Niviventer Marshall, 1976

Nishioka, Yuichiro, Nakagawa, Ryohei, Nunami, Shin & Hirasawa, Satoshi, 2016, Small Mammalian Remains from the Late Holocene Deposits on Ishigaki and Yonaguni Islands, Southwestern Japan, Zoological Studies 55 (5), pp. 1-21 : 15-18

Genus Niviventer Marshall, 1976

Material examined: A left maxilla (MPM-Fo 2913); three left M 1 (MPM-Fo 2915-2917); two left M 2 (MPM-Fo 2918-2919); a right M 2 (MPM-Fo 2920); a left M 3 (MPM-Fo 2921); a right mandible with M 1 and M 2 (MPM-Fo 2922); six left M 1 (MPM- Fo 2902, 2923-2927); four right M 1 (MPM-Fo 2928- 2931); a left M 2 (MPM-Fo 2911); a right M 2 (MPM- Fo 2912); two left M 3 (MPM-Fo 2932-2933); a right M 3 (MPM-Fo 2934).

Locality: Sabichi Fissure, Ishigaki Island; Layers 5-6 and indeterminate layer of Umabana-zaki Fissure, Yonaguni Island.

Measurements: See Tables 4 and 5.

Description: The crown of M 1 is higher than the radical part, in MPM-Fo 2915 and 2916. MPM-Fo 2917 preserves only a crown part due to being young or broken, but is as high as the other specimens of M 1. The occlusal surface has a slender, oval-shaped outline, and comprises three transverse laminae. The anterior lamina forms an asymmetric chevron, because the labial anterocone is more reduced than the anterostyle. The anterostyle shows a circle without a posterior spur. The labial anterocone also lacks a posterior spur. The middle lamina is similar to the anterior lamina, but is symmetrical. The occlusal outline is slender (bucco-lingually narrow) because the middle lamina bends at a right or acute angle. There is no spur on the enterostyle and paracone of the middle lamina. The posterior lamina is simple, comprising large hypocone and small metacone. Either posterostyle or posterior cingulum is absent. The hypocone expands antero-posteriorly, as a rounded diamond shape. The metacone is very small, and separated from the paracone by a gap. The number of roots is five: large anterior root, large posterior root, small buccal root, and two small lingual roots.

The crown of M 2 (MPM-Fo 2920) is as high as the longest root of the tooth. It has a rounded triangular outline in the occlusal view. The occlusal pattern is simple, without any additional spurs or cusps. The labial anterocone is always absent. The anterostyle is well-developed, with a circular or semi-circular outline. The anterostyle is isolated from the first lamina, even at a strong wear stage. The first lamina of M 2 is similar to the middle lamina of M 1. The joint between the enterostyle and the protocone is clearly constricted. Each enterostyle and paracone of MPM-Fo 2919 connects with the posterior lamina, or the hypocone, due to wear. The hypocone forms a circular shape, without either posterostyle or posterior cingulum. The number of roots is basically four.

The occlusal surface of M 3 (MPM-Fo 2921) is strongly worn, showing an inverted triangular outline. There is a weak sinus between the anterostyle and the enterostyle. The enterostyle, paracone, and hypocone are fused to one another, and form an isolated fold on the postero-buccal part. There are three roots.

The crown of M 1 is comparatively high even at a strong wear point. The occlusal surface, with a rounded rectangular outline, comprises three laminae and a posterior cingulum. The anterior lamina is composed of two cusps, i.e. the labial anteroconid and the lingual anteroconid. The anterior lamina shows a semicircular shape in the occlusal view, owing to strong wear. The lingual anteroconid is slightly larger than the labial anteroconid. The middle lamina forms a chevron shape by the protoconid and the metaconid, and connects to the anterior lamina at the center. The protoconid is as large as the metaconid. The posterior lamina is also chevron-shaped, comprising the hypoconid and the entoconid. This lamina has a mesial mure between the cusps. MPM-Fo 2902 has a cingulum at the buccal corner of the hypoconid, instead of the accessory cusps. MPM-Fo 2928 has neither accessory cusp nor cingulum on the occlusal surface. The posterior cingulum is large, isolated, and elongated bucco-lingually. There are four roots: the anterior and posterior roots are large, and the buccal and lingual roots are small.

The crown of M 2 is considerably higher, relative to its radical part. The occlusal surface shows a rounded quadrilateral outline, and comprises two chevron-shaped laminae and a posterior cingulum. The labial anteroconid is vestigial or absent at an early wear stage. The anterior chevron comprises the protoconid and the metaconid: the former is larger than the latter. The protoconid almost connects to the labial anteroconid. The posterior chevron is isolated from the first chevron and the posterior cingulum by deep gaps. The hypoconid is slightly larger than the entoconid. There is a weak cingulum on the buccal side of the hypoconid of MPM-Fo 2911, but any accessory cusps are absent in all specimens. The posterior cingulum shows a diamond shape in the occlusal view. There are two large roots.

The occlusal surface of M 3 shows an inverted triangular shape, elongated antero-posteriorly. The enamel pattern is composed of two transverse laminae. The labial anteroconid and any accessory cusps are absent. There are two roots: the anterior root is elongated transversely, drawing an arc, and the posterior root is well-developed antero-posteriorly.

Remarks: Rodents on Ishigaki and Yonaguni Islands are currently composed only of cosmopolitan (domestic) species, such as Rattus rattus , R. norvegicus , and Mus musculus ( Motokawa 2000) . The referred specimens belong to a medium-sized rat, and are characterized by slender outline, chevron-shaped laminae without spurs, small labial anterocone on M 2, five roots on M 1, and four roots on M 1. These characteristics are usually represented by common rats, such as R. rattus and R. norvegicus ( Musser 1981) . However, most of the molars are higher than those of Rattus , therefore, the form from Sabichi-do Cave and Umabana-zaki Fissure is different from both R. rattus and R. norvegicus occurring on Ishigaki and Yonaguni Islands. The Late Pleistocene sediments on Miyako Island recovered extinct murids, Rattus miyakoensis ( Kawaguchi et al. 2009) or Diplothrix sp. ( Nakagawa et al. 2012). This species resembles Diplothrix legata currently occurring on Amami, Tokunoshima, and Okinawa Islands ( Hasegawa 1985; Iwasa 2009b), but they are clearly larger than the specimens examined in this study.

More than 30 species of rats and mice live in the Indomalayan Region ( Corbet and Hill 1992), and some of them share tooth characters with Rattus . Chaimanee (1998) indicated that Rattus was phylogenetically close to Maxomys , Berylmys , Bandicota , Niviventer , and Leopoldamys , based on molar morphology. Furthermore, Zheng (1993) reported some extinct genera of rats from the Pleistocene of Central China, such as Qianomys and Wushanomys . Among these murids, the pattern of M 2 is the best similar to white-bellied rats, Niviventer , rather than the other genera in having no labial anteroconid, no buccal accessory cusp, and a single anterior root ( Musser 1981; Zheng 1993).

Musser (1981) listed tooth diagnosis of Niviventer as follow: (1) four roots beneath M 1 in all species; (2) lamina shaped like a chevron on M 2; (3) lower molars simple; (4) anterolabial cusp (labial anteroconid in this paper) on M 2 usually absent; (5) anterior lamina on M 1 composed of two small cusps, usually connecting to each other and forming an oblong or triangular lamina that is much narrower than the middle lamina. All of these features are observed in the specimens described in this study. Moreover, his comparisons indicated that the combination of slender outline and asymmetrical first lamina of M 1 always distinguish between Niviventer and Rattus .

Niviventer is one well-diversified genus in Asia, and includes 18 extant species and one extinct species: N. andersoni , N. brahma , N. cameroni , N. confucianus , N. coning , N. cremoriventer , N. culturatus , N. eha , N. excelsior , N. fratemus , N. fulvescens , N. hinpoon , N. langbinis , N. lepturus , N. lotipes , N. niviventer , N. precofucianus , N. rapit , and N. tenaster ( Musser 1981; Zheng 1993; Musser and Carleton 2005). According to these studies, all Niviventer species have small differences on upper molars, comparing with the fossil form from Umabana-zaki Fissure: N. andersoni and N. brahma have a distinct labial anterocone; N. eha has a posterior spur with the hypocone; N. excelsior and N. fulvescens have a prominent lingual anterocone; N. confucianus and N. hinpoon have the enterostyle extending posteriorly; N. coninga and N. culturatus have the protocone extending anteriorly and the enterostyle and paracone extending posteriorly; N. preconfucianus , which was found from the Pleistocene of China ( Zheng 1993), is similar in shape but smaller.

Niviventer is not currently distributed in Japan, including the Ryukyu Islands, but the referred specimens share many molar characteristics with Niviventer rather than with Rattus . In recent years, the Late Pleistocene and Holocene deposits on Ishigaki Island (Shiraho-Saonetabaru cave site) yielded a great deal of rodent remains, and most of these were also classified into a species of Niviventer ( Kawamura and Kawamura 2013) . This form is conspecific basically with the specimens described in this study.