History of Commensal Rodents on Ishigaki Island (Southern Ryukyus) Reconstructed from Holocene Fossils, Including the First Reli

Quaternary International xxx (2015) 1e11

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History of commensal rodents on Ishigaki Island (southern Ryukyus) reconstructed from Holocene fossils, including the ﬁrst reliable fossil record of the house mouse Mus musculus in Japan

Ai Kawamura

Department of Geosciences, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan article info abstract

Article history: The history of commensal rodents in mainland Japan and the Ryukyu Islands (Ryukyus) has been poorly Available online xxx understood owing to the paucity of reliable fossil records. Recent excavation of the Shiraho-Saonetabaru cave site on Ishigaki Island of the southern Ryukyus yielded a large number of rodent fossils from well- Keywords: dated sediments ranging from the Late Pleistocene to the Holocene. Some of the fossils were prelimi- Ishigaki Island narily assigned to Mus musculus and Rattus sp. They are described here to confirm their taxonomic Holocene positions. These forms are considered to be commensal rodents. Commensal rodent A history of commensal rodents on Ishigaki Island is reconstructed based on the stratigraphic position Systematics Fossil and relative abundance of the commensal forms as well as the non-commensal native rodent, Niviventer History sp., which is very abundant in most of the horizons of the sediments. Before the end of the Pleistocene, no commensal forms inhabited the island, and the rodent fauna consisted only of the non-commensal form. In the early Holocene, one of the commensal forms, M. musculus, invaded the island, but the continued abundance of the non-commensal form afterward indicates that the invasion did not affect the fauna. In the late Holocene, a second invasion, by Rattus sp., probably caused the rapid decline and extinction of the non-commensal form. Since the extinction, the rodent fauna of the island has comprised only the commensal forms. In Japan, there are no known reliable fossil occurrences of M. musculus,incontrasttoseveral occurrences of Rattus at Middle Pleistocene to Holocene fossil localities. Thus, the fossil of M. musculus described here records the first reliable occurrence of the species in Japan, and is very important for documenting the first appearance of M. musculus in the southern Ryukyus in the early Holocene. The species is considered to have appeared much later in mainland Japan and the central Ryukyus. © 2015 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction archeologists have paid little attention to Quaternary fossils of these species, and because many allocations of these fossils to Three commensal rodent species (rodents closely associated species have been incorrect. On Ishigaki Island of the Ryukyus with humans) are now distributed all over Japan. They are the (Ryukyu Islands; Fig. 1A), the current rodent fauna comprises only house mouse Mus musculus, the ship or black rat Rattus rattus, and these commensal species, and lacks any non-commensal native common or brown rat Rattus norvegicus, all of which have world- species. The commensal species must have invaded the island wide distributions. The species' histories, based on fossil records, sometime in the Quaternary, and replaced native species now have been obscure in Japan because paleontologists and extinct on the island. Until 2010, however, no reliable fossil records of the commensal species had been known from the island. In 2010, an extensive excavation was carried out at the Shiraho-Saonetabaru cave site on Ishigaki Island (Fig. 1B), because the site had yielded reliably-dated Pleistocene human bones, which had been rare in Japan (Nakagawa et al., 2010). E-mail address: [email protected]. Speciﬁcally, the excavation revealed stratigraphic sequences of http://dx.doi.org/10.1016/j.quaint.2015.01.024 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.

Fig. 1. Maps showing (A) the location of Ishigaki Island and the fossil localities in mainland Japan discussed in the text, and (B) the topography of Ishigaki Island and the locations of the fossil localities discussed in the text.

the site's sediments, and produced abundant collections of hu- (Fig. 1B). The northern half of the main part is largely moun- man bones, non-human mammal fossils, and artifacts (Nakaza tainous, with the highest peak 525.8 m above sea level, and is et al., 2013). The geological age of the sediments was formed primarily by pre-Neogene basement rocks comprising confirmed to range from the Late Pleistocene to the Holocene by sedimentary, igneous, and metamorphic rocks. The topography a number of radiocarbon dates as well as archeological analyses and geology continue along the peninsula. In contrast, the of the artifacts. southern half of the main part is mostly hills and terraces, and is Among the mammal fossils collected, those of small mam- formed primarily by Pleistocene limestone and clastic sediments, mals were especially abundant, and were preliminarily studied collectively named the Ohama Formation by Nakagawa et al. by Kawamura and Kawamura (2013). Some of the fossils studied (1982). They overlie the basement rocks. The Shiraho- are referable to M. musculus and Rattus sp., and are considered Saonetabaru cave site is located in the northeastern area of the to record the presence of commensal rodents on this island. In southern half, about 0.7 km inland (24 240100N, 124 1404500 E; this paper, detailed systematic descriptions are given to the Fig. 1B). fossils referred to these two forms, to confirm their taxonomic Around this site, long and largely horizontal caves have positions. Using taxonomy and the chronological distributions of developed in the limestone of the Ohama Formation along the all the rodent forms from the site, this paper reconstructs a unconformable boundary between the limestone and meta- history of the commensal rodents on Ishigaki Island, and relates morphic basement rocks, the latter named the Tomuru Forma- it to the extinct native rodent form that was assigned to tion by Foster (1965). The site is situated in one of the caves, Niviventer sp. by Kawamura and Kawamura (2013). Furthermore, called Shiraho-Saonetabaru Cave by Nakagawa et al. (2010). this paper discusses the significance of the M. musculus fossil Yamasaki and Nakazato (2013) inferred that the ceiling of this from the site, the first reliable fossil record of this species in cave had collapsed with the enlargement of the cave, thereby Japan. forming a doline-like depression in which sediments had been continuously deposited from the Late Pleistocene to modern times. 2. Geographical and geological settings The generalized stratigraphic sequence of the sediments is described by Katagiri and Yamasaki (2013), and is outlined in Ishigaki Island is situated in the southwestern part of the Table 1. The sediments are divided into five layers: 0 to IV, in Ryukyu Islands, which form an island arc between Kyushu and descending order. Layers III and IV are subdivided into Layers IIIA Taiwan (Fig. 1A). The island is closer to Taiwan than to Okinawa to IIIE and Z, and Layers IVA and IVB, respectively. Among the Island, the largest central island of the arc. Ishigaki Island consists layers, Layer 0 represents the topmost artificial sediments of of a square-like main part and a northeast-extending peninsula modern times. Layer I is composed of dark brown sandy mud

ments indicative of the Shimotabaru Period roughly corre- B sponding to the Late Jomon Period in mainland Japan. Layer Z is a limestone breccia bed interposed between Layers IIIE and IVA. Layer IV is also composed of brown mud. Yoneda et al. (2013) dated a number of samples from Layers I to IVB by the radio- Artiodactyla carbon method using accelerator mass spectrometry, and ob- Felis e e eee e e sp.

tained dates ranging from ca. 0.3 ka to ca. 24 ka (uncalibrated C dates). occurrence shown in the text of ¼ Carnivora

Table 1 Niviventer a a r a arc ara ara ee ee a Generalized stratigraphic sequence of the sediments in the Shiraho-Saonetabaru sp. C cave site, with lithology, geological age, and the age estimation by Katagiri and ve specimens). ? ﬁ Yamasaki (2013). In the age estimation column, the Nakamori and Mudoki Periods Rattus e r r roughly correspond to the 14th to 17th centuries, and the Heian to Yayoi Periods in sp. tive abundances of small mammals, larger mammals (carnivores and ar- mainland Japan, respectively, and numerical ages represent estimated ranges of B uncalibrated radiocarbon dates.

Geological Age estimation by Generalized Lithology rare (less than ¼ Mus ee ee ee ee ee e ee age Katagiri and stratigraphic musculus B

Yamasaki (2013) sequence no comparable species in the present fauna of Ishigaki Island, open circle ¼ Holocene Modern Layer 0 Artiﬁcial sediments Nakamori Period Layer I Dark brown sandy

mud Rodentia Nakamori to Mudoki Layer II Brown sand and Periods gravel Homo a c a a ar r a ca.1.7e4.0 ka Layer III IIIA Brown mud sapiens B e ve to nine specimens), r

ca.8.5 9.5 ka IIIB Brown mud ﬁ

Late Pleistocene ca.16e18 ka IIIC Brown mud

not dated IIID Brown mud , respectively. Solid circle Primates ca.24 ka IIIE Brown mud common (

not dated Z Limestone breccia ¼ ca.20e24 ka Layer IV IVA Brown mud IVA þ IVB IVB Brown mud Miniopterus e e sp. Doi et al. (2013) B and Myotis ee ee ee ee sp. C 3. Faunal setting

Fossils of land mammals including humans occur in all the Hipposideros r r layers except Layer 0. Table 2 shows the stratigraphic distri- sp. bution of each form and its relative abundance, based on data B from Doi et al. (2013), Kawamura and Kawamura (2013),and abundant (more than ten specimens), c ¼ Namiki (2013). There are 12 forms in the fossil fauna, of which ﬁve are ﬂying mammals (chiropterans), two are the commensal Rhinolophus rr cc r ee ee sp.

forms described and discussed here (M. musculus and Rattus B sp.), and one is a domestic species (Bos taurus). Besides the 12 forms, Kawamura and Kawamura (2013) reported rare occur- Pteropus ee e ee e ee e e e e ee ee e ee e e rence of Suncus cf. murinus from Layer A. This form is the only sp. B representative of soricomorphs in the fauna of this site, but is Kawamura and Kawamura (2013), Namiki (2013) not shown in Table 2, because Layer A has very limited distribution in the site, and is uncertain in correlation with the sequence in Table 1 (Katagiri and Yamasaki, 2013). Chiroptera Among the 12 forms, Myotis sp., Niviventer sp., and Felis sp. do not have comparable species now inhabiting Ishigaki Island, and Layer IVB Layer IVA Layer Z Layer IIIE Layer IIID Layer IIIC r r a r Layer IIIB r c a probably became extinct sometime in the late Holocene (solid cir- Layer II Layer IIIA a a a r r c cle in Table 2). However, the remaining nine forms are the same as or comparable to the species now inhabiting Ishigaki Island (open circle in Table 2). Pleistocene Holocene Layer I r Late tiodactyls), and humans are based on Table 2 Stratigraphic distributions of land mammals, including humans, in the Shiraho-Saonetabaru cave site with relative abundance of each form. The rela comparable species in the present fauna of Ishigaki Island. a but not in his tables.

4. Method and material

During and after the excavation, a large amount of sediment collected from the site was washed using ﬁne mesh screens according to the method in Katagiri (2013) and Kawamura and Kawamura (2013). A large number of small mammal fossils were obtained, including the fossils of M. musculus and Rattus sp. described here. These fossils were compared with fossil and extant specimens of murine rodents stored in the following institutions: AUE ¼ Aichi University of Education, Kariya, Aichi Prefecture, Japan (extant and fossil specimens). IVPP ¼ Institute of Vertebrate Paleontology and Paleoanthro- pology, Chinese Academy of Sciences, Beijing, China (fossil specimens). IZ ¼ Institute of Zoology, Chinese Academy of Sciences, Beijing, China (extant specimens). NMNS ¼ National Museum of Natural Science, Taichung, ROC (extant specimens). The terminology and measuring methods (see Fig. 2) are from Kawamura (1988). This terminology follows the system of Jacobs (1978). The measurements were taken using a Nikon measure- scope (MM-11) with a Nikon electric digital counter (CM-6S). All the fossils described here are stored in Okinawa Prefectural Archaeological Center, Nishihara, Okinawa Prefecture, Japan.

5. Systematic paleontology

5.1. Genus Mus Linnaeus, 1758

Remarks: The genus Mus, now distributed mainly in Africa and South Asia, comprises a number of small murine species. Musser and Carleton (2005) listed 38 extant species, which include a commensal and cosmopolitan species, M. musculus. Only three extant species of Mus are known from East Asia (except southernmost China): M. caroli, M. musculus, and M. pahari. Fossils of M. musculus have been described from several Middle Pleistocene localities of Zhoukoudian in Beijing (Young, 1934; Pei, 1936 and others; revised by; Zhang et al., 1993), and from Middle Pleistocene localities of Jinniushan in Liaoning (Zhang et al., 1993) and Sun- jiashan in Shandong (Zheng et al.,1997). Fossils of M. pahari or M. cf. pahari have been recorded from the Early Pleistocene at Long- gudong Cave in Hubei (Zheng, 2004) and Sanhe Cave in Guangxi (Wang et al., 2009); the Middle Pleistocene at Baiyanjiao Cave in Guizhou (Zheng, 1993); the Middle or Late Pleistocene at Wuyun Cave in Guangxi (Chen et al., 2002); and the Late Pleistocene at the Lower cave of Pingba in Chongqing (Zheng, 1993) and Mulan

Mountain in Guangxi (Wang et al., 2010). Additionally, fossils of Fig. 2. Terminology and measurements used for murine lower molars (M1,M2, and ﬁ e ¼ Mus were reported from the Early Pleistocene at Longgupo in M3). Partly modi ed from Kawamura (1988).C0C5 buccal accessory cusps, L ¼ length of crown, W ¼ width of crown. Chongqing (Zheng and Zhang, 1991; Zheng, 1993) and the Late Pleistocene at Migong Cave also in Chongqing (Huang et al., 2000). These localities are all in China, but there are no reliable fossil re- Shiraho-Saonetabaru cave site. It is referred to M. musculus as cords of Mus for Japan, Korea, or Taiwan, except a record of Mus sp. described below. from the Holocene (?) of Miyako Island, Japan (Nakagawa et al., 2012). The following dental traits are recognized for the genus Mus, 5.2. Mus musculus Linnaeus, 1758 based on extant and fossil specimens at AUE, IZ, IVPP, and NMNS as well as the published literature (e. g., Miller, 1912; Aoki and Tanaka, Synonym: Many species names have been proposed for this 1941; Tokuda, 1941; Kowalski and Ruprecht, 1964; Misonne, 1969). species because of its close association with humans (see Musser The upper incisor is noticeably notched. M1 and M2 have no post- and Carleton, 2005). 3 2 erostyle. M and M3 are reduced and much smaller than M and M2. Material: 1 left mandible with M1,M2, and M3 (HF718). See M1 shows an X-pattern formed by the wear of the anterior and Fig. 3 (1ae1c) and Fig. 4 (1) for photographs and sketch, middle laminas. M1 has no medial anteroconid, and no buccal respectively. accessory cusps in most species. M2 has no buccal accessory cusps. Locality and horizon: Layer IIIB of the Shiraho-Saonetabaru A mandible with molars showing the above-mentioned traits of cave site, Shiraho, Ishigaki, Okinawa Prefecture. Mus was found in the small mammal fossil assemblage of the Age: early Holocene.

Fig. 3. Fossils of Mus musculus and Rattus sp. from the Shiraho-Saonetabaru cave site. 1 ¼ left mandible with M1,M2, and M3 of Mus musculus, numbered HF718 (1a ¼ buccal view, 1b ¼ occlusal view, 1c ¼ lingual view); 2 ¼ right mandible with M1,M2, and M3 of Rattus sp., numbered HF1776 (2a ¼ buccal view, 2b ¼ occlusal view, 2c ¼ lingual view); 3 ¼ left M1 of Rattus sp., numbered HF1873 (3a ¼ buccal view, 3b ¼ occlusal view, 3c ¼ lingual view).

Fig. 4. Detailed sketches showing the occlusal patterns of the molars of Mus musculus and Rattus sp. from the Shiraho-Saonetabaru cave site. 1 ¼ left M1,M2, and M3 of Mus musculus (HF718), 2 ¼ right M1,M2, and M3 of Rattus sp. (HF1776), 3 ¼ left M1 of Rattus sp. (HF1873).

Description: The overall size is much smaller than the compa- Molar measurements: rable fossils of Rattus sp. described below and Niviventer sp. from the same site; however, it is nearly as large as or somewhat larger than those of comparable Micromys species. The crowns of the molars are much lower than those similarly-worn molars of Niviventer sp., but nearly as high as those of Rattus sp. In lateral view, the cusps of the molars are inﬂated anteroposteriorly as in Rattus sp. and Niviventer sp., in contrast to the slenderer cusps of Micromys species. MandibledThe mental foramen is large, and opens on the buccal face of the diastema considerably anterior to the anterior end of M1. In buccal view, the upper border of the diastema gradually ascends posteriorly, and does not form a steep line just anterior to M1. The lower masseteric crest is strong, but no upper masseteric crest is seen on the buccal face of the horizontal ramus. M1dThe occlusal outline of the crown is a rounded rectangle, which is broader linguobuccally than that of Micromys minutus. The anterior lamina is formed by the lingual anteroconid and labial Comparison and taxonomic position: The fossil described here anteroconid, of which the former is considerably larger than the has a small size comparable to those of extant Mus and Micromys latter. The anterior wall of this lamina is strongly emarginated, so species. In the fossil, M1 shows the X-pattern, and has neither that the two cusps are clearly distinguished from each other. This medial anteroconid nor buccal accessory cusp; M2 also lacks buccal emargination is biased buccally from the medial line of the crown. accessory cusps; and M3 is small relative to M2. As mentioned The anterior lamina has no medial anteroconid. The middle lamina, above, these dental traits are peculiar to the genus Mus, and a chevron shape, is formed by the protoconid and metaconid, which distinguish it from other murine rodents including Micromys, Rat- are nearly same size. This lamina is somewhat broader than the tus, and Niviventer. Thus the fossil can be allocated to the genus anterior lamina in occlusal view. The wear surface of the middle Mus. It is compared with three representative species now lamina is connected to that of the anterior lamina just posterior to distributed in East Asia: M. caroli (extant specimens from Taiwan the labial anteroconid, so that the X-pattern formed by the anterior stored in NMNS), M. pahari (those from China in IZ), and and middle laminas is asymmetrical, and is biased buccally. The M. musculus (those from Japan and Taiwan in AUE and NMNS). valley between the lingual anteroconid and metaconid is much M. caroli differs from the fossil in having a somewhat smaller M1 deeper linguobuccally than that between the labial anteroconid with different morphology. In M1 of M. caroli, the vertical groove on and protoconid in occlusal view. The posterior lamina, consisting of the anterior wall of the anterior lamina is weaker and shorter, so the hypoconid and entoconid, is well separated from the middle that the outline of the anterior wall shows no emargination in lamina. The two cusps are nearly the same size, giving the lamina occlusal view. The labial anteroconid is larger relative to the lingual symmetrical chevron shape. The lamina is nearly the same width as anteroconid, and the anterior lamina is connected to the middle the middle lamina in occlusal view. The posterior cingulum is much lamina on the medial line of the crown. Consequently, the X- lower than the posterior lamina. Its occlusal outline is an elongated pattern is more symmetrical in M. caroli. ellipse linguobuccally. There are no accessory cusps on the buccal M. pahari is considerably larger than the fossil. M1 of M. pahari wall of the crown. This tooth has two roots. has a strong C2 on the buccal side of the crown, while this cusp is M2dThe occlusal outline is a rounded trapezoid. The anterior completely absent in M1 of the fossil. In the same tooth of M. pahari, lamina formed by the protoconid and metaconid are chevron the middle lamina is remote from the posterior lamina, while the shaped. The protoconid is somewhat larger than the metaconid. former is nearer to the latter in M1 of the fossil. The anterior wall of the protoconid has a short cingulum, which The extant specimens of M. musculus resemble the fossil in size corresponds to the labial anteroconid. The anterior lamina is well and dental morphology, especially that of M1, where the anterior separated from the posterior lamina by a deep transverse valley. wall of anterior lamina has a strong vertical groove, and the X- The posterior lamina, consisting of the hypoconid and entoconid, pattern is asymmetrical. Of the three representative Mus species also is chevron shaped, and is considerably narrower linguo- from East Asia, such similarities justify the allocation of the fossil to buccally and anteroposteriorly than the anterior lamina. This M. musculus. chevron is symmetrical owing to nearly the same size of the two cusps. The posterior cingulum is much lower than the posterior 5.3. Genus Rattus Fischer de Waldheim, 1803 lamina, and has an elliptical shape in occlusal view. No accessory cusps are present on the buccal wall of the crown. This tooth has Remarks: The genus Rattus, now distributed mainly in South- two roots. In lingual view, the anterior and posterior roots are east Asia, Australia, and New Guinea, comprises a number of nearly equal in size. middle-sized murine species. Musser and Carleton (2005) listed 66 M3dThis tooth is much smaller than M2, and the size difference extant species in the genus, including two commensal and between the teeth is greater in the present fossil than in extant M. cosmopolitan species such as R. norvegicus and R. rattus (including minutus as well as Rattus sp. and Niviventer sp. from this site. The R. tanezumi), and recognized six species groups: R. norvegicus occlusal outline of the crown is a rounded triangle. The crown species group, R. exulans species group, R. rattus species group, comprises the anterior lamina and entoconid. The anterior lamina R. fuscipes species group, R. leucopus species group, and R. xanthurus is formed by same-sized protoconid and metaconid, and is a sym- species group. In East Asia, there are relatively few species of Rattus, metrical chevron shape. The entoconid has a semicircular shape, because that region is outside of the genus' main distribution. Be- and the anterior margin is slightly concave in occlusal view. There sides the cosmopolitan species, three species are naturally found in are neither labial anteroconid nor accessory cusps. This tooth has East Asia (except southernmost China): R. losea of the R. rattus two roots, the anterior, smaller one extending almost vertically and species group, R. exulans of the R. exulans species group, and the posterior one extending posteroventrally. R. nitidus of the R. norvegicus species group. In addition to the

Please cite this article in press as: Kawamura, A., History of commensal rodents on Ishigaki Island (southern Ryukyus) reconstructed from Holocene fossils, including the first reliable fossil record of the house mouse Mus musculus in Japan, Quaternary International (2015), http:// dx.doi.org/10.1016/j.quaint.2015.01.024 A. Kawamura / Quaternary International xxx (2015) 1e11 7 cosmopolitan species, it was necessary to compare the fossils from form is comparable to that of Niviventer sp. from this site, but much the Shiraho-Saonetabaru cave site, which are referred to Rattus, larger than M. musculus described above. The crowns of the molars with these species. are much lower than those of Niviventer sp. Fossil materials previously described as Rattus in China were MandibledThe mental foramen opens on the buccal face of the revised by Zheng (1993), who transferred some of them to other diastema just anterior to the anterior end of M1. In buccal view, the genera such as Leopoldamys and Niviventer. In regard to fossils from upper border of the diastema abruptly ascends posteriorly to form a Locality 1 of Zhoukoudian in Beijing, for example, those allocated to steep line just anterior to M1. The lower masseteric crest forms a Epimys (¼ Rattus) rattus by Young (1934) were re-identified as sharp ridge on the buccal face of the horizontal ramus, while no Niviventer confusianus. According to Zheng (1993), fossils of upper masseteric crest is present on the same face. R. norvegicus occurred from the Middle and Late Pleistocene at M1dThe occlusal outline of the crown is a rounded rectangle Zhoukoudian. They were originally assigned to “Epimys rattus” by similar to that of M. musculus described above, but is generally Pei (1936, 1940). The same species is recorded from the Early broader than that of Niviventer sp. from the same site. The anterior Pleistocene at Sanhe Cave in Guangxi (Wang et al., 2009) and from lamina has an emargination on its anterior wall that is observable the Middle Pleistocene at Hexian in Anhui (Zheng, 1983), and from in the less worn specimen (HF1873), but is absent from the worn the Late Pleistocene at the Lower cave of Pingba in Chongqing specimen (HF1776). This emargination distinguishes the lingual (Zheng, 1993) and Mulan Mountain in Guangxi (Wang et al., 2010). anteroconid from the labial anteroconid. The former cusp is larger Additionally, R. losea is reported from the Late Pleistocene at than the latter cusp, so that the emargination is biased somewhat Wuyun Cave and Mulan Mountain in Guangxi (Chen et al., 2002; buccally. The anterior lamina has no medial anterocoid. The middle Wang et al., 2010). lamina is a very broad-angled chevron in occlusal view. In this In Japan, fossils of Rattus are known from the Middle and Late lamina, the protoconid approximates the metaconid in size. The Pleistocene, and from the Holocene (Kawamura, 1988, 1989, 1991). middle lamina is somewhat broader than the anterior lamina in Besides the fossils treated in these papers, those of Rattus are occlusal view. The transverse valley between the anterior and recorded from the Middle Pleistocene at Tarumi NT Cave (Inada middle laminas is deep. The X-pattern seen in M1 of Mus is not and Kawamura, 2004), and from the Holocene at the Shizukawa formed even in the worn specimen, in which the two laminas are 22 site (Kawamura, 2002), Taishaku-Oburo Cave Site (Niwa and connected on both lingual and buccal sides. The valley between the Kawamura, 2000) and Mumyono-ana Cave (Nakagawa et al., middle and posterior laminas is also deep, distinctly separating 2012). these laminas even in the worn specimen. C2 is clearly observed at The following dental traits are recognized for the genus Rattus, the buccal entrance of this valley in the less worn specimen. In the based on extant and fossil specimens at AUE, IZ, IVPP, and NMNS as worn specimen, this cusp becomes a projection at the anterobuccal well as the published literature (e. g., Miller, 1912; Aoki and Tanaka, corner of the posterior lamina. The posterior lamina is a broad- 1941; Tokuda, 1941; Kowalski and Ruprecht, 1964; Misonne, 1969; angled chevron in occlusal view. In this lamina, the hypoconid is Musser, 1981; Musser and Newcomb, 1983; Zheng, 1993). The nearly the same size as the entoconid. The posterior lamina is crown heights of the molars are normal for murids. M1 and M2 have slightly broader than or nearly as broad as the middle lamina in relatively broad crowns, and have neither a posterostyle nor a occlusal view. The posterior cingulum is relatively small and much posterior cingulum. The metacones of M1 and M2 are relatively lower than the posterior lamina. It has an elliptical shape, elon- large and recognizable even in worn specimens. The labial ante- gating transversely in occlusal view. rocone of M1 is not reduced in most species, and is usually present In lingual, buccal, and occlusal views, the enamel surfaces of in M2.M1,M2, and M3 have five, four, and three roots, respectively. the cusps are partly wrinkled in the less worn specimen. The cusps M1 has no medial anteroconid, and usually lacks the X-pattern are not so strongly tilted anteriorly as those of Niviventer sp., and typically observed in Mus. One or two buccal accessory cusps are the valleys between the cusps are very broad. This tooth has four usually present in M1.M2 usually has one buccal accessory cusp. roots, as observed in the worn specimen. The anterior and pos- The labial anteroconid is present in M2 and M3. The posterior terior roots are much larger than the two roots interposed be- cingulum is relatively small in M1 and M2.M1,M2, and M3 have four, tween them. three, and three roots, respectively. M2dThis tooth can be observed only in the worn specimen In the small mammal fossil assemblage of the Shiraho- (HF1776). The occlusal outline is a rounded square, with the ante- Saonetabaru cave site, only two specimens are referred to the robuccal corner slightly projecting anteriorly. This indicates that genus Rattus. They are easily distinguishable from the above- the labial anteroconid was originally present at this corner. The mentioned fossil of M. musculus by their larger size and dental anterior and posterior laminas produce a very broad-angled traits described below. The traits also distinguish them from fossils chevron in occlusal view. These laminas have nearly the same lin- of Niviventer sp. from the same site, although the specimens are guobuccal width. The anterior margin of the posterior lamina nearly as large as those of N. sp. draws a straight line, the buccal end of which protrudes anteriorly. This indicates that C2 was present at this position. The posterior 5.4. Rattus sp. cingulum is semi-elliptical, elongating transversely. This tooth has three roots, with the posterior root much larger than the two Material: 1 right mandible with M1,M2, and M3 from Layer I anterior roots. (HF1776); 1 left M1 from Layer II (HF1873). See Fig. 3 (2ae2c and M3dThis tooth is present in the worn specimen (HF1776). The 3ae3c) and Fig. 4 (2 and 3) for photographs and sketches, occlusal outline is a rounded triangle. Its anterobuccal corner pro- respectively. jects slightly anteriorly, which seems to indicate the presence of the Locality and horizon: Layers I and II of the Shiraho- labial anteroconid in a less worn stage. This tooth is smaller than Saonetabaru cave site, Shiraho, Ishigaki, Okinawa Prefecture. M2, but the size difference between the teeth is lesser than in Mus. Age: late Holocene. The anterior lamina is a broad-angled chevron in occlusal view. The Description: Of the two specimens referred to this form, the entoconid is semicircular in the same view, and is considerably mandible (HF1776) is white in color and less fossilized. The molars biased lingually from the medial line of the crown. This tooth has anchored on this mandible are considerably worn, while the de- three roots, with the two anterior ones being much smaller than tached M1 (HF1873) is less worn and lacks roots. The size of this the posterior root. All the roots extend posteroventrally.

Molar measurements: 6. Discussion

6.1. Other fossil localities

Besides the Shiraho-Saonetabaru cave site, rodent fossils have been reported from only two localities on Ishigaki Island, where the fossils were obtained from Quaternary sediments infilling fissures formed in limestone; one fissure near Sabichi-do Cave and several fissures at Ishisuku-yama (Fig. 1B). Nishioka and Nakagawa (2012) reported mammalian fossils, including “R. rattus”, and a radiocarbon date of a rat femur indicating the late Holocene from the former fissure. Unfortunately, no taxonomic information on “R. rattus” is available, because they have not published its systematic description. In regard to Ishisuku-yama, Hasegawa and Nohara (1978) reported fossils of two non-flying mammal species such as “R. rattus” and “Metacervulus astylodon” (¼ Cervus astylodon) from the fissures. Judging from their published photographs (Pl. II-10, Figs. 9e17), those fossils are probably not assignable to R. rattus, but rather to the same form as Niviventer sp. from the Shiraho- Saonetabaru cave site, because the fossils show the following Comparison and taxonomic position: The following dental dental traits. In M1, the labial anterocone is remarkably reduced, so traits distinguish the specimens from Niviventer sp. from this site. that the buccal margin of the crown draws a straight line extending The molars are less hypsodont. M1 has a somewhat broader anteroposteriorly. Also, the anterior lamina of M1 is narrower crown relative to its length, and has a more developed C2; its relative to the middle lamina, and the middle and posterior laminas middle lamina shows a broader-angled chevron; its posterior form chevrons with narrower angles than those in M1 of Rattus. cingulum is somewhat smaller; the lateral faces of the crown are Therefore, it is inferred that no commensal rodent species are wrinkled in a less worn stage; and the valleys between the lam- present at the Ishisuku-yama fissures. Moreover, judging from the inas are much broader. M2 has three roots instead of two roots in other published photographs (Pl.II-7, Figs.5, 6), the best-preserved Niviventer sp. antler of “Metacervulus astylodon” appears to lack a typical The traits observed in the molars of the present fossils are feature of that species (the second tine branches off posterodorsally consistent with those described for Rattus (Section 5.3, above), and at the point relatively near to the first fork in “M. astylodon”). This thus the specimens are assigned to Rattus. The fossils have been antler is rather similar to that of Cervus nippon, but its specific compared with the three extant species of Rattus occurring in East determination should be reserved owing to insufficient informa- Asia (R. losea, R. exulans, and R. nitidus) as well as the two tion on the fossils from Ishisuku-yama. In this paper, therefore, commensal and cosmopolitan species (R. rattus and R. norvegicus). “M. astylodon” from Ishisuku-yama is treated as Cervus sp. Among them, extant specimens of R. losea observed at NMNS and Such revisions result in Niviventer sp. and Cervus sp. being the illustrated by Aoki and Tanaka (1941) have larger M2 and M3 recognized taxa for those fossils from Ishisuku-yama instead of “R. relative to M1 than the corresponding molars of the present fossils. rattus” and “Metacervulus astylodon”. These forms are now extinct In comparison with extant specimens of R. exulans stored in on Ishigaki Island. On the other hand, no reliable chronological data NMNS, the molars of R. exulans are considerably smaller than have been obtained from the sediments and fossils of Ishisuku- those of the present fossils, although these species' molars do not yama. However, comparison with the well-dated Late Pleistocene differ much morphologically. Judging from figure (Pl. XXI, Fig. 125) to Holocene fauna of the Shiraho-Saonetabaru cave site shown in of Misonne (1969),M1 of R. nitidus has the X-pattern, and thus Table 2 provides a biostratigraphic resolution on the chronological R. nitidus differs from the present fossils. Extant specimens of position of the fauna of Ishisuku-yama. The fossils of the Shiraho- R. norvegicus and R. rattus stored in AUE and NMNS are consistent Saonetabaru cave site include abundant specimens of extant Sus with the present fossils in size, and their molars are morpholog- scrofa, but none of extinct Cervus sp. By contrast, the fossils of ically similar to those of the fossils. R. norvegicus is distinguishable Ishisuku-yama are characterized by an abundance of Cervus sp., but from R. rattus by the morphology of M1, where the labial ante- no Sus scrofa. It is, therefore, inferred that the fauna of Ishisuku- rocone is more reduced in R. norvegicus. Unfortunately, there is no yama is earlier than that of the Shiraho-Saonetabaru cave site M1 in the present fossils. Although it has been proposed that the (Fig. 5), and is dated to a time earlier than 24 ka. In conclusion, no buccal accessory cusps of M1 are more developed in R. rattus than commensal rodents inhabited Ishigaki Island from the time of the in R. norvegicus (e. g., Miller, 1912; Kowalski and Ruprecht, 1964), fauna of Ishisuku-yama to the end of the Pleistocene (Fig. 5). this character varies among individuals of both species, judging from data of Miyao (1960), Miyao et al. (1966), and Miyao and 6.2. History of the commensal rodents Ikeda (1969). In the present fossils, M1 has only one buccal accessory cusp (C2), a feature observed in both R. rattus and A history of the commensal rodents on Ishigaki Island is R. norvegicus. reconstructed using the fossil records from the Shiraho- In conclusion, the fossils are distinguishable from R. losea, Saonetabaru cave site as well as Ishisuku-yama (Fig. 5). Until the R. exulans, and R. nitidus, but not from the commensal species end of the Pleistocene, the only rodent on the island had been the R. norvegicus and R. rattus. Thus the fossils are indeterminate at the native form, Niviventer sp. The fossil records shown in Table 2 species level, and are referred to Rattus sp. herein; however, they indicate that humans have undoubtedly inhabited the island at have affinity to R. rattus and R. norvegicus. It is probable that they least since the time of Layers IVA and IVB (uncalibrated radiocarbon belong to one or both species, and they are considered to represent dates of ca. 20e24 ka; Table 1), but commensal rodents had been a commensal form. absent until the appearance of M. musculus in the early Holocene.

Fig. 5. Generalized chronological distribution of the rodents and humans on Ishigaki Island with faunal characters and events inferred. Solid circle ¼ reliable occurrence, open circle ¼ originally assigned to “Rattus rattus”, but emended here as Niviventer sp.

This appearance is evidenced by the fossil of M. musculus from human settlements, which seems to have been different from that Layer IIIB (Table 2). This species is the first commensal rodent to of Niviventer sp. It is also inferred that the habitat was very sparse in invade the island. As mentioned in the section on systematic the early Holocene, because archeological sites of this period are paleontology (Section 5), the fossil records from China suggest that almost absent on the island. The interspecific size and habitat dif- the genus Mus, including M. musculus, has commonly inhabited ferences, and the sparseness of M. musculus habitat at that time, China since the Early Pleistocene. Thus it is likely that M. musculus explain why the invasion of that species did not affect Niviventer sp. invaded Ishigaki Island directly from China or indirectly via Taiwan. Harris (2009) reviewed native small mammal extinctions It is inferred that sea level and the island's geography in the early attributable to introduced rodents on islands since AD 1500, and Holocene were almost identical to the present condition (e.g., Ota listed 12 cases in which the extinctions were caused mainly by and Yonekura, 1987; Chinzei and Machida, 2001). Thus, it is likely commensal rodents such as M. musculus, R. rattus, and R. exulans. that M. musculus came to the island on the boats or rafts of Only in one case was M. musculus responsible for an extinction of a migrating humans. native rodent: the lava mouse Malpaisomys insularis on the Canary The invasion of M. musculus does not appear to have affected the Islands. Boye et al. (1992) inferred the mechanism of its extinction rodent fauna of the island, because the rodent fossil assemblages of to be a disease carried by M. musculus and to which populations of Layer IIIB and the overlying Layer IIIA are almost the same as those the lava mouse were vulnerable. In most of the cases listed by of the Late Pleistocene layers, in which fossils of Niviventer sp. are Harris (2009), however, R. rattus, with its much larger size, is generally very abundant (Table 2). Sometime between 1.7 ka (un- considered to have caused the extinction of native small mammals calibrated radiocarbon date) and the Nakamori Period, the rodent mostly by interspecific competition or parasite/disease infection. fauna was drastically changed by the replacement of Niviventer sp. The second invasion onto Ishigaki Island in the late Holocene by the invading Rattus sp., which is recorded in the faunal difference was made by Rattus sp., which is nearly the same size as Niviventer between the assemblage of Layer IIIA and those of Layers II and I sp. Taking the above-mentioned list of Harris (2009) into consid- (Table 2). The invasion probably occurred in association with eration, it is probable that Rattus sp., with affinity to R. rattus and frequent human movements among islands of the Ryukyus, main- R. norvegicus, caused the extinction of Niviventer sp. soon after its land Japan, Taiwan, and China in the late Holocene, especially in the invasion. The mechanism of the extinction cannot be proposed here Historic Period. The extinction of Niviventer sp. resulted in the ro- owing to insufficient data from the Shiraho-Saonetabaru cave site, dent fauna of Ishigaki Island becoming composed exclusively of especially those from Layers I and II. commensal species. This situation has continued to the present day. 6.4. Previous fossil records of “M. musculus” in Japan 6.3. Invasion and extinction No reliable fossils of M. musculus had been known from Japan, The first invasion of commensal rodents in the early Holocene until Kawamura and Kawamura (2012, 2013) preliminarily reported was of M. musculus, which is much smaller than Niviventer sp. the fossil described here. Previous records of the species (Naora, M. musculus seems to have occupied a habitat mainly in and around 1954, 1972) are probably erroneous, as explained below.

Naora (1954) reported fossils of two murine forms such as “Mus Until the end of the Pleistocene, no commensal forms had molossinus molossinus or Mus molossinus” (¼ M. musculus) and inhabited the island, whose rodent fauna had been composed “Apodemus sylvaticus speciosus” (¼ A. speciosus) from Quaternary only of the non-commensal form. In the early Holocene, one of cave and fissure sediments of Takanosuzawa Cave and Shimizu- the commensal rodents (M. musculus) first invaded the island, Sekkai Quarry in the Kuzuu area, Tochigi Prefecture (Fig. 1A). He but this invasion did not change the rodent fauna. In the late did not publish systematic descriptions of these forms. Subse- Holocene, a second invasion by another commensal rodent quently, Kowalski and Hasegawa (1976) described rodent fossils (Rattus sp.) probably caused the extinction of the non-commensal collected from Takanosuzawa Cave, and recognized two murine form. Since the extinction, the fauna has been composed only of species (A. speciosus and A. argenteus) in the fossils. However, they the commensal forms. The fossil of M. musculus from this site mentioned that no remains of Mus had been found from the cave. records the first reliable occurrence of this species in Japan, and Some of the fossils studied by Naora (1954) are now stored in indicates its distribution in the southern Ryukyus in the early the National Museum of Japanese History, Sakura, including rodent Holocene, although its invasion seem to be much later elsewhere fossils from Takanosuzawa Cave. Reexamination of them in April in Japan. 2014 has revealed that the fossils contain no M. musculus specimens, but rather A. argenteus and A. speciosus,asKowalski and Acknowledgments Hasegawa (1976) mentioned. It is, therefore, inferred that Naora (1954) misidentified fossils of A. argenteus from the two localities This study has been conducted as a part of the excavation in the Kuzuu area as “M. molossinus”. research project for the Shiraho-Saonetabaru cave site organized by On the other hand, Naora (1972) published a systematic Okinawa Prefectural Archaeological Center. Thanks are due to Y. description of two mandibles of “Mus cfr. musculus molossinus” Kawamura and M. Mitamura for discussion on rodent systematics from Site B in the Shiriya area, Aomori Prefecture (Fig. 1A). This site and Quaternary geology, and improving the draft, and to the is equivalent to Shiriya quarry of Kowalski and Hasegawa (1976) or members of the project including H. Nakaza, C. Katagiri, M. Fujita, S. Locality 3 of Hasegawa et al. (1988), and its fossil-bearing sedi- Yamasaki, N. Doi, M. Yoneda, K. Yoshimura, and M. Namiki for ments are dated to the early Late Pleistocene. Naora (1972) providing facilities and information. As regards the comparisons described that M1 on the mandibles had buccal accessory cusps with fossil and living murine rodent specimens, thanks are also due forming a bank-like row. However, M1 of the genus Mus lacks such a to: staffs of IVPP including C. Z. Jin, Y. Q. Zhang, S. H. Zheng, Y. Wang, row, while it is observed in M1 of the genus Apodemus. The sizes of A. L. Yan; those of IZ including X. P. Cao, Z. J. Feng, Y. Yang; those of M1 and M2 measured by Naora (1972) are consistent with those of NMNS including C. H. Chang, Y. J. Chen, and H. N. Chang; those of A. argenteus given by Kawamura (1989). Furthermore, Kowalski and the National Museum of Japanese History including H. Harunari, Y. Hasegawa (1976) described murine rodents from this site, and Kudo and N. Kami for giving permission, facilities, and information. allocated them to A. speciosus, A. argenteus, and R. norvegicus I thank Y. Nishioka and an anonymous reviewer for reviewing the instead of “A. speciosus speciosus”, “M. cfr. musculus molossinus”, and manuscript. “Rattus sp.” by Naora (1972). As in the case of Takanozawa Cave, it is probable that Naora (1972) erroneously referred the mandibles of References A. argenteus to “M. cfr. musculus molossinus”. Aoki, B., Tanaka, R., 1941. The rats and mice of Formosa illustrated. Memoirs of the 6.5. Significance of the fossil of M. musculus Faculty of Science and Agriculture, Taihoku Imperial University 23 (4), 121e191. Boye, P., Hutterer, R., Lopez-Martínez, N., Michaux, J., 1992. A reconstruction of the lava mouse (Malpaisomys insularis), an extinct rodent of the Canary Islands. The above-mentioned revision of the previous records confirms Zeitschrift der Saugetierkunde€ 57, 29e38. that the fossil from the Shiraho-Saonetabaru cave site represents Chen, G.J., Wang, W., Mo, J.Y., Huang, Z.T., Tian, F., Huang, W.W., 2002. 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