Biology:Nesorhinus hayasakai

Short description: Species of mammal (fossil)

Nesorhinus hayasakai Temporal range: 0.9–0.45 Ma PreЄ Є O S D C P T J K Pg N ↓
Skeletal mount of N. hayasakai
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Rhinocerotidae
Genus:	†Nesorhinus
Species:	†N. hayasakai
Binomial name
†Nesorhinus hayasakai (Otsuka & Lin, 1984)

Nesorhinus hayasakai was originally classified as sub-species of the extinct rhinoceros species Rhinoceros sinensis, but in 2021, it was reclassified as a distinct and newly named species, known as Nesorhinus. Nesorhinus fossils were unearthed in Daxi DistrictNeizha in Taoyuan City as well as Zuojhen District and Cailiao River in Tainan City. Suspected fossil records have also been found in Dakeng, Taichung and Qijin inKaohsiung.

Research history

The earliest documented evidence of rhinoceros fossils in Taiwan can be traced back to the year 1926, when Sato collected mandibular bone and molar from Neizha of Daxi Street, Daxi District, Shinchiku Prefecture (present-day grate in Daxi District, Taoyuan City). A few years later, Ichiro Hayasaka of Taihoku Imperial University also rediscovered a more complete mandible near the site of Sato's specimen. In 1942, he pointed out that in Taiwan, in addition to Daxi, there were other sites in Dakeng, Taichu Prefecture (present-day Dakeng, Beitun District, Taichung City), In Zuojhenzhuang, Xinhua District, Tainan State, and Zuozhenzhuang (present-day Zuojhen District, Tainan City) and Qihou, Takao City, Takao Prefecture (present-day Qijin District, Kaohsiung City) are all suspected to have records of new fossil species of Rhinoceros.

In 1971, when the famous fossil collector Chang-Wu Pan of Tainan visited Zuojhen, he received several fossilized rhinoceros teeth from a local elementary school student, Shih-Ching Chen, who had found them in the riverbed of Cailiao River. Pan and Chen conducted an inquiry into the origin of the rhinoceros’ teeth. Their speculation suggested that the fossils had been dislodged from the mud rock layer along the riverbed, prompting intriguing possibilities of yet undiscovered rhinoceros remains concealed within the geological strata. This remarkable finding was brought to the attention of Prof. Chao-Chi Lin from National Taiwan University. In December 1971, a team was formed by Taiwan Provincial Museum, including Prof. Chao-Chi Lin, Chang-Wu Pan, and Chun-Mu Chen, a well-known fossil collector from Zuojhen, to conduct the first excavation in Zuojhen to excavate rhinoceros fossils including teeth and limb bones, but some limb bones were still preserved in the original stratum. In 1972, the team was joined by Japanese paleontologists Tokio Shikama and Hiroyuki Otsuka for the second excavation, and the remaining limb bones were excavated.

In 1984, Hiroyuki Otsuka and Prof.Chao-Chi Lin named the specimens collected by Ichiro Hayasaka from Daxi, the specimens excavated by the Provincial Museum of Taiwan, and the rhinoceros fossil specimens collected from Zuojhen in the Tainan County Fossil Museum as Rhinoceros sinensis hayasakai to commemorate Ichiro Hayasaka's research and discovery of rhinoceroses fossils in Taiwan.

In 2021, the Rhinoceros philippinensis found in Philippines was revisited and studied and found to share distinct features with the Rhinoceros sinensis hayasakai, unlike the Asian rhinoceros and  dicerorhinus, a distinctive clade of East Asia, were established together as the new genus Nesorhinus, and the Rhinoceros sinensis hayasakai was thus reclassified as a new combined species, Nesorhinus hayasakai.

Morphological description

Nesorhinus is a medium-sized rhinoceros that differs from other species of the rhinoceros family by six distinguishing features, including the complete separation of the roots of the upper cheek teeth, the usual crochet on P2-4, the absence of the crista on P3, the absence of protocone constriction on M1-2, depressed posterior half of ectoloph, trigonid angular on lower cheek teeth, supraglenoid tubercle raised posteriorly on the scapula.

Nesorhinus hayasakai has three autapomorphic features, all of which are located on the teeth: crochet on P2-4, absence of lingual cingulum on the upper molars, and trigonid on the lower cheek teeth, which are acutely dihedral in the occlusal plane.

Although no nasal or frontal have been found in specimens of Nesorhinus hayasakai and Nesorhinus philippinensis, they are classified as rhinocerotina and rhinoceroti based on the reconstruction of the phylogenetic tree, and with reference to the plesiomorphy and symplesiomorphy of this group. It is likely that the Nesorhinus had only a single branchial horn (i.e., only a nasal horn without a frontal horn).

Based on the tooth dimensions, the Nesorhinus hayasakai was estimated to have an average weight of approximately 1263 kg, with the upper and lower limits falling between 1018-1670 kg, and 1306 kg based on the size of the radius . By comparing the size with extant rhinoceroses forelimbs, it is assumed that the Nesorhinus hayasakai is about 1.31 m tall, similar in size to the extant Sumatran rhinoceroses, and slightly larger than the rhinoceros of the same genus, the Nesorhinus philippinensis.

Phylogenetic tree

The phylogenetic tree of Rhinocerotinae reconstructed by Antoine et al. in 2022 indicates that the Nesorhinus genus and the Dicerorhinus are more closely related to the one-horned rhinoceros.

Tapirus terrestris     †Hyrachyus eximius       †Trigonias       †Ronzotherium     † Elasmotheriinae †Subhyracodon occidentalis   Rhinocerotinae   Rhinocerotini Rhinocerotina Diceroti               Ceratotherium simum     Diceros bicornis       † Ceratotherium neumayri       †Coelodonta antiquitatis       †Stephanorhinus       † Stephanorhinus etruscus       †Dihoplus pikermiensis       †Dihoplus schleiermacheri     Rhinoceroti     †Lartetotherium sansaniense     †Gaindatherium browni         Dicerorhinus   Dicerorhinus sumatrensis     †Dicerorhinus     Rhinoceros         †Rhinoceros platyrhinus     †Rhinoceros kendengindicus       Rhinoceros unicornis       †中Rhinoceros sinensis       Rhinoceros sondaicus       †Nesorhinus   †Nesorhinus philippinensis     † Nesorhinus hayasakai           † Teleoceratina   † Brachypotherium perimense     † Teleoceras fossiger       † Aceratheriini   † Alicornops       †Hoploaceratherium tetradactylum     † Aceratherium incisivum           † Plesiaceratherium

†

Spatiotemporal Distribution

The genus Nesorhinus is mainly distributed in Taiwan and the Philippines from the Early to Middle Pleistocene. Within the context of the Pleistocene era, three primary hypotheses exist regarding the mechanisms of biological dispersal in Southeast Asia, including:

(1) Dispersal from Siwaliks through the ice-age exposed Sunda Shelf (Siva-Malayan route), and

(2) Dispersal from China to Indonesia via Thailand (China-Malayan route) and

(3) From Taiwan to the west and south via the Philippines across the ocean barrier.

The sibling reconstruction and paleobiogeographic analyses suggest that the common ancestor of the Nesorhinus, Rhinoceros and Dicerorhinus) probably originated from the Asian continent, and the ancestral roots of the Nesorhinus genus trace back to regions such as Southeast Asia or South Asia as opposed to the Sunda region, and has a higher probability of spreading across the sea from Taiwan to the Philippines (i.e., hypothesis (3) above). The extant Asian rhinoceros has the ability to cross rivers and marine corridors ^[5][6], and the closest distance between Taiwan and Luzon Island in the Philippines was only about 57 km during the glacial maximum period when the sea level dropped about 120 m during the Miocene or Pleistocene. Considering the swimming ability of extant rhinoceroses, the sea level drop during the ice age and the prevailing southward blowing Southeast Asian winds, it cannot be ruled out that the Nesorhinus swam across the Luzon Strait and spread southward at that time.

The lighter weight and slimmer limbs of the Nesorhinus philippinensis may be the result of insular dwarfism. Whether such an evolutionary phenomenon also occurred in the Nesorhinus of Taiwan is still unknown.

Paleoecology

The fossils from Zuojhen belong to the Chiting Formation, which is a sandshale interbedding of marine phasesediment, with a geological age of about 990,000 to 460,000 years ago (Early to Middle Pleistocene). . The fossils from the Chiting Formation suggest that the habitat of the Nesorhinus hayasakai was probably an open grassland, and the animals in the ecosystem are collectively known as the Zuojhen fauna, which includes large animals such as mammoths, elaphurus, wild boars, golden snub-nosed monkeys, homotherium, Toyotamaphimeia and other large animals that cohabitated with the Nesorhinus hayasakai.

References

Wikidata ☰ Q65060579 entry