RESEARCH ARTICLE
Fish fossils from the Middle Pleistocene Saginota Formation of the Ihara Group, Fuji, Shizuoka, Japan
2026 年 30 巻 p. 209-225
詳細
2026 年 30 巻 p. 209-225
While the Middle Pleistocene (Chibanian) ichthyofauna in Japan has been reconstructed from only a few fossil records to date, three well-preserved, articulated marine fish fossils were newly found from the middle part (ca. 0.58–0.57 Ma) of the Middle Pleistocene Saginota Formation of the Ihara Group in Shizuoka, central Japan. The fossil assemblage from the Saginota Formation comprises two fish species (Lateolabrax cf. japonicus and Mugil sp.) and two mollusks (Musculista senhousia and Corbicula japonica). In one fossiliferous block, the fossil remains identified as Lateolabrax cf. japonicus are characterized by: (1) ctenoid scales with a subrectangular outline and rounded corners; (2) a dentary bifurcating posterodorsally with a swollen upper ramus; and (3) a preopercle bearing five or more spines along the ventral margin. The skeletal remains of the other specimens are identified as Mugil sp. based on the presence of: (1) a pelvic-fin formula of I,5; (2) a pectoral fin consisting of 16 soft rays; and (3) an opercle that is rounded-subtriangular, with a small process anterior to the articular grooves and posteriorly radiating striae. The burial conditions of the fossil specimens, combined with sedimentary-facies observations, indicate that the deposit of the Saginota Formation occurred in a semi-enclosed brackish setting with intermittent marine inflow and possibly with dysoxic or anoxic bottom waters. This marine environment is supported by the occurrence of extant fish genera, Lateolabrax and Mugil, primarily inhabiting inner-bay to brackish areas.
Glacial-interglacial climate oscillations and attendant sea-level fluctuations had a major influence on the distributions and population connectivity of marine organisms around the Japanese Archipelago during the Pleistocene (e.g. Avise, 2000; Liu et al., 2006; Schwarzhans and Ohe, 2019). Molecular studies have suggested that sea level-driven range contractions and expansions influenced the genetic diversity of a variety of coastal fishes (e.g. Lecomte et al., 2004; Zardoya et al., 2004). Paleontological studies have documented more than forty fossil localities in Japan yielding otoliths and isolated fish teeth (Miyata et al., 2024), and these materials have long been used for paleobiogeographic and paleoenvironmental reconstructions (e.g. Kawase and Nishimatsu, 2016; Mitsui et al., 2021). In contrast, skeletal remains are known from only about ten reported localities (Miyata et al., 2024). Skeletal remains, especially well-preserved articulated specimens, provide important information for reconstructing the evolutionary history, paleobiogeography, and paleoenvironmental settings of fish fossil assemblages (Yabumoto, 2019). Additional articulated fish fossil specimens from the Pleistocene of Japan are therefore needed to clarify the origin and establishment of the modern fish fauna.
The Katanoyama Formation (ca. 1.3 ± 0.2 Ma; Calabrian), which is exposed on the island Tanegashima in Kagoshima, southwestern Japan, has yielded abundant articulated fish fossils representing at least 12 orders and 16 families (Saheki, 1929; Yabumoto and Uyeno, 1990, 1994). Articulated fish fossils from central Japan are relatively rare, representing only two orders and four families (Ohe, 1993; Matsuura, 1996; Ohe and Shinya, 2006; Yokoyama et al., 2013a, b).
In Shizuoka, central Japan, Pleistocene teleost fossils have been reported mainly from two strata: the Lower Pleistocene (Gelasian) Kakegawa Group in Kakegawa and Fukuroi (Ohe, 1993; Ohe and Shinya, 2006; Kitamura, 2014, 2016) and the Middle Pleistocene (Chibanian) Ihara Group in Fuji (Yokoyama et al., 2013a, b). The Upper Kakegawa Group has yielded adundant otoliths representing 31 families and 51 genera (Kitamura, 2016), as well as skeletal remains of macrourids and sparids, such as Dentex sp. (Ohe, 1993). Only a few teleost fossils have been recognized from the Ihara Group, but these include well-preserved skeletal remains representing Engraulis cf. japonicus, Clupeidae gen. et sp. indet., and Dorosomatinae gen. et sp. indet., demonstrating that these taxa were present along the Pacific coast of central Japan during the Middle Pleistocene (Yokoyama et al., 2013a, b). Although teleost fossils from the Ihara Group are relatively scarce and their taxonomy remains unresolved, they nevertheless provide paleontological evidence that helps fill stratigraphic and geographic gaps in the teleost fossil record relative to the distribution of their extant marine relatives in central Japan.
Here, we describe a new fossil assemblage comprising teleost fish and bivalves from the Middle Pleistocene Saginota Formation of the Ihara Group in Fuji, Shizuoka, Japan. These specimens provide important information for understanding the Middle Pleistocene faunal composition of the Ihara Group. In addition, we present a preliminary interpretation of the depositional environment of the fossil assemblage based primarily on habitat preferences and taphonomic observations.
Institutional abbreviations.—KPM, Kanagawa Prefectural Museum of Natural History, Odawara, Kanagawa, Japan; SPMN, Museum of Natural and Environmental History, Shizuoka, Shizuoka, Shizuoka, Japan.
The examined fossils were recovered from outcrops of the Saginota Formation of the Lower–Middle Pleistocene Ihara Group in Minami-Matsuno, Fuji, Shizuoka, Japan (Figure 1). The Ihara Group is distributed across the Kanbara (Iwabuchi), Habuna and Hoshiyama hills in the lower Fuji River area and was previously divided into the Kanbara Formation and the Iwabuchi Formation, in increasing stratigraphic order. The Kanbara Formation is composed mainly of conglomerate, whereas the Iwabuchi Formation consists of andesitic volcanic rocks intercalated with sand-gravel beds (Shiba et al., 1990; Shiba, 1991). Within the Kanbara Hills, the sand-gravel beds of the Iwabuchi Formation were further subdivided into three members: the Minami-Matsuno Gravel Member, the Sensui Gravel Member along the northwestern margin, and the Saginota Gravel Member in the northern sector. Subsequent studies demonstrated that these three members, although locally separated by minor unconformities, are broadly coeval with the volcanic rocks of the Iwabuchi Formation (Sugiyama et al., 2010; Ozaki et al., 2016). As a result, the volcanic rocks and collective gravel beds formally grouped within the “Iwabuchi Formation” of Shiba et al. (1990) are now commonly divided into separate lithologic units: the Iwabuchi Volcanic Rocks and the Saginota Formation, respectively (Ozaki et al., 2016).
The Saginota Formation in Minami-Matsuno is divided lithologically into the lower, middle, and upper parts based on key massive conglomerate beds (Yokoyama et al., 2013a). The lower and middle parts are exposed along Urushinosawa and another small valley to the northeast, both of which are E–W-trending tributaries of the river Ketsuryugawa in the Hirashimizu area (Figure 1C). The lower part consists mainly of conglomerate (3.5 m thick) and sandstone (2.3 m thick). The conglomerate is composed of well-rounded, poorly sorted pebbles in a brownish coarse-sand matrix, and the sandstone locally shows cross-lamination. The middle part is dominated by massive mudstones or fossiliferous rhythmites composed of thinly interbedded bluish-gray silt and white muddy layers. In the lowermost ~6 m of this part, a 0.7 m-thick cross-laminated sand bed and two intercalated conglomerate beds (2.5 m and 1.5 m thick) occur. Immediately above these, a ~2.5-m-thick, strongly folded mass is present. The steep axial planes of its folds indicate a slump formed in relatively unconsolidated submarine sediments. The middle part further intercalates with a tuff bed that is correlated with the Hiwaki Tephra (Hwk), which was deposited during the Chibanian (MIS 15, ca. 0.58–0.57 Ma) (Machida and Arai, 2003; Yamazaki, 2006). The fossil-bearing horizons in the rhythmite lie a few meters above and below this tuff bed and yield abundant plants (Shiba et al., 1990), insects (ditto), marine invertebrates such as Theora lubrica and Echinocardium cordatum (Kubota, 1978; Shiba et al., 1990), and teleosts (Yokoyama et al., 2013a, b). A molar of the proboscidean mammal Stegodon orientalis was also recorded from the one of these horizons (Tsuchi, 1961).
The fish and molluscan fossils described herein were collected from the middle part of the Saginota Formation along Urushinosawa and another small valley to its northeast in the Hirashimizu area of Minami-Matsuno (Figure 1). The fossiliferous blocks yielding teleost fish skeletons were collected from horizons P1 and P3 (Figure 2). P1 is located in the major rhythmite directly below the tuff bed (ca. 0.58–0.57 Ma) and P3 lies within the massive mudstone corresponding approximately to the P1 horizon. According to Yokoyama et al. (2013a, b), P1 has previously yielded remains of clupeid and dorosomatid clupeiforms, and horizon P3 correlates with layers yielding the engraulid clupeiform Engraulis cf. japonicus. Among the three teleost specimens described here, one block preserving a dense concentration of disarticulated skeletal remains (SPMN-FL 20004) was collected from horizon P3 in 2006 by Ichiro Miyazawa (Network for Shizuoka Prefecture Museum of Natural and Environmental History). Two additional specimens were collected by the authors during 2024–2025 field research. Molluscan fossils are rare at the locality, but the authors also found some blocks including bivalves with their outer shells from the P1 and P2 horizons. All fossil materials are housed at the Museum of Natural and Environmental History, Shizuoka (SPMN) in Suruga Ward, Shizuoka, Shizuoka, Japan.
The anatomical terminology of fish skeletons, scales, and caudal region follows Suda (1991), Kobayashi (1958) and Kimura (2024), respectively. Fin ray counts were performed following the methods of Nakabo (2013), and scale length (ScL) and scale width (ScW) measurements represent maximum anteroposterior length and maximum dorsoventral width, respectively. Other measurements were obtained from three to four points on each part. The classification and taxonomic names at higher levels follow Nelson et al. (2016). The taxonomy of molluscan fossils follows Habe (1977) and Okutani (2000).
To evaluate the taxonomy of the teleost specimens, we compared them with extant taxa. These included sea bass (Lateolabrax japonicus and L. latus) and mullet (Mugil cephalus) specimens that the authors collected from Suruga Bay and Lake Hamana in Shizuoka, Japan, as well as specimens of Chelon haematocheilus (KPM-NI 11889, KPM-NI 11890 and KPM-NI 44479) and Mugil cephalus (KPM-NI 23424) housed at the Kanagawa Prefectural Museum of Natural History in Kanagawa, Japan.
Class Actinopterygii Woodward, 1891
Division Teleostei Müller, 1845
Order Perciformes Linnaeus, 1758
Family Lateolabracidae Ghedotti et al., 2018
Genus Lateolabrax Bleeker, 1855
Lateolabrax cf. japonicus (Cuvier, 1828)
Material.—SPMN-FL 25019 (disarticulated cranial and postcranial elements from a single individual).
Locality.—Hirashimizu, Minami-Matsuno, Fuji, Shizuoka, Japan.
Horizon.—Middle part of the Saginota Formation, Ihara Group; Middle Pleistocene.
Measurements.—See Tables 1 and 2.
| M1 | M2 | M3 | M4 | |||
|---|---|---|---|---|---|---|
| Lateolabrax cf. japonicus | ||||||
| SPMN-FL 25019 | maxilla | Figure 4a | 48.2 | 38.1 | 15.4 | 3.3 |
| quadrate | Figure 4d | 38.6 | 36.3 | 15.5 | — | |
| preopercle | Figure 4e | 47.2 | 39.3 | 29.8 | 14.0 | |
| caudal fin | Figure 4h | 67.1 | 64.3 | 24.1 | 56.1 | |
| Mugil sp. | ||||||
| SPMN-FL 20004 | opercle | Figure 6b | 51.1 | 23.9 | 44.4 | — |
| preopercle | Figure 6c | 53.3 | 34.4 | 17.2 | 26.3 | |
| cleithrum | Figure 6d | 67.1 | 34.4 | 17.2 | 26.7 | |
| vertabra | Figure 6e | 11.4 | 14.2 | 17.2 | 17.6 | |
| SPMN-FL 25020 | pectoral fin | Figure 6f | 54.1 | 36.5 | 33.5 | — |
| n | ScL | ScW | |
|---|---|---|---|
| Lateolabrax cf. japonicus | |||
| SPMN-FL 25019 | |||
| Type 1 | 62 | 4.8–8.6 (mean 6.0) | 5.2–9.3 (mean 7.5) |
| Type 2 | 5 | 2.2–3.9 (mean 3.1) | 2.2–3.6 (mean 3.0) |
| Mugil sp. | |||
| SPMN-FL 20004 | |||
| Type 1 | 8 | 9.9–13.9 (mean 11.7) | 9.3–22.4 (mean 13.2) |
| Type 2 | 7 | 8.3–18.7 (mean 12.7) | 11.2–18.8 (mean 15.5) |
| SPMN-FL 25020 | |||
| Type 2 | 1 | 16.9 | 11.9 |
Description.—SPMN-FL 25019 comprises disarticulated cranial and postcranial elements together with abundant scales preserved on part (SPMN-FL 25019-1) and counterpart (SPMN-FL 25019-2) slabs, of which SPMN-FL 25019-1 exhibits the clearer relief. Identified skeletal elements include a preopercle, supracleithrum, scapula, angular, maxilla, dentary, four branchiostegal rays, quadrate, coracoid, urohyal, epihyal, autopalatine, four ribs, five vertebrae, a pelvic fin (formula I,5) and a caudal fin with 17 soft rays. 62 scales preserve well-defined external morphology.
The left maxilla lacks its posterior tip, but its anterior part is intact (Figure 4A, a). The maxilla body narrows at its anterior end and gradually widens posteriorly, slightly curving ventrally.
The left dentary (Figure 4B, b) is missing its anterior half, so the number of oblong foramina located anteriorly on the lateral surface cannot be confirmed. The posterior part is deeply forked dorsally with a swollen upper ramus, and the mandibular canal is present at the point of bifurcation. The dentary bears a dense patch of villiform teeth along the oral margin. The partial left angular missing its anterior third is triangular with dorsal and ventral posterior processes (Figure 4C, c). A median groove on the internal surface extends posteriorly to a saddle-shaped suspensorial articulation facet.
The left autopalatine bears impressions of a finger-like, posteriorly directed maxillary process and two anterior grooves. There is no evidence of a palatine tooth row. SPMN-FL 25019-2 preserves a left quadrate (Figure 4D, d) and a left preopercle (Figure 4E, e). The quadrate is fan shaped, and its thickness increases posteriorly.
The preopercle is crescent shaped. There are six large serrations along its posterior margin, which extend from the preopercular angle to the ventral margin. Several small spines are clustered at the preopercular angle. The lowermost spine is the longest and sharpest. The spaces between adjacent spines are deeply incised. The middle portion of the anterior margin is flared outward, forming a wing-like projection.
The urohyal bears paired lateral ridges along its ventral margin, a bifid anterior tip, and a slightly emarginate posterior margin. The left ceratohyal and epihyal are preserved in articulation. Four branchiostegal rays supporting the branchiostegal membrane are present along the ventral margin (Figure 4F, f). The ceratohyal is narrow anteriorly and broadens posteriorly. Its ventral margin is more strongly curved than the dorsal margin. The epihyal is subtriangular with a broad anterior end and a depression on the posterior end, which forms a facet for articulation with the interhyal. The branchiostegal series comprises four rays. Rays 1–3 articulate with the ceratohyal and are arched and flattened, and ray 4 articulates with the epihyal and is the longest and flattest.
The supracleithrum is almost completely preserved and has a nearly straight outline. A well-developed bony sensory canal forms tubular structure along the posterior margin of the dorsal half the bone. The scapula is a thin, laminar element with an elongate oval shape, a sharp posterior margin, and a large circular fenestra occupies the central portion (scapular foramen). However, it cortical surface is poorly preserved, and there is no evidence of ornamentation. The paired basipterygia are preserved in articulation with the pelvic fins and lie closely apposed at the fin base (Figure 4G, g). Each basipterygium bears an anterior process, and these opposing processes meet at the midline, forming an acute angle and delimiting a shallow median groove along their medial surfaces.
Eight compressed and flattened vertebral fragments are preserved. No transverse processes are visible on any of the preserved fragments, but haemal spines are present. In anterior view, a foramen between the neural spines (neural-arch portal) is present in some of the fragments.
The pelvic fin (Figure 4G, g) is preserved in articulation with the paired basipterygia. The left fin contains a spine and five soft rays (I,5). The caudal fin (Figure 4H, h) is detached from the caudal skeleton but is nearly complete. It comprises 17 principal rays, and the posterior margin is deeply forked, forming a symmetrical V-shaped outline with a pronounced median emargination. The lateral margin of the fin is nearly straight.
The scales are classified into two morphotypes (Type 1 and Type 2) based on shape and size. Regular body scales (Type 1) are mostly subrectangular, with four acute angles (Figure 4I, i). They are consistently longer than wide (ScL = 4.8–8.6 mm, ScW = 5.2–9.3 mm: see also Table 2). Prominent concentric ridges, arranged annularly, ornament the anterior portion of each scale, and five to ten grooves extend from the focus toward the anterior margin. Small ctenii are arranged in a single row along the posterior margin, indicating that the scales are ctenoid. Discoid-shaped scales (Type 2) are relatively rare, small, and wider than long (ScL = 2.2–3.9 mm, ScW = 2.2–3.6 mm: see also Table 2). The focus is as clear as in Type 1 scales, but small ctenii are arranged in one row along the posterior margin.
Comparisons.—The scales and skeletal elements of SPMN-FL 25019 belong to a single species, which can be referred to the family Lateolabracidae based on the following characters: (1) ctenoid scales with a subrectangular outline and rounded corners, (2) a dentary bifurcated on its posterodorsal side with a swollen upper ramus, and (3) a preopercle with five or more spines along its ventral edge (Tanaka et al., 1966; Matsui, 2008; Nakamura et al., 2022). Based on the scale of the preserved skeletal elements, SPMN-FL 25019 is estimated to have had a total body length of several tens of centimeters, comparable to species of the extant sea bass genus Lateolabrax.
Lateolabrax is the only genus of the family Lateolabracidae, and it comprises three extant species (L. japonicus, L. latus and L. maculatus) (Nelson et al., 2016). The preopercle of SPMN-FL 25019 is similar in shape and serration pattern to L. japonicus, which is characterized by five major spines ventrally directed from the angle to the ventral edge. Moreover, the preopercle of SPMN-FL 25019 is most similar to that of L. japonicus, which also bears deeply incised spaces between adjacent posterior spines and a relatively enlarged ventralmost spine. The shape of the caudal fin of SPMN-FL 25019 is also most similar to that of L. japonicus, which has a caudal fin with a moderately inwardly incised posterior margin that is more distinctly forked than those of the other extant species (Yokogawa, 2019).
Although SPMN-FL 25019 is too fragmentary to compare fin-ray patterns and/or total proportions with extant species for a robust taxonomic assignment, the combination of characters present in its preserved skeletal elements and scales suggests that it is most similar to Lateolabrax japonicus. We therefore tentatively assign it to that species.
Order Mugiliformes Linnaeus, 1758
Family Mugilidae Linnaeus, 1758
Genus Mugil Linnaeus, 1758
Mugil sp.
Material.—SPMN-FL 20004; SPMN-FL 25020.
Locality.—Hirashimizu, Minami-Matsuno, Fuji, Shizuoka, Japan.
Horizon.—Middle part of the Saginota Formation, Ihara Group; Middle Pleistocene.
Measurements.—See Tables 1 and 2.
Description.—SPMN-FL 20004 represents a concentration of at least four individuals, inferred from the number of opercles. Some cranial elements are articulated, but most bones and scales are dispersed. Identifiable elements include a quadrate, four preopercles, five opercles, three subopercles, a cleithrum, two pelvic-girdle elements, two caudal vertebrae, three pelvic fin soft rays, and two dorsal-fin soft rays. There are also abundant scales, eight over which preserve the external morphology particularly well. SPMN-FL 25020 is an articulated abdomen with associated scales preserved on part (SPMN-FL 25020-1) and counterpart (SPMN-FL 25020-2) slabs. The pectoral fin is better-preserved in SPMN-FL 25020-1 than in SPMN-FL 25020-2. Identifiable elements include a complete pelvic girdle, a pelvic fin, and a scale. Both specimens were collected from the same outcrop and their scales, pelvic fins, and pelvic girdles are similar in size and shape.
The neurocranium of SPMN-FL 20004 is preserved in articulation (Figure 6A, a), and only the dorsal surface is exposed. Boundaries between elements are difficult to discern, but the parasphenoid is preserved among the midline elements. The parasphenoid is anteroposteriorly elongated and bears three rami on its posterior third: the elongated lateral rami broaden anteriorly into flattened lobes, whereas the median posterior ramus is stout.
The right quadrate lacks its dorsal half. The preserved anterior portion is fan-shaped, and the posterior margin bears three blunt, wave-like undulations. A shallow central embayment is present, and the posterior edge is thick. The opercle (Figure 6B, b) is represented by three right and two left elements. It is subtrianguar and convex anterior to the articular fossa, and the surface of the dorsal portion is ornamented by several grooves. The preopercle (Figure 6C, c) is represented by one left and three right elements. It is L-shaped and tapers acutely toward the posterior margin. The subopercle is represented by one left and two right elements. All are incomplete, and even the best-preserved examples lack their anterior half. The subopercle is thin and laminar in shape, narrowing posteriorly to an acute posteroventral tip. There are several fine, subparallel striae along the posterior margin. An anterodorsal process is not preserved.
The cleithrum (Figure 6D, d) is almost completely preserved. Its shape is a thin lamina that broadens and curves posteroventrally. From about the upper one-third downward, the medial surface bears a flange-like shelf. Dorsally, the cleithrum flares laterally, and a rounded process is present on the anterosuperior margin. SPMN-FL 20004 preserves the paired basipterygia in articulation with the pelvic fins. The left and right girdles lie closely apposed at the fin bases. Each basipterygium bears a sharply hooked anterior process. The opposing processes meet at the midline, forming an acute angle and a shallow median groove on the medial surface. In addition, the basipterygium bears a well-developed median subpelvic process that projects straight anteriorly. SPMN-FL 25020 lacks the anterior half of the pelvic girdle. Although the basipterygium is preserved in articulation with the pelvic fin, its elements are poorly preserved (obscured by dark pigment).
Two caudal vertebrae are preserved (Figure 6E, e). Both are laterally compressed and flattened. The neural spines are completely preserved, whereas the haemal spines are only partially preserved.
SPMN-FL 20004 preserves many fin elements, but most are fragmentary. SPMN-FL 25020 preserves the left pectoral fin, although its basal portion is missing. The fin bears 16 soft rays (Figure 6F, f), which are segmented and distally branched. The fin shape broadens distally, with the longest rays positioned dorsally. The pelvic region preserves a soft ray of the right pelvic fin and a spine and five soft rays (I,5) of the left pelvic fin (Figure 6G, g).
SPMN-FL 20004 preserves a few isolated scales. The scales are relatively large and classified into two morphotypes (Type 1 and Type 2) based on their shape and the presence or absence of grooves. The shape of scale Type 1 (Figure 6H, h) is subquadrangular and slightly wider than long (ScL = 9.9–13.9 mm, ScW = 9.3–22.4 mm: see also Table 2). The anterior region bears densely spaced, annular circuli, and the focus lies slightly to the posterior side. The posterior margin is strongly abraded, and neither ctenii nor a regular row of ctenial bases is visible.
The shape of scale Type 2 (Figure 6I, i) is discoid and longer than wide (ScL = 8.3–18.7 mm, ScW = 11.2–18.8 mm: see also Table 2). The anterior margin has no ctenii, which indicates that it is a cycloid scale. The anterior region does not have grooves. The surface bears densely spaced, annular circuli, and the focus is approximately central. The scales of SPMN-FL 25020 are similar in morphology to the Type 2 scales of SPMN-FL 20004.
Comparisons.—The scales and skeletal elements of SPMN-FL 20004 and SPMN-FL 25020 belong to a single species, which can be referred to the family Mugilidae based on the following characters: (1) paired pelvic fins with I,5 elements; (2) a pectoral fin consisting of 16 soft rays; and (3) a rounded-subtriangular opercle with a small process on the anterior end forward from the articular grooves and posteriorly radiating striae (Senou, 2013; Miyata et al., 2024; Kimura, 2024). Extant mugilids distributed around Japan comprise 8 genera and 16 species (Nelson et al., 2016).
Compared with extant mugilid species, SPMN-FL 20004 shares characteristics of the opercle with and is similar in size to adults of Mugil cephalus and Chelon haematocheilus. Of those two species, the opercle of SPMN-FL 20004 is most similar to that of M. cephalus in that (1) it has a weak or no convex postero-dorsal margin, (2) the articulation socket is oriented ventrally, and (3) the shape is relatively elongated anteroposteriorly.
In the Mugilidae, the distribution of cycloid and ctenoid scales varies between genera as well across different as regions of the body (Thieme and Moritz, 2021). A fully ctenoid condition, characterized by two to five rows of independent ctenii in the posterior field, is typical of some species of Mugil (e.g. M. cephalus) (Roberts, 1993). Although the independent ctenii in the posterior field are obscure in SPMN-FL 20004 and SPMN-FL 25020, the two scale types found in both specimens also present in in the extant M. cephalus. The combination of skeletal and scale characteristics indicates that the SPMN-FL 20004 and SPMN-FL 25020 represent at least one species of Mugil. However, we leave their specific identifications unresolved because they lack any diagnostic features that distinguish extant mugilid species, such as fin-ray patterns.
Class Bivalvia Linnaeus, 1758
Order Mytilida A. Férussac, 1822
Family Mytilidae Rafinesque, 1815
Genus Musculista Yamamoto and Habe, 1958
Musculista senhousia (Benson, 1842)
Material.—SPMN-FL 25021.
Locality.—Hirashimizu, Minami-Matsuno, Fuji, Shizuoka, Japan.
Horizon.—Middle part of the Saginota Formation, Ihara Group; Middle Pleistocene.
Measurements.—Shell length = 13.3 mm; shell height = 3.9 mm.
Description.—SPMN-FL 25021 preserves a dense accumulation of multiple individuals, mostly represented by fragmented shells and single valves. The valve remains are thin and long-ovate, with exterior radial ribbing and numerous chestnut-brown stripes oriented perpendicular to the ribs.
Remarks.—SPMN-FL 25021 is identified as Musculista senhousia based on its thin, long-ovate shell and the preservation of characteristic chestnut-brown stripes on the exterior surface (Habe, 1977; Okutani, 2000). M. senhousia is a well-known opportunistic species that currently inhabits intertidal to shallow subtidal muddy bottoms across the Northwest Pacific, from the Kuril Islands to Southeast Asia (Habe, 1977).
Order Venerida Gray, 1854
Family Cyrenidae Gray, 1840
Genus Corbicula Megerle von Mühlfeld, 1811
Corbicula japonica Prime, 1864
Material.—SPMN-FL 25022.
Locality.—Hirashimizu, Minami-Matsuno, Fuji, Shizuoka, Japan.
Horizon.—Middle part of the Saginota Formation, Ihara Group; Middle Pleistocene.
Measurements.—Shell length = 9.2 mm; shell height = 5.1 mm.
Description.—SPMN-FL 25022 preserves a dense accumulation of multiple individuals, represented mostly by single valves. The outer shell layer of each is partially dissolved, and the internal sculpture is only faintly preserved. The valve remains are thick, nearly symmetrical, and rounded subtriangular, with densely spaced commarginal growth ribs. Both the anterior and posterior lateral teeth are elongate.
Remarks.—SPMN-FL 25022 is identified as Corbicula japonica based on its rounded subtriangular shell, relatively thick valves, and elongate lateral teeth (Habe, 1977; Okutani, 2000). Corbicula japonica is a common euryhaline bivalve that inhabits brackish-water environments, such as estuaries and lagoons, across the Japanese archipelago, Sakhalin, and the Korean Peninsula (Okutani, 2000).
The teleost fossil assemblage from the Saginota Formation currently includes five taxa: Perciformes (Lateolabrax cf. japonicus), Mugiliformes (Mugil sp.), and Clupeiformes (Engraulis cf. japonicus, Dorosomatidae gen. et sp. indet., and Clupeidae gen. et sp. indet.) (Yokoyama et al., 2013a, b; this study). Although the alpha taxonomy of this assemblage remains unresolved, its composition is consistent with the present-day ichthyofauna of inner-bay to estuarine brackish environments under temperate to warm-temperate conditions (Yabumoto and Uyeno, 1990; Nakabo, 2013).
Along the Pacific coast of Japan, marine ecosystems are broadly classified according to the influence of the Kuroshio (warm-water or lower-latitude) and Oyashio (cold-water or higher-latitude) currents (e.g. Yatsu et al., 2013). The Middle Pleistocene (Chibanian) Miyata Formation (Kazusa Group) in Kanagawa has yielded at least 1,389 fish fossil specimens representing 62 taxa (20 orders and 31 families: Mitsui et al., 2021). This assemblage is dominated by temperate-water species from an open-shelf environment at an estimated water depth of 100–200 m. However, the co-occurrence of subarctic species, such as Clupea pallasii and Theragra chalcogramma, within the same fossil assemblage suggests that it represents a mixed-water regime influenced by both the Kuroshio and Oyashio currents, which allowed cold-water species to temporarily disperse southward. This interpretation is consistent with the hypotheses that the warm Kuroshio Current migrated northward and reached its present latitude of around 35°N by ~3 Ma (Gallagher et al., 2015), and that the subarctic front separating the Oyashio from the Kuroshio migrated meridionally along the southeastern coast of Japan during the Chibanian (Suganuma et al., 2018). Similarly, the paleo-Suruga Bay may have developed comparable mixed surface waters. However, the fossil assemblage from the Saginota Formation is characterized by the dominance of L. cf. japonicus and Mugil sp., which are more typical taxa of shallower, stratified inner-bay or estuarine regions rather than of the outer-shelf environment (to ~200 m) inferred for the Miyata Formation. Moreover, cold-water indicators (e.g. Clupea pallasii) are absent from the Saginota Formation. This may reflect differences in depositional environment (e.g. water temperature), age, or both between the Saginota and Miyata formations (Mitsui et al., 2021).
In modern oceans, Lateolabrax and Mugil are abundant around the Japanese Archipelago, but their fossils are rare, except at archaeological sites dating from the Jomon period onward. A fossil of Lateolabrax cf. japonicus was discovered from the Katanoyama Formation (Calabrian) on Tanegashima (Yabumoto and Uyeno, 1990), and the SPMN-FL 25019 from the Saginota Formation represents the second fossil record of the genus from Japan. There are two previous fossil records of Mugiliformes from Japan: one is a large mugiliform from the Katanoyama Formation (Yabumoto and Uyeno, 1990) and the other is Mugilidae gen. et sp. indet., described from isolated opercular and supracleithral elements from the Renkoji Formation (Calabrian) along the Tama River in Tokyo (Miyata et al., 2024).
Depositional environment of the fish fossilsThe fish fossils from the Saginota Formation were recovered from rhythmites, which are generally formed in restricted bodies of water with low kinetic energy or deposited rapidly without bottom disturbance by waves or currents (Asano et al., 2012; Yokoyama et al., 2013a). Yokoyama et al. (2013a) also noted that the abundance of Theora lubrica in these rhythmites indicates a poorly ventilated depositional environment either on an inner-bay floor or a brackish-lake bottom. Moreover, such a closed water-environment is supported by the presence of Musculista senhousia, described in this study, which is tolerant of eutrophic conditions in inner-bay and brackish-water environments (Nambu et al., 2006).
The scarcity of benthic invertebrates and trace fossils in the Saginota Formation further implies the presence of dysoxic to anoxic bottom waters at the time of deposition. The coexistence of Lateolabrax cf. japonicus and Mugil sp. is consistent with an inner-bay or brackish-water environment. Thus, the accumulation of four individuals of Mugil sp. may have resulted from a mass mortality event caused by the expansion of dysoxic waters. Anoxic water masses in eutrophic, restricted embayments commonly occur in modern Japanese waters (e.g. Maruo and Yokota, 2012). In addition to the presence of pollution-indicating bivalves, all mollusk specimens collected in this study are all remarkably small compared to their extant counterparts and represent juvenile individuals. This implies that these mollusks were episodically transported into an environment unsuitable for competitive benthic fauna, such as a dysoxic bottom-water environment, where they temporarily survived and reproduced before dying off in large numbers. The short residence time and apparent absence of persistent marine inflow suggest that the area did not constitute an ideal long-term habitat for marine benthos. Therefore, the depositional environment of the Saginota Formation was likely a semi-enclosed brackish system lacking steady marine replenishment, rather than an open inner-bay setting with continuous exchange. Although the plant fossils from the Saginota Formation will be described in a future study, we observed abundant terrestrial plant remains and the freshwater macrophyte Myriophyllum spicatum, suggesting fluvial influx into the basin, as would be expected if it was a closed-water environment.
The nature of the fish fossil assemblage of the Saginota Formation closely resembles that of the Miocene Tottori Group (Asano et al., 2012; Hamada et al., 2026). The fish fossils from the Tottori Group were likely deposited in a lagoon, based on the following evidence: (1) interbedded rhythmites and intervening black claystone, reflecting strongly reducing conditions (Hirao et al., 2012); (2) a near-shore freshwater to marine-influenced lake/lagoon setting (Yamana, 1963); (3) the abundance of shallow-marine fishes (Yamana, 1963; Uyeno and Hirao, 2000); and (4) the scarcity of benthic fauna and absence of any trace fossils, implying dysoxic to anoxic bottom waters. According to Asano et al. (2012), the exceptional preservation of those fish fossils can be described by a four-stage depositional process, as described below (see also Figure 8).
1. Fair-weather conditions.—A stable halocline developed in the lagoon, with a low-salinity surface layer overlying a high-salinity brackish bottom layer. This produced density stratification and bottom conditions unsuitable for benthic colonization.
2. Storm conditions.—During storms, storm surges and high waves forced large volumes of seawater into the lagoon as a salt wedge, carrying shallow-marine fishes landward.
3. Post-storm phase 1.—After the storm abated, salt-wedge intrusion ceased. In the stratified bottom waters, oxygen was rapidly consumed through the decomposition of settling organic matter, sediment reduction, the mixing of brackish water masses, and respiration by the intruding fauna. This led to the morbidity and mortality of shallow-marine fishes.
4. Post-storm phase 2.—The more than 100 fish specimens collected were entombed within very fine-grained sand rather than on bedding surfaces or within silt. This indicates that they were buried rapidly after the storm as the fine sand settled. Rapid burial protected the carcasses from decay and destruction, resulting in their exceptional preservation.
By analogy with the Tottori model, the fish-bearing beds at Hirashimizu probably formed in a semi-enclosed brackish system lacking steady marine inflow. Episodic storm surges and wave action transported shallow-marine fishes landward. Subsequent isolation of the basin and oxygen depletion in the bottom waters caused mass mortality, while limited bioturbation at the sediment–water interface promoted the preservation of articulated skeletons (Figure 8).
Specimen-specific postmortem features strongly support this scenario. The lateolabracid SPMN-FL 25019 exhibits a well-preserved caudal fin that is disarticulated from the caudal skeleton, selective preservation of left lateral cranial elements, and preferential loss of right lateral scales. The mugilid specimens SPMN-FL 20004 and SPMN-FL 25020 exhibit selective preservation of one element from paired cranial bones and the co-occurrence of articulated scales and cranial elements. These patterns suggest that after seafloor emplacement, the carcasses underwent a period of decay before burial and were exposed to weak bottom currents that preferentially winnowed the more exposed side. This interpretation is consistent with the assertion of Yokoyama et al. (2013a, b) that articulated fish remains decayed on the seafloor without significant scavenger disturbance or dispersal of disarticulated elements. In summary, the fish-bearing beds of the Saginota Formation represent a semi-enclosed brackish environment lacking persistent marine inflow, in which rare storm events transported marine fishes into the basin, followed by rapid bottom-water oxygen depletion. Limited benthic activity and burial within fine-grained sediments subsequently promoted the exceptional preservation of articulated fish fossils.
The faunal assemblage from the middle part of the Saginota Formation (Chibanian, MIS 15) comprises two articulated teleosts, Lateolabrax cf. japonicus and Mugil sp., and two brackish bivalves, Musculista senhousia and Corbicula japonica. The teleost assemblage is closely comparable to the present-day ichthyofauna of inner-bay to estuarine brackish environments under temperate to warm-temperate conditions, and its taxonomic composition indicates a generally temperate climatic setting. Comparison with the coeval Miyata Formation fish fauna along the Pacific coast further suggests that paleo-Suruga Bay was influenced by a mixed Kuroshio–Oyashio surface-water regime during the Chibanian. The taxa documented in this study, together with sedimentilogical observations, indicate that the fossil-bearing beds of the Saginota Formation were deposited in a semi-enclosed brackish environment with intermittent marine inflow. This environment was characterized by dysoxic to anoxic bottom waters, as indicated by the development of rhythmites and the concentration of juvenile brackish bivalves. The fish remains were probably buried under these dysoxic conditions with minimal disturbance and limited bioturbation. This study contributes to our understanding of the paleobiogeography of marine fishes during the Chibanian and the establishment of the modern fish fauna around central Japan.
We thank I. Miyazawa for collecting the fossil specimen SPMN-FL 20004 described herein. We are grateful to H. Senou and H. Wada (Kanagawa Prefectural Museum of Natural History) for providing access to the collections at their institution and for their helpful comments on fish taxonomy. We also thank A. Kitamura (Shizuoka University) and T. Sasaki (The University Museum, The University of Tokyo) for their helpful comments on molluscan taxonomy. Special thanks are given to M. Shiba (Museum of Natural and Environmental History, Shizuoka) for providing geological information and valuable suggestions regarding the fossil locality. We thank the anonymous reviewers for constructive comments and suggestions that improved the manuscript.
S.H. and Y.N. initiated the study and were primarily responsible for the taxonomic aspects. K.Y., Y.N. and S.H. conducted field surveys in the study area and collected geological data. J.L. supervised the study and contributed to the preparation and revision of the manuscript. All authors contributed to the writing of the paper. All authors contributed to the writing of this paper.
