure of the giant waterlily (Victoria). An enlarged nasal horn without other compensation would make support impossible, even with an ossified nasal septum, so the nasal horn has to shift towards the frontal bone.