RESEARCH ARTICLE
A new Miocene smelt fish, Spirinchus izumoensis sp. nov. (Osmeriformes, Osmeridae) from Hiebara in Izumo, Shimane, Japan
2026 年 30 巻 p. 66-76
詳細
2026 年 30 巻 p. 66-76
A new smelt fish, Spirinchus izumoensis sp. nov., is described from the Lower to Middle Miocene Kawai Formation in Shimane, southwestern Japan. The new species differs from its congeners in possessing the unique combination of the following characteristics: enlarged and thickened anal fin rays and enlarged anterior proximal pterygiophores; non-parallel anterior half proximal pterygiophores and parallel posterior proximal pterygiophores of the anal fin; short proximal pterygiophores at the middle part of the anal fin, almost as short as or shorter than their associated haemal spines; and total of 48 vertebrae. The new species was found along bedding planes with well-developed lamination, suggesting that it was deposited under low-oxygen bottom-water conditions. It is inferred that a brackish estuarine environment influenced by both freshwater and marine conditions prevailed during the deposition of mudstone in the Kawai Formation.
ZooBank registration: urn:lsid:zoobank.org:act:CA1A427E-7EFE-438B-B1F9-D5E68F479395
The genus Spirinchus is a group of teleostean fishes classified within the family Osmeridae. This genus includes three extant species (Spirinchus lanceolatus, S. starksi and S. thaleichthys) that inhabit the subarctic to boreal lakes, rivers and coastal marine environments of the North Pacific and its adjoining river systems (McAllister, 1963). Two of these species (S. lanceolatus and S. thaleichthys) are known to undertake upstream spawning migrations (McAllister, 1963). Like other members of the family Osmeridae, Spirinchus species are small (generally <400 mm in standard length, most commonly <200 mm), possess a slender and shallow body, and typically form schools in freshwater, brackish, and marine environments (McAllister, 1963; Nelson et al., 2016).
The family Osmeridae includes six extant genera with 15 species distributed across the Pacific, Atlantic, and Arctic oceans in the Northern Hemisphere. Numerous fossil species of this family from the Oligocene to Miocene have also been reported (e.g. Weiler, 1963; Gaudant and Burkhardt, 1984; Uyeno and Sakamoto, 1999; Reichenbacher, 2000; Prokofiev, 2005; Nelson et al., 2016). However, fossil records of the genus Spirinchus are extremely limited, with only two species from Japan reported to date: Spirinchus akagii Uyeno and Sakamoto, 1999 from Tottori and Spirinchus sp. from Kyoto (Uyeno and Sakamoto, 1999; Yasuno, 2017).
In this paper, we describe a new species of Spirinchus based on a single specimen recovered from the Lower to Middle Miocene Kawai Formation exposed at Okitani in Hiebara, Izumo, Shimane, southwestern Japan and discuss the environment in which it was deposited. The type specimen in part and counter part is reposited in the collection of the Shimane University Museum (SMU-IC).
(by M. Hamada and Y. Yabumoto)
Order Osmeriformes Regan, 1913
Family Osmeridae Regan, 1913
Genus Spirinchus Jordan and Evermann, 1896
Type species.—Spirinchus thaleichthys (Ayres, 1860).
Spirinchus izumoensis sp. nov.
(New Japanese name: Izumo shishamo)
ZooBank lsid: urn:lsid:zoobank.org:act:CA1A427E-7EFE-438B-B1F9-D5E68F479395
Holotype.—Shimane University Museum catalog number SMU-IC-F0061a, part and SMU-IC-F0061b, counterpart, nearly complete skeleton, 39 mm in standard length, presumed male.
Locality, geological setting, and age.—The holotype specimen was collected from the Kawai Formation at Okitani in Hiebara, Izumo, Shimane, Japan (Figure 1) (35°18′53.3″N, 132°49′56.5″E). Its age is latest Early to earliest Middle Miocene.
In Hiebara Town, Neogene sedimentary rocks and volcanic rocks (Kamishima Volcanic Group) unconformably overlie the basement rocks, which consist of rhyolitic tuff and quartz porphyry (Hiyodori Granite). This region has been the focus of several stratigraphic investigations since the 1950s (e.g. Katto and Nakamura, 1952; Okamoto, 1959; Hojyo, 1964; Matsuura et al., 2005; Arita et al., 2017).
The Lower to Middle Miocene Kawai Formation in Hiebara exhibits significant lateral variation in facies. The formation is composed of arkosic sandstone or pebbly sandstone and laminated mudstone containing lignite with multiple intercalated beds of pyroxene andesite. The Kawai Formation includes plant macrofossils correlated with the Daijima-type flora, marine molluscan fossils, and fish fossils (Okamoto, 1959; Hojyo, 1964; Takao, 2016; Arita et al., 2017).
The holotype was discovered at Okitani, where mudstone from the Kawai Formation is exposed as a limited outcrop. This mudstone is pale yellowish-brown with a grayish tint and is marked by well-developed planar parallel lamination. In the upper part of the outcrop, white tuffaceous mudstone is observed.
The age of the Kawai Formation in Hiebara remains unclear. However, Sawada et al. (2013) conducted K–Ar dating of Neogene volcanic and plutonic rocks in eastern Shimane and reported a whole-rock K–Ar age of 15.4 ± 0.6 Ma for the Kawai Formation. Hayashi et al. (2018) analyzed planktonic foraminifers and calcareous nannofossils from the Susa area of Izumo and concluded that the age of the upper Kawai Formation to the overlying lower Kuri Formation corresponds to the planktonic foraminiferal zones N.8 to N.9 of Blow (1969) and the calcareous nannofossil zones CN3 to CN4 of Okada and Bukry (1980). Additionally, Hayashi et al. (2013) examined planktonic foraminifers near the type locality of the Kuri Formation and correlated the assemblage with the planktonic foraminiferal Zone N.8 of Blow (1969) and the calcareous nannofossil Zone CN3 of Okada and Bukry (1980). Thus, the age of the Kawai Formation is estimated to be Early to Middle Miocene (Figure 2).
Etymology.—Izumo is an ancient place name from Japanese mythology associated with the founding of Japan. The city of Izumo, where the fossil was found, is also part of the Izumo region in Shimane.
Diagnosis.—The new species is distinguished from other species of the genus Spirinchus in having short proximal pterygiophores at the middle part of the anal fin that are almost as long as or shorter than the associated haemal spines and a total of 48 vertebrae.
DescriptionThe holotype consists of a part and counterpart (Figure 3).
Both the part and counterpart show the lateral sides of the body. The body is slender. The standard length is approximately 3.5 times the head length and approximately 7.7 times the maximum body depth, the latter measured at the level of the pelvic fin insertion.
Skull.—The head and snout are long (Figures 3, 4). The upper jaw is not preserved. The distance between the anterior end of the lower jaw and the posterior end of the opercle is approximately 2.1 times the distance between the anterior end of the lower jaw and the anterior margin of the orbit and 3.5 times the orbital diameter.
The lower jaw is well preserved (Figure 4). The dentary is deep with a round dorsal margin and with a groove along the ventral margin. The length of the dentary is approximately 2.6 times its depth. The teeth are unobservable.
The angular is triangular and smaller than the dentary. The articulation of the angular and quadrate is just under the anterior margin of the orbit (Figure 4).
The eye is large and the eyeball is preserved as being black in color. The length of the head behind the orbit is slightly longer than the orbit diameter. Although the outlines of the infraorbital bones are not clear, the bones surround the orbit (Figure 4).
The anterior and ventral margins of the quadrate are clearly exposed, but the dorsal and posterior margins are not visible because they are covered by the infraorbital. The quadrate contacts the ectopterygoid anteriorly. The hyomandibular attaches to the anterior margin of the dorsal arm of the preopercle. The hyomandibular consists of a wide dorsal part and a narrow ventral part. Other bones of the suspensorium are not visible.
The preopercle is L-shaped (Figure 4). Its dorsal arm is slightly longer than its ventral arm. The opercle is large and approximately quadrangular in shape. The width of the opercle is shorter than its depth. The subopercle is relatively large and its depth is approximately 25% of the depth of the opercle. The interopercle is not clearly exposed.
Vertebrae and ribs.—Both the abdominal and caudal vertebrae numbers are 24 (Figure 3). The length of each abdominal vertebra is almost the same as its depth. The length of each caudal vertebra is slightly longer than its depth. The ribs are long, extending to the ventral margin of the body, and their total number is 22.
Caudal fin and skeleton.—The caudal fin is forked (Figure 5). The long unbranched dorsalmost and ventralmost fin rays are thick, and the segments are long. There are eight branched caudal fin rays in the upper lobe and seven in the lower lobe. Five or six hypurals, the parhypural, and at least one epural are visible (Figure 5). The first hypural is the largest in size, followed by the third or fourth hypural.
Dorsal and anal fins.—The dorsal fin is situated at the middle of the body. It originates slightly behind the site of insertion of the pelvic fin. The dorsal fin base is shorter than the anal fin base. The number of dorsal fin rays is 14. The dorsal fin pterygiophores are not clearly preserved.
The anal fin originates behind the end of the dorsal fin base. The anal fin base is long. The number of anal fin rays is 17 (Figure 6), and the number of anal fin proximal pterygiophores is 17. The three anterior proximal pterygiophores of the anal fin are enlarged. The eight anterior proximal pterygiophores are non-parallel and the 8th to 16th proximal pterygiophores are parallel. The proximal pterygiophores at the middle part of the anal fin are short, and those behind the 6th proximal pterygiophore are shorter than their associated haemal spines (Figure 6).
Pectoral and pelvic fins.—The pectoral fins are large and located ventrally. Twenty-two pectoral fin rays can be counted (Figure 3). The pectoral fin extends to the point under the 13th abdominal centrum.
The cleithrum is well preserved. The part along the anterior margin of the cleithrum is thick and has a posterior-wide flange. The coracoid is slender, and its anterior part is curved downward.
The pelvic fin is located slightly behind the midpoint of the abdomen (Figure 3). The pelvic fin rays are long and extend beyond the posterior end of the dorsal fin base. The number of pelvic fin rays is 7 or 8. The basipterygium is slightly longer than the length of the four centra and is located anterior to the dorsal origin.
The new species described in this paper is most closely related to Spirinchus akagii Uyeno and Sakamoto, 1999 from the Lower to Middle Miocene Iwami Formation at Miyanoshita in Kokufu, Tottori City, Tottori Prefecture. This assertion is based on it sharing almost the same meristic characteristics, as well as enlarged and thickened anal fin rays and enlarged anterior proximal pterygiophores, non-parallel anterior half proximal pterygiophores, and parallel posterior proximal pterygiophores of the anal fin.
Uyeno and Sakamoto (1999) referred S. akagii to the order Osmeriformes based on its greatly enlarged and thickened anal fin rays and proximal pterygiophores in presumed male specimens, given that a sexually dimorphic anal fin is found only in osmeriform fishes of the Salangidae and several genera of the Osmeridae (McAllister, 1963; Wilson and Williams, 1991). Furthermore, they referred S. akagii to the family Osmeridae rather than the Salangidae based on its general appearance and discussed the characteristics that distinguish it from other Osmeridae species (Uyeno and Sakamoto, 1999). Spirinchus akagii is most similar to Spirinchus lanceolatus, with the most enlarged and thickened anal fin rays and anterior proximal pterygiophores among all osmerids classified into this family to date (Uyeno and Sakamoto, 1999).
The holotype of Spirinchus izumoensis sp. nov. is assumed to be male because of its enlarged and thickened anal fin rays, and it is referred to the family Osmeridae because it possesses anal fin rays and anterior proximal pterygiophores similar to those of S. akagii. However, this new species differs from S. akagii in having proximal pterygiophores that are almost as short as or shorter than their associated haemal spines at the middle part of the anal fin (Figure 6). In Spirinchus akagii, the proximal pterygiophores are almost twice as long as their associated haemal spines at the middle of the anal fin. See Table 1 for a complete comparison of diagnostic characters among recent and fossil species of Spirinchus.
| Characters | Spirinchus izumoensis sp. nov. | Spirinchus akagii (male) | Spirinchus akagii (female) | Spirinchus lanceolatus |
|---|---|---|---|---|
| vertebra | 48 | 48–50 | 49 | 60–65 |
| Abdominal vertebra | 24 | 23 | 24 | 32–38 |
| Caudal vertebra | 24 | 27 | 25 | 24–28 |
| dorsal fin rays | 14 | 13 | 12 | 10–11 |
| proximal pterygiophores of the dorsal fin | ? | 13 | 11 | – |
| anal fin rays | 17 | 16 | 17 | 17–20 |
| proximal pterygiophores of the anal fin | 17 | 15 | 16 | – |
| anterior proximal pterygiophores of the anal fin | not parallel (male) | not parallel | not parallel | parallel (both male and female) |
| middle proximal pterygiophores of the anal fin | not parallel (male) | not parallel | not parallel | parallel (both male and female) |
| posterior proximal pterygiophores of the anal fin | parallel (male) | parallel | parallel | parallel (both male and female) |
| ratio of length of the proximal pterygiophores to associated haemal spines at the middle part of the anal fin | 1:1 | 2:1 | 1:1 | 2.6:1 |
| pelvic fin rays | 7–8 | 8 | 8 | 9 |
| origin of pelvic fin | in advance of origin of the dorsal fin | in advance of origin of the dorsal fin | in advance of origin of the dorsal fin | anterior to dorsal origin |
To infer the paleoenvironment that prevailed at the time when the holotype of Spirinchus izumoensis sp. nov. was deposited, the depositional setting at Okitani in Hiebara, Shimane was examined and compared with that of the Iwami Formation at Miyanoshita in Kokufu, Tottori City, Tottori Prefecture (where the closely related species S. akagii has been reported).
Fish fossils from the Kawai Formation in Okitani, including Spirinchus izumoensis sp. nov., are preserved along parallel laminae and exhibit excellent preservation, with no evidence of postmortem scavenging by other organisms. These features suggest deposition in a low-oxygen or anoxic bottom-water environment. Furthermore, the presence of molluscan fossils from intertidal to shallow marine species in the underlying sandstone indicates that there was probably a brackish estuarine environment influenced by both freshwater and marine conditions at the time of mudstone deposition in the Kawai Formation.
The depositional environment of the Iwami Formation at Miyanoshita was described in detail by Asano et al. (2012a, b). Specifically, Asano et al. (2012b) reported the fish fossil-bearing beds at Miyanoshita as thinly interbedded fine-grained sediments yielding shallow marine to coastal fish fossils, while notably lacking both benthic invertebrate fossils and trace fossils. Based on these features, they inferred that the dead fishes were deposited under low-energy flow conditions within a shallow marine to coastal lagoon setting. They also suggested that this lagoon was relatively deep and enclosed, promoting the development of a strong halocline, which in turn resulted in persistently anoxic conditions in the highly saline brackish bottom waters. Asano et al. (2012a) regarded the fish fossil-bearing beds at Miyanoshita as the basal part of the Tochimoto Shale Member of the Iwami Formation and estimated that it was deposited between 16.5 and 16 Ma based on previous studies (e.g. Hirao et al., 2005). Subsequently, Haji et al. (2022) re-evaluated the age of the Iwami Formation around Miyanoshita and reported a U–Pb zircon age of 17.44 ± 0.16 Ma from a tuff horizon above the beds yielding fossils of S. akagii.
As noted above, the depositional environment of the Lower to Middle Miocene Kawai Formation distributed at Okitani in Hiebara, Shimane is similar to that of the Lower to Middle Miocene Iwami Formation at Miyanoshita in Kokufu, Tottori, suggesting that the sediments were deposited in a lagoon-like setting. Although the precise depositional age of the Kawai Formation remains unclear, consideration of the age of the Iwami Formation yielding S. akagii indicates an age difference of approximately 2 to 1.5 million years, showing that the holotype of S. izumoensis sp. nov. was deposited during a geologically younger period (Figure 7).
We would like to thank Seiji Miyazaki (Shimane University) for support in collecting fossils. We are grateful to reviewers Jennifer A. Lane and Shinya Miyata for their critical reading and constructive comments on the manuscript. This work was supported by JST SPRING, Grant Number JPMJSP2155.
M.H. and Y.Y. mainly designed and performed this study and conducted morphological observations. N.A. collected the holotype and conducted the field survey. M.H., N.A., and T.I. conducted the field survey and discussed stratigraphy and taphonomy. A.T. facilitated access to the Hiebara area and collected fossil specimens. M.H., Y.Y., and T.I. contributed to writing the manuscript. All authors contributed writing the paper.
