Saturday 26 April 2014

A tandem-horned Elasmothere Rhinoceros from the Late Miocene of Gansu Province, northwest China.

Rhinoceroses are iconic members of the modern Mammalian megafauna, distinguished by their large bulk, thick hides and horns. There are five modern species of Rhinoceros from Africa and Asia, three of which are considered to be Critically Endangered under the terms of the International Union for the Conservation of Nature's Red List of Endangered Species, with the two remaining being considered Vulnerable and Near Threatened. The earliest Rhinoceroses appear in the fossil record in the Early Eocene in North America. These animals were more Horse-like than Rhinoceros-like in appearance, and the smallest were no bigger than a Dog.  Elasmothere Rhinoceros first appeared in South Asia in the Early Miocene and persisted till the Late Pleistocene.

In a paper published in the Chinese Science Bulletin in May 2013 Deng Tao of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and the Department of Geology at Northwest University, and Wang ShiQi and Hou SuKuan also of the Key Laboratory of Evolutionary Systematics of Vertebrates, describe the partial skull of an Elasmothere Rhinoceros from the Late Miocene Liushu Formation at Huaigou in Guanghe County, Gansu.

The skull is assigned to the previously described species Sinotherium lagrelii, but contains details not previously known in this species. Previously specimens of Sinotherium lagrelii, from Shanxi Province, Mongolia and Kazakhstan were known only from isolated teeth and fragments of mandible (jawbone) and cranium (skull), so while the Gansu specimen is incomplete, it reveals a lot more about the skull morphology of the species than was previously  known. 

Skull of Sinotherium lagrelii from the Linxia Basin. (a) Dorsal view; (b) lateral view; (c) ventral view. alf, anterior lacerate foramen; bt, basal tuberosity; cf, condyloid fossa; eam, external auditory meatus; fhb, frontal horn boss; gf, glenoid fossa; hf, hypoglossal foramen; in, intercondyloid notch; plf, posterior lacerate foramen; lt, lacrimal tubercle; mp, muscular process; nhb, nasofrontal horn boss; nt, nuchal tuberosity; oc, occipital condyle; pc, parietal crest; pgc, pterygoid canal; pgp, postglenoid process; pop, paroccipital process; pp, postorbital process; ptp, posttympanic process; st, supraorbital tuberosity; tc, temporal condyle; tpc, temporal crest; za, zygomatic arch. Deng et al. (2013).

The Huaigou specimen comprises the rear portion of the skull, including the posterior part of the nasal and maxilla. The preserved portion of the nasal supports a large elevated boss, with a rough surface that is associated with the presence of a horn in Rhinoceros skulls (Rhinocerous horns are comprised of keratin – hair – rather than bone, and do not necessarily survive even in conditions good for bone preservation). Behind this is a smaller depression also with a rough surface. Deng et al. interpret this as evidence for the presence of two horns high on the head of Sinotherium lagrelii.

Among Elasmothere Rhinoceros the large, Pleistocene, Elasmotherium sibiricum had a distinctive two meter horn on its forehead, while all other species in which the positioning of the horns is known have a single large horn on the front portion of the nasal (i.e. the end of the nose) similar to that seen in modern Indian and Javan Rhinoceros. Deng et al. speculate that the twin horned Sinotherium lagrelii might be intermediate between these two states, although since both horns of Sinotherium lagrelii are located on the forehead, and the presence or absence of a horn on the tip of the nose is unknown, this seems a slightly tenuous claim.

A series of six Elasmothere species from the Middle Miocene to the Late Pleistocene. They display an increase in skull size and development from a nasal horn to a frontal horn. These skulls are reconstructed based on specimens from Tunggur in Inner Mongolia, Middle Miocene for Hispanotherium tungurense, Houshan in Guanghe, Gansu, Late Miocene for Iranotherium morgani, Guonigou in Dongxiang, Gansu, Late Miocene for Parelasmotherium linxiaense, Guonigou in Dongxiang, Gansu, Late Miocene for Ningxiatherium euryrhinus, Huaigou in Guanghe, Gansu, Late Miocene for Sinotherium lagrelii, and Sarepta in Russia, Late Pleistocene for Elasmotherium sibiricum. Deng et al. (2013).

The Liushu Formation of the Linxia Basin comprises a 100 m thick sequence of light yellowish brown carbonate-cemented siltstones intercalated with a few thin beds of mudstones and marls. It has been dated to between 11 and 6.4 million years old using palaeomagnetic dating (the Earth undergoes occasional reversals of its polarity, which in some circumstances is recorded by the alignment of iron particles in sedimentary rocks; since these reversals are irregular and global, a pattern can be recognised in rocks from very different locations, and used to establish a dating sequence), with the Huaigou locality having a palaeomagnetically derived age of about 7 million years. As well as Sinotherium lagrelii the site has yielded a Bear, a Badger-like Mustelid, three Hyenas, three Felids, a Chalicothere, a Horse, a Deer, a Giraffe and two Bovids. The Bovids and Horse belong to species that have been found in Late Miocene deposits elsewhere. Analysis of pollen from the site suggests that the area was probably an arid grassland with occasional stands of broad-leaved trees.

Habitat reconstruction of Sinotherium lagrelii in the Linxia Basin during the Late Miocene. Deng et al. (2013).

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