Japaneomys yasunoi, Kimura & Tomida & Kalthoff & Casanovas-Vilar & Mörs, 2019

Kimura, Yuri, Tomida, Yukimitsu, Kalthoff, Daniela C., Casanovas-Vilar, Isaac & Mörs, Thomas, 2019, A new endemic genus of eomyid rodents from the early Miocene of Japan, Acta Palaeontologica Polonica 64 (2), pp. 303-312 : 306-308

Japaneomys yasunoi sp. nov.

Figs. 2–4 .

ZooBank LSID: urn:lsid:zoobank.org:pub:525E9D08-16F7-42B8-BCE4-6DBB1AC14B14

1994 Pseudotheridomys sp. ; Tomida and Setoguchi 1994: 191.

2011 Eomyidae gen. et sp. indet.; Tomida 2011: 2.

Etymology: Named after Toshikatsu Yasuno, who discovered the fossils while removing the matrix in search of fish fossils.

Type material: Holotype:NMNS-PV19995, isolated right m1. Paratype: NMNS-PV19994 fragmentary right dentary with incisor and p4 ( Fig. 2 ).

Type locality: Dota Town, Kani City , Gifu Prefecture, central Japan .

Type horizon: Near the uppermost level of the Nakamura Formation, Mizunami Group, early Miocene (~18.5 Ma).

Diagnosis.— Japaneomys yasunoi possesses a combination of dental characters that are rare in the Eomyidae ( Fig. 2 ): the presence of four roots in m1 and the hypolophid extending anteriorly to connect to the posterior ectolophid (≈ the anterior arm of hypoconid) on p4 and m1. These characters are present only in Asianeomys , Keramidomys , and Estramomys , the descendant genus of Keramidomys .

Differing from Asianeomys , Japaneomys yasunoi has a combination of the following characters: more bunodont pattern with lower lophids (i.e., very weak metalophid of p4 merged into the posterior wall of the metaconid and protoconid; mesoconid and ectolophid lower than hypoconid on p4; metalophid and hypolophid of m1, which run along pointy metaconid and entoconid, respectively, becoming very low in the talonid basin; hypolophid of m1 lower than the ectolophid), yet transverse lophids are complete, extending to the edge of the tooth; two rooted p4 (rather than three roots as in all species of Asianeomys but A. junggarensis ); anterior lobe narrower than posterior lobe on m1 (correspondingly, synclinid I of m1 much shallower and shorter than half the length of synclinid IV); hypolophid anteriorly concave on m1.

Sharedwith Asianeomys butdifferingfrom Keramidomys : absence of anteroconid on p4; anterior ectolophid connecting to the posterior wall of the protoconid rather than the occlusal surface of the protoconid on p4. Similar in size to Keramidomys , smaller than Asianeomys ( Fig. 3 ).

Description. —The right mandible is heavily damaged, with only the anterior half of the corpus preserved, retaining complete p 4 in the tooth socket and a portion of the incisor. The

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lower incisor is oval in cross section ( Fig. 4B ). The outer enamel surface is smooth and shows a shallow groove towards the labial side of the tooth. The lower m1 was isolated from the jaw probably during the preparation work, but the tooth sockets of m1 and m2 are preserved. The m2 and m3 were not discovered. The diastema is relatively shallow. The mental foramen is not preserved. The eomyid mandible is very uniform ( Engesser 1999), and that of Japaneomys yasunoi appears to be similar to that of other genera such as Eomys .

Lower fourth premolar (p4): Slightly worn specimen. Length 0.81 mm, width 0.63 mm in the anterior lobe, and 0.80 mm in the posterior lobe ( Fig. 3 ). Bunodont tooth with two roots, which are not bifurcated even at the apex of each root as confirmed in CT image. Trapezoidal outline with round corners in occlusal view. The width of the posterior lobe is almost equal to the length of the tooth. There is no indication of the anteroconid. The metaconid and protoconid are of the same size and are aligned transversely without relative displacement of either cusp. These cusps are very weakly connected posteriorly by an extremely weak and low metalophid that is merged into the posterior wall of the metaconid and the protoconid ( Fig. 2 ). The posterior side of the metaconid and protoconid form a single wall without being individualized. The anterior ectolophid is connected to the base of the protoconid at the posterolingual corner. The sinusid is nearly M-shaped. A long mesolophid is directed anterolingually and reaches the posterolingual corner of the metaconid, but it is extremely low. A mesostylid is present. The mesoconid has a bulbous base, which leaves the anterior and posterior ectolophid extremely short. The occlusal surface of the mesoconid and the ectolophid are lower than that of the hypoconid. The hypolophid runs downward steeply along the anteroposteriorly-compressed entoconid and becomes flat and low in the talonid basin, weakly connecting to the posterior ectolophid. The hypoconid is less compressed than the entoconid and is lower than the level of the metaconid and protoconid. A robust posterolophid is connected with the posterolingual corner of the hypoconid without narrowing its width and extends along the tooth edge to the posterolingual corner of the entoconid. All synclinids are narrow, forming V-shaped valleys. The synclinids II and IV are closed by the mesolophid and posterolophid, respectively. The synclinid IV is deeper than the synclinids II and III due to the strong posterolophid.

Lower first molar (m1): Slightly worn specimen. Length 0.84 mm, width 0.77 mm in the anterior lobe, and 0.88 mm in the posterior lobe ( Fig. 3 ). Bunodont tooth with four roots. Quadrate outline in occlusal view, the longitudinal axis of the tooth is in the same length as the width of the tooth. The anterior lobe (formed by the metaconid, protoconid, and anterolophid) is transversely narrower than the posterior lobe (formed by the entoconid, hypoconid, and posterolophid) by ~10% ( Fig. 2 ). The metaconid and entoconid are slightly displaced anteriorly compared to the protoconid and hypoconid, respectively. Due to the slight displacement of metaconid and the narrow anterior lobe, the synclinid I is significantly shallower and shorter than the synclinid IV by 67%, and the anterolophid is only half the length of the posterolophid although it reaches to the lingual side of the tooth, closing the synclinid I. The anterolophid is separated from the protoconid by an extremely shallow and narrow groove, making the labial side of the synclinid I closed. The anteroposteriorly-compressed metaconid and entoconid are high and pointy, resulting in the metalophid and hypolophid running steeply downward along the cusps and becoming flat and low on the talonid basin before they connect to the center of the protoconid and the posterior ectolophid, respectively. In the talonid basin, the hypolophid is lower than the level of the ecotolophid and the labial cusps, whereas the metalophid is at the same level as them ( Fig. 2C ). The metalophid is straight, whereas the hypolophid is anteriorly concave, directed posterolabially on the slope of the hypocone and anterolabially on the talonid basin, leaving the syncinid III narrower and shallower than the synclinid II on the labial side. The synclinid II is about half the length of the synclinid III because a low mesolophid is directed anterolingually to the posterior wall of the metaconid and closes the synclinid II with the mesostylid. A mesoconid is present but does not have a bulbous base. A well-developed posterolophid slightly lowers its height lingually but reaches the lingual side of the tooth, attaching to the posterior base of the hypocone and closing the synclinid IV.

Incisor enamel microstructure: A small fragment of the lower incisor of the paratype specimen of Japaneomys yasunoi ( NMNS-PV19994 ) was sectioned transversally ( Figs. 2B , 4B ). The enamel band is thin and measures about 52 μm thick in its center. The enamel band slightly thickens laterally, thus forming a very shallow groove along the long axis of the tooth (arrow in Fig. 4B ). As it is typical for eomyid incisors, the Portio interna ( PI) consists of longitudinally oriented uniserial Hunter-Schreger bands ( HSB), and the thin Portio externa ( PE) is composed of radial enamel (about 9 μm in thickness, 17% of the enamel band thickness). The PI is split into three well-separated layers: the interprismatic matrix (IPM) runs perpendicular to the prisms in the innermost layer (about 20 μm, 38%) at the enamel-dentine-junction (EDJ) as well as in the outermost layer (about 9 μm, 17%) at the PI/PE junction, while the IPM is oriented parallel to the prisms in the middle layer (about 14 μm, 27%). Although being uniserial, the HSB frequently show double bands. The individual prisms are mostly round in the innermost and outermost layer of the PI, oval-shaped in the middle layer of the PI and lancet-shaped in the PE. No starting zone is present at the EDJ.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 2. Eomyid rodent Japaneomys yasunoi gen. et sp. nov. from the Dota locality, early Miocene. A. NMNS-PV19994, paratype, right p4. B. NMNS-PV19995, holotype, m1. In occlusal view (A1, A2, B1, B2); SEM images (A1, B1), HDR images (A2, B2). Labial view of the right mandible with p4 A3). Surface model of m1: isometric (B3), occlusal (B4), bottom (B5), anterior (B6), labial (B7), lingual (B8), and posterior (B9) views. Scale bars 1 mm.

Fig. 3. Tooth measurements of Japaneomys, Asianeomys, and Keramidomys. Each symbol represents the mean of measurements, which were taken from the primary literature (Wang and Emry 1991; Wang 2002; Wu et al. 2006; Maridet et al. 2015; Qiu and Li 2016). Error bars represent the range of the measurements.

Fig. 4. Lower incisor (NMNS-PV19994, paratype) of eomyid rodent Japaneomys yasunoi gen. et sp. nov. A. SEM image of the enamel microstructure. Transverse section showing the four-layered schmelzmuster with a thick, tripartite Portio interna (PI) and a thin Portio externa (PE); EDJ, enamel-dentine-junction; OES, outer enamel surface. This section is located directly right (medial) to the arrow in B. B. Schematic drawing of the incisor cross section. The enamel band is shown in black, the solid arrow points to the shallow groove. The magnification is about ×26.