Iranotherium

Iranotherium ("Beast of Iran") is an extinct genus of large elasmotheriine rhinocerotids, comparable in size to a modern white rhino. It is known from the Late Miocene (Tortonian) of Maragha, Iran and the middle part of the Liushu formation of northwestern China. It was a precursor to the related Sinotherium and may have been ultimately outcompeted by its descendant. This species is most well known for showing unique sexual dimorphism among rhinos.

Iranotherium is only known from cranial material, and rare postcranial elements.

The skull is long and dorsally concave, with a length of 775 mm. The parietal crests are broad, the nasal bone is long and wide, with a shallow U-shaped nasal notch, the orbit is prominent and has a high position close to the skull roof, the first half of the zygomatic arch is narrow. The skulls also show distinct sexual dimorphism: The base from which the horn grows is large and strong in the male, while it is small and weak in the female, and there is strong and rough rugosities present on the back half part of the zygomatic arch of the male. The back half of the zygoma is also uplifted and higher than the skull roof.

Iranotherium teeth are hypsodont, (they were covered and filled by cement), meaning that Iranotherium was a grazer that fed on grasses. Its upper incisors are completely absent and the premolars are significantly shortened, however, the cheek teeth are longer in the male than in the female.

When Iranotherium was originally discovered from teeth, it was recognized that this species was similar to the rhino Elasmotherium and was the ancestor of the latter (This was before Sinotherium was discovered), however in 1908, when it was published as a new species, it was strangely referred to the genus Rhinoceros. In 1924, the material of R. morgani from Maragha was further described. Ringström suggested that this species did not belong in Rhinoceros, referring it to his new genus Iranotherium as I. morgani.

Iranotherium as a genus was originally described in 1908 by Mecquenem where no holotype was designated for the genus. Specimen MNHN 1905–10, and atlas from Maragha, Iran was chosen as the Lectotype for this genus.

In 2005, three specimens of Iranotherium were described by Tao Deng. Two of these specimens were complete skulls that showed evidence of unique sexual dimorphism. These specimens are: HMV 0979, a complete young male skull, HMV 1098, a complete adult female skull and finally HMV 1099, known only from a mandible belonging to an adult that is missing the ascending ramus. All specimens were found from the lower part of the upper Miocene Liushu Formation in the Linxia Basin (Gansu, China). HMV 0979 and HMV 1099 were discovered at Houshan village, and HMV 1098 at Shanzhuang village.

The studied skulls and mandible from the Linxia Basin, especially HMV 0979 and HMV 1099, are found to be generally similar to the skull and mandible of Iranotherium morgani from Maragha, with minor differences, like the length ratio of lower premolars to molars. This difference is considered to be due to individual or sexual variations and is not sufficient for establishing a new species. Hence, all three specimens described have been assigned to the genus Iranotherium by Deng. Based on the two skulls from the study, the Maragha specimen is identified as a male due to the presence of hemispherical rugosities on its zygoma.