Nesorhinus hayasakai
# Nesorhinus hayasakai

Nesorhinus hayasakai is an extinct species of rhinoceroti. It was originally diagnosed as the sub-species of Rhinoceros sinensis, but was revised as a newly named Nesorhinus in 2021. N. hayasakai fossils were discovered in Neizha, Daxi District in Taoyuan City as well as Cailiao River , Zuojhen District in Tainan City. Potential fossil records have also been found in Dakeng, Taichung and Qijin, Kaohsiung.

## Research history

The earliest record of rhino fossils in Taiwan dates back to 1926, when Sato collected mandibular bone and molar from Neizha of Daxi Street (present-day 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, Hayasaka pointed out that in addition to Daxi, there were other sites like Dakeng (present-day Dakeng, Beitun District, Taichung City), Zuojhenzhuang (present-day Zuojhen District, Tainan City) and Qihou (present-day Qijin District, Kaohsiung City) are all suspected to have records of new fossil species of Rhinoceros.

In 1971, when the fossil collector Chang-Wu Pan 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 investigated where the rhinoceros teeth were found and speculated that the fossils had been washed out of the mudrock layer along the riverbed, and that there might be remaining rhinoceros fossils in the rock layer. This discovery was conveyed to Prof. Chao-Chi Lin of 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 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, Rhinoceros philippinensis found in Philippines was revisited and studied. It was found to share distinct features with the Rhinoceros sinensis hayasakai, unlike other Asian taxa Rhinoceros and Dicerorhinus, should belongs to a endemic clade of East Asia. Both species were established together as the new genus Nesorhinus, and the Rhinoceros sinensis hayasakai was thus revisited 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 root fully isolated on upper cheek teeth, the crochet usually present on P2-4, the absence of the crista on P3, the absence of protocone constriction and depressed posterior ectoloph concave on M1-2, trigonid angular on lower cheek teeth, supraglenoid tubercle convex and salient on the scapula.

Nesorhinus hayasakai has three autapomorphic features, all of which are present on the teeth: present of crochet on P2-4, absence of lingual cingulum on the upper molars, and trigonid on the lower cheek teeth 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 recovered as rhinocerotina and rhinoceroti based on the reconstruction of the phylogenetic tree, and according to the plesiomorphy and symplesiomorphy of this group, it is likely that the Nesorhinus had only a one horn (i.e., only a nasal horn and no frontal horn).

The average weight of the Nesorhinus hayasakai was estimated to be about 1263 kg based on the size of the teeth, with the upper and lower limits range 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 rhinoceros, and slightly larger than Nesorhinus philippinensis.

## Phylogenetic tree

The phylogenetic tree of Rhinocerotinae reconstructed by Antoine et al. in 2022 indicates that the Nesorhinus are more closely related to the Dicerorhinus and the Rhinoceros:

## Spatiotemporal Distribution

Nesorhinus is mainly distributed in Taiwan and the Philippines from the Early to Middle Pleistocene. There are three main hypotheses for the pathways of biological dispersal in Southeast Asia during the Pleistocene, including:

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

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

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

The phylogenetic reconstruction and paleobiogeographic analyses suggest that the common ancestor of the Nesorhinus, Rhinoceros and Dicerorhinus) probably originated from the Asian continent, and the common ancestor of the Nesorhinus is more likely to have originated from regions such as Southeast Asia or South Asia rather than 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. 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 fall about 120 m during the Miocene or Pleistocene. Therefore, considering the swimming ability of extant rhinoceroses, the sea level fall 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 slender limbs of the Nesorhinus philippinensis may be the result of insular dwarfism. Whether such an evolutionary adaptation also occurred in the Nesorhinus of Taiwan is still unknown.

## Paleoecology

The fossils of Nesorhinus hayasakai from Zuojhen belong to the Chiting Formation, which is a interbeded sandstone and shale facies, 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 Chochen fauna, which includes large animals such as Mammoths, Elaphurus, wild boars, Rhinopithecus, Homotherium, Toyotamaphimeia that cohabitated with the Nesorhinus hayasakai.

