Sinotherium

The type species of Sinotherium is S. lagrelii. It is also known to have an additional species from the Zaisan depression of Kazakhstan called S. zaisanensis, however, doubt has been raised on its validity.[1]

Finds of Sinotherium are rather rare and often only fragmentary. The first fossils, which also led to the description of the rhinoceros genus, came to light at the beginning of the 20th century and were discovered by JG Andersson in the Baode district in the Chinese province of Shanxi in deposits from the Upper Miocene. These mainly consisted of isolated teeth, but also an upper jaw fragment with the preserved row of teeth from the second premolar to the penultimate molar and a lower jaw fragment.[2][3] From north-western Mongolia near Chono-Khariakha, a 72 cm long, well-preserved lower jaw was discovered which dates to the Lower Pliocene.[4] Other individual finds are known from Kazakhstan, including a rear part of the skull with part of the teeth and several skeletal elements of the body.[1] The most complete skull to date was found in the upper area of the Liushu Formation near Houaigou in the Guanghe District of Gansu Province. The Liushu Formation is about 100 m thick and over wide ranges of Linxia basin digested. This section is dated to about 7 to 6.4 million years and thus belongs to the end of the Miocene. The geological deposits of the Linxia Basin have already produced numerous well-preserved fossil rhinoceros remains, including numerous representatives of the Elasmotheriinae. Only the part of the snout is missing from the skull and it provided evidence of the location of the horns in Sinotherium.[5]

The horns of older elasmotheriines are present on their nasals (nose), whereas the horn of Sinotherium's descendant Elasmotherium is present on its frontals (forehead), Sinotherium shows a unique condition in which its horn is present in an intermediate "naso-frontal" position.[5] This represents the horn shifting from its ancestral nasal position to the derived frontal position, eventually resulting in the completely frontal restricted position of Elasmotherium.[5]

In addition to the nasofrontal horn, Sinotherium also preserves a rugosity on its forehead, just behind the nasofrontal horn, which implies that the animal had two horns.

Early elasmotheriine genera of the line leading to Elasmotherium, have a dolichocephalic skull supporting a horn growing on their nose, just like any other Rhinocerotid, however, Elasmotherium was the sole member of Elasmotheriinae that had a brachycephalic skull and supported a horn on its forehead instead.[5] A transition between this state of nose-horned to forehead-horned elasmotheriines remained missing until 2012, when the first cranial remains of Sinotherium lagrelii, (specimen IVPP V 18539, a partial skull housed at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in Beijing) was described, demonstrating a very important transition from nose-horned elasmotheriines like Ningxiatherium to forehead-horned elasmotheriines like Elasmotherium.[5]

While the horns of Ningxiatherium-like elasmotheriines are present on their nasals, and the horn of Elasmotherium on its frontals, Sinotherium shows its horn to be present in a "nasofrontal" position, (present on both the nose and the forehead).[5] This shows the horn shifting from its ancestral nasal position to a more derived frontal position, eventually resulting in the completely frontal restricted position of Elasmotherium.[5]