The Omma-Manganji ostracod fauna (Plio-Pleistocene) of Japan and the zoogeography of circumpolar species

The Omma-Manganji fauna of Japan signifies a time during the late Pliocene and Pleistocene when arctic-subarctic species migrated far south of their present geographic range in response to oceanographic changes. Omma-Manganji deposits exposed on Hokkaido, northern Honshu, and Sado Islands yielded about 224 species of marine Ostracoda. At least 26 are circumpolar species known previously from Arctic seas off the British Isles, eastern North America, Scandinavia and Europe, comprising between 14 and 47% of the ostracod assemblage in eight of ten formations studied. The 26 circumpolar species and 21 other western Pacific cryophilic species are illustrated and their distribution in Japanese deposits is discussed.

Unlike the North Atlantic, cryophilic ostracods from Neogene and Quaternary deposits of the North Pacific are not so well known, despite their potential in palaeoclimatology and, as pointed out by Hanai (1977), in evolutionary studies. The Japanese Islands, located between 46"N and 24"N latitudes, are characterised by relatively steep water temperature gradients and include cold temperate to subtropical marine climatic zones. The Omma-Manganji fauna, originally applied to cold water molluscan assemblages in the Pliocene of Japan (Otuka, 1939;Chinzei, 1978), represents one or several periods during which high-latitude species migrated southward and offers an excellent opportunity to investigate cryophilic ostracods in Japan. Early studies by Hanai (1957aHanai ( , b, c, 1959Hanai ( a, b, 1970 laid the taxonomic groundwork on Omma-Manganji ostracods, showing that typical high-latitude genera such as Palmenella, Cythere and Semicytherura occurred as fossils in the Sawane and Setana Formations of Japan. Ishizaki (1966Ishizaki ( , 1971) also described Japanese ostracod species having similarities with arctic forms. Recently, Okada (1979) and Ishizaki & Matoba (1985) illustrated additional ostracods from Plio-Pleistocene deposits of Akita Prefecture having affinities with North Atlantic species. Similarly.  found several previously unrecognised cryophilic ostracod genera in the Daishaka Formation of northern Honshu and Hayashi (in press) gave a useful discussion of the stratigraphy and ostracods of the Setana Formation.
In our study of Omma-Manganji ostracods, we recognised at least 26 cryophilic species which either live today in high-latitude seas off the British Isles, northeastern North America, Europe and Scandinavia and/or occur in late Pleistocene deposits of that region. Although the taxonomy of some Omma-Manganji ostracods is in a preliminary state, we felt the discovery of well known cryophilic species merited special attention because of the zoogeographic and evolutionary implications. Consequently, our primary goals are 1) to record the distribution of circumpolar species in Japanese Neogene and Quaternary Omma-Manganji deposits from the Oshima Peninsula of Hokkaido, Sad0 Island in the Sea of Japan, the Oga Peninsula and the Hokuriku area on the Japan Sea side of Honshu (Fig. l), 2) to illustrate circumpolar species and other cryophilic species not known from seas off Europe and North America and 3) to briefly discuss some evolutionary patterns in cold water genera.

MATERIAL AND METHODS
A total of 53 ostracod-bearing samples form the basis of this paper. Most were collected by the authors during the summer of 1984 from the Oshima Peninsula, Sado Island, and the Hokuriku province, those from the Oga Peninsula and Gojonome region, Akita Prefecture, collected in 1985 by N. Schmidt and Dr. K.
Ishizaki were also examined.
Between 50 and l00g of dried sedimcnt were washed through nested sieves in U . S . Gcological Survey (USGS) laboratories in Reston, Virginia. Most sediments were unconsolidated and disaggregated easily. Sodium carbonate (NaC03) or varsol, a kerosene product, were used to break down indurated samples. When dry, sediment coarser t h a n 1SOpm was sieved; each size fraction was sprinkled on a tray and ostracods were picked and placed on micropaleontologic slides.
Collections in the Smithsonian's Museum of Natural History, the U.S. Geological Survey, the University Museum, Tokyo University, and the Institute of Geosciences, Shizuoka University were examined to identify species. Scanning electron photomicrographs were taken on a JEOL 35-C at the USGS.

STRATIGRAPHY
The stratigraphic relationships of the geological formations studied are shown in Fig. 2 as based on biostratigraphic, palaeomagnetic and radiometric data. In some cases, the age of a formation is not precisely known and the correlations are considered approximate. Table 1 gives locality information for stratigraphic sections yielding ostracods. The 18 localities studied are shown on location maps in Figs. 3 and 4; stratigraphic sections for each are shown in Fig. 5.

Oshima Peninsula, Hokkaido
The Setana Formation is exposed in the Yakumo, Imagane, and Kuromatsunai areas of the Oshima Peninsula. Samples were collected along the Soibetsu River, a tributary of the Shubuto River in the Kuromatsunai area; and along the Penkerupeshube and Sakkurubetsu Rivers, which are tributaries of the Yurappu River in the Yakumo area, and the Kuroiwa River, a tributary of the Toshibetsu River in the Imagane area  Hayashi (1982) discussed the stratigraphy of these areas in detail. The Setana Formation represents planktonic foraminifera1 zone N22 (early Pleistocene) and Chitoku (1983) correlated the Setana in the Imagane and Kuromatsunai areas to coccolith subzone CN14a (0.9-0.3 my) based on calcareous nannofossils Gephyrocapsa spp. and Pseudoemiliania lacunosa.
The Tomikawa Formation, located in southwestern Hokkaido, consists of fossiliferous conglomerate, pebbly sandstone and coarse to medium sandstones. Samples were collected for ostracods from sections exposed on the Muneyama River 6km northwest of Kamiiso and the Hosokomata-zawa River about 3 km northwest of Tomikawa (Fig. 3d). Although previously considered Pliocene in age, Akiba (1981) found the diatom Rhizosolenia curvirostris in this formation indicating a lower to middle Pleistocene age.
(Figs. 4b-d) correspond to those described by Ishizaki & Matoba (1985). The stratigraphy of these deposits has been described in detail by Kitazato (1975) for the former area and Matsui (1981Matsui ( , 1985 for the latter.

Sad0 Island, Niigata
On Sado Island in the Sea of Japan, the Sawane Formation, including (in ascending order) the Kawachi, Kaidate and Shichiba Members, is exposed in seacliffs on the west coast of Mano Bay. Several sections (numbers 13-16, Table 1) exposing laminated to massive siltstones and shelly pebbly sands were collected from these cliffs and small tributaries nearby

Hokuriku Province, Toyamahhikawa
The Himi Group includes fossiliferous deposits in the Hokuriku Province on the Japan Sea side of central Honshu, including the namesake of Japanese cold water faunasthe Omma Formation. Ostracods were collected from the Junicho Formation (section 17) exposed near the town of Himi, Toyama Perfecture and the Omma Formation (section 18) along the Sai-gawa River 4km south of Kanazawa, Ishikawa Prefecture ( Formations correspond respectively to sections 28 and 3 of Hasegawa (1979) who demonstrated a late Pliocene to Pleistocene age for the Junicho Formation and a middle Pleistocene age for the Omma Formation.

THE OMMA-MANGANJI OSTRACOD FAUNA
A total of 224 ostracod species were identified from the ten formations. Many of these were rare, constituted small percentages of the total assemblages, and were difficult to identify as previously described species. Consequently, from the 224 total species, 135 species occurring in three or more samples were studied in detail and the following analyses refer only to these species. Table 2 summarises the total numbers of samples, specimens, and species for each formation. Almost 9000 specimens of the 135 common species were examined. The average number of specimens per sample was 170 valves and/or carapaces.
We calculated Jaccard similarity coefficients to compare the overall similarities between the ostracod assemblages from the ten formations studied based on the presence or absence of the 135 common species (Table 3). The Jaccard coefficient is calculated as C/N1 + N 2 -C, where C is the number of species common to two formations being compared and N , and N2 are the total numbers of species in the two formations (Hazel, 1970b). The highest Jaccard coefficients are for the Sawane and Setana Formation (.582), which have 64 species in common, and the Setana and Tomikawa Formations (.562), with 59 species in Omrna-Manganji ostracods of Japan kilometers ", , , common. Relatively high values werc also obtained for thc tollowing pairs of formations: Sctana-Sasaoka, Sawanc-Sasaoka and Sasaoka-Junicho. 'I'he lowest Jaccard coefficients werc obtained for the Oinma and Kitaura Formations (. 120) with only 9 spccics in common, and the Omma and Wakimoto Formations (.130) with only 10 in common. Other low Jaccard values were obtained for the Kitaura-Junicho, the Kitaura-Anden and Wakimoto-Anden Formations.
High similarities are generally interpreted to signify similar palaeoenvironments, in particular, similar water temperatures and depths. Likewise, the low similarities are believed to signify faunal differences associated with differences in these environmental parameters. For example, the low similarities between the Omma Formation and most other formations may reflect a deeper water environment, probably mid to outer shelf, in contrast to inner shelf or bay for other formations and Ogasawara (1981) has postulated a complex sequence of oceanographic changes for Omma-Mangan ji deposits.
Because our focus was on cryophilic species, we divided the total ostracod fauna into three categories of species. First, the 26 species we refer to as "circumpolar" (Table 4) are well documented in Recent and/or fossil deposits of the North Atlantic and adjacent Arctic seas. Herein, the term "circumpolar" refers only to the Arctic, not the Antarctic. We do not imply necessarily that these circumpolar species live in the Arctic region today, only that they are known from fossil and/or modern sediments from high-latitude northern hemisphere seas.
Many circumpolar species are reported here for the first time from the Pacific. In some cases, morphological differences between Pacific and other Arctic regions (particularly the North Atlantic) populations may only signify intraspecific variation. For example, Baffinicythere howei and Cytheropteron champlainum are morphologically distinct from Atlantic forms, but the overall similarities appear to be so strong that we believe they merit subspecies status. Conversely, in other cases, such as Hemicytherura clathrata, Heter-ocyprideis fascis, Sarsicytheridea bradii, Palmenella limicola, Hemicythere emarginata, Semicytherura affinis and others, Pacific specimens are within the range of normal intra-population variability for Atlantic forms. Taxonomic affinities are discussed briefly in the systematic palaeontology section below.
The third group includes the remaining 88 species which we refer to as "endemic temperate" to distinguish them as taxa that are typical of the temperate climatic zones in the western Pacific. Many are living along Japanese coasts today; others are probably extinct. Detailed discussion of these species is beyond the scope of this paper. However, it is noteworthy that circumpolar species co-occur with species that are apparently more thermophilic in habitat in species-rich assemblages that contrast with typical low diversity, post-glacial assemblages from the North Atlantic (Neale & Howe, 1975;Cronin, 1981; in press).
These three categories of species are admittedly somewhat arbitrary and, with additional work on modern North Pacific faunas, a better zoogeographic zonation will be available. Nonetheless, they conveniently subdivide the fauna for the purpose of discussion. Table 2 lists and Fig, 6 graphically displays the percentages of circumpolar, cryophilic and temperate species in each formation.
The Tomikawa Formation contains 21 of the 26 circumpolar species, comprising 46.3% of the toal Tomikawa assemblage. It is thc only formation in Japan to yield Heterocyprideis fuscis and Sarsicyther- endemic temperate species in the total ostracod assemblages from the ten formations studied.
idea macrolaminata. Rabilimis septentrionalis occurs in the Tomikawa, Wakimoto and Shibikawa Formations, while Hemicytherura clathrata occurs in the Tomikawa and Setana Formations. Sarsicytheridea bradii occurs in the Tomikawa and Shibikawa Formations. The presence of several circumpolar species of Cytheropterotz and the absence of such temperate genera as Cornucoquimba, Loxoconcha, Aurila and Bythoceratina also reflects the cold water environment of the Tomikawa. The Setana Formation contains a more diverse ostracod assemblage including 20 circumpolar species, which comprise 23.1% of the total. Baffinicythere howei, Finmarchinella angulata, Acanthocythereis dunelmensis and circumpolar species of Semicytherura, Cytheropteron, characterise this unit, which also contains abundant Finmarchinella, Hemicythere, and Cythere.
The Junicho, Omma and Anden formations contain few circumpolar species (7,6 and 4 species respectively, representing 6.0, 18.4 and 1.596, Table 2) and, as suggested by the Jaccard coefficients, are faunally quite distinct from other Omma-Manganji assemblages. Temporal changes in the ostracod assemblages were examined in the Tomikawa (section 5), Sawane (section 14), and Omma (section 18) Formations and in the composite sequence of the Kitaura, Wakimoto, Shibikawa and Anden Formations from the Oga Peninsula (sections 7, 8 and 9) (Figs. 7a-d). The percentage of circumpolar species ranges from moderate in the lower part of the Tomikawa to very high (60-85%) in the middle part and dropping back to moderate in the upper part (Fig. 7a). The four samples in section 14 show little change in ostracod assemblages for this part of the Sawane Formation (Fig. 7c). The Omma Formation sequence (Fig. 7d) shows widely fluctuating percentages but this is partly attributable to small numbers of specimens and also perhaps to the complex oceanography (Ogasawara, 1981). In the Oga sequence (Fig. 7b), the Kitaura and Wakimoto Formations show moderate percentages and the overlying Shibikawa and Anden Formations show significant decreases in circumpolar species (see also Okada, 1979).
Figs. 8a and 8b show the percentages of each category for the nine and six samples respectively from the Setana and Sasaoka formations suggesting little variation within each formation in the contribution of circumpolar, cryophilic and temperate species.

DISCUSSION
Hanai's early studies on selected genera of Setana and Sawane ostracods clearly showed the cold water affinities of the fauna.  convincingly demonstrated that the Daishaka Formation (also part of the Omma-Manganji fauna) ostracods include many genera and species in common with other high-latitude assemblages, especially those from seas off the British Isles, nothern Europe, Scandinavia and eastern North America. Our data builds on these studies and three conclusions can be made at the present time.
First, the Omma-Manganji ostracod fauna actually consists of a heterogeneous array of species assemblages signifying different palaeoenvironments and water temperatures. Despite the preliminary nature of ostracod data for several formations, important similarities and differences can already be identified. The Setana and Sawane Formations, for example, show strong similarities in their high species diversity and the overall composition of the faunas. In general, water temperature, depth and substrate were probably the most important factors influencing the particular species occurring in each formation. This conclusion corroborates results derived from Omma-Manganji molluscan assemblages, which include a variety of species associations that have been subdivided by  Chinzei (1978) into two coastal associations and one offshore muddy bottom association. However, more detailed sampling than we were able to obtain is required to determine short-term oceanographic changes within a single formation. Second, we expand the growing list of ostracod species common to the North Pacific and North Atlantic Neogene and Quaternary. Of the 26 circumpolar species discussed above, Tabuki (1 986) recorded 10 from the Daishaka Formation: Acanthocythereis dunelmensis, Argilloecia conoidea, Baffinicythere howei, Cytheropteron nodosoalatum (= C. yajimai , part, PI. 17, fig. 13), Elofsonella concinna, Finmarchinella angulata, Hemicythere emarginata, Munseyella hatatatensis, Palmenella limicola, Semicytherura subundata, and perhaps Sclerochilus contortus.  also found Cluthia japonica and Cytheropteron angulatum (= C. tsugaruense . We add 16 species to the list of circumpolar species occurring as fossils in Japan and first occurrences in the Pacific region for three genera, Heterocyp-rideis, Rabilimis, and Sarsicytheridea. Third, we propose the hypothesis, on the basis of our qualitative assessment of cryophilic genera from different high-latitude seas, that during periods of climatic oscillations, populations of stenotopic species give rise more frequently to new species than do eurytopic species. We also suggest that, for unknown reasons, certain genera diversified to greater degrees in the Pacific compared with the Atlantic despite the fact that both regions were influenced by the same global climatic changes. It is well known that global climatic changes frequently altered North Pacific (Sancetta & Silvestri, 1986) and North Atlantic (Shackleton et al., 1984) palaeoceanography, affecting the zoogeography of temperature-sensitive species. Climatic cooling caused range expansion of cryophilic species into lower latitudes and, climatic warming, range contraction to higher latitudes. For example, southward shifts in species' zoogeographic ranges are known from the occurrence of cryophilic species in post-glacial deposits in the British Isles (Brady, Crosskey &L Robertson, 1874), and eastern North America (Brady & Crosskey, 1871;Cronin, 1981, in press). The Omma-Manganji fauna demonstrates that similar range shifts occurred in the Northern Pacific. Further, the late Pliocenemiddle Pleistocene age predates the late Pleistocene (15,000-10,000 years) age of most North Atlantic faunas. By extending the stratigraphic ranges of cryophilic species, we can assume these species were subjected to many climatic cycles and that populations of cryophilic species have repeatedly shifted their ranges during glacial-interglacial cycles. In theory, range expansion and contraction might result in evolutionary divergence and the evolution of new species in the distal-most populations (Hanai, 1977).
Some cryophilic genera contain one or several seemingly ubiquitous species, while the opposite is true of other genera that diversified during the last few million years and include species endemic to different regions. Those in the former category include more eurytopic taxa able to withstand broad temperature and depth ranges. For example, Palmenella Iimicola is common in high-latitude deposits in the Atlantic and Pacific representing many types of palaeoenvironments. Elofson (1941) and Hazel (1970a) found this species tolerant of a wide range of depths and temperatures, Similarly, Sarsicytheridea bradii, S. macrolaminata and Heterocyprideis fascis are eurytopic species tolerant of a wide range of environmental conditions.
Other cryophilic genera have been undergoing active speciation during the Neogene/Quaternary. Among them, several are more diverse in the Pacific: Finmarchinella, Robertsonites, Hemicythere, and Cythere, and include species endemic to the Pacific. Cold water members of the genera Cytheropteron and Semicytherura also diversified during the last few million years along Asia and North America and Europe, although it is not yet clear how many species in these genera are endemic to the Atlantic, Pacific or both.

-100
Family Cytherideidae Sars, 1925Heterocyprideis Elofson, 1941 Heterocyprideis fascis (Brady & Norman, 1889) (Pl. 1, fig. 15) Remarks. H . fascis is very rare in the Tomikawa Formation. It is known from Pleistocene deposits off the eastern United States (Hazel, 1968), in eastern Canada (Cronin, in press). and in Norway (Lord, 1980(Lord, ) al., 1983, where it was recorded as H. sorbyana. Masson & Whatley (1977) described a closely related specis, H . macrotuberculata from the Quaternary of the North Sea, which we think might be conspecific with H. comparison of high-latitude species from different regions and the integration of climatic change into our phylogenetic analyses.

NOTES ON THE SYSTEMATIC PALAEONTOLOGY
This section provides brief comments about the 26 circumpolar, the 21 cryophilic species and several other species deserving special attention regarding their affinities with North Atlantic species. Illustrated specimens are deposited in the Department of Paleobiology, U.S. Museum of Natural History, Washington, D.C.
Genus Munseyella van den Bold, 1957 Munseyella hatatatensis Ishizaki, 1966 (Pl. 3, fig. 16) Remarks. We believe M. hatatatensis is conspecific with M. mananensis Hazel & Valentine, 1969, a North Atlantic species common off northeastern North America and in Pleistocene deposits in eastern Canada (Cronin, in press 1, fig. 12) Remarks. S. bradii shows no obvious differences with specimens from the North Atlantic. Distribution. Tomikawa, abundant in the Shibikawa Form at ion. Material. 83 specimens.
Family Hemicytheridae Puri, 1953 Genus Baffinicythere Hazel, 1967Baffinicythere howei Hazel, 1967 fig. 16) Remarks. This species is represented in Japan by several morphotypes ranging from coarsely reticulate to variably tuberculate. Tubercles are often well developed in the posterior half. Japanese specimens resemble B. howei from the North Atlantic in overall shape, size, and oriiatnent and in the strong sexual dimorphism of the carapace, but they have a much stronger ridge along the anterior margin and, in the posterior area, a less conspicuous triangular shaped area formed by ridges. At present we consider the Japanese form a distinct subspecies of B. howei. Distribution. Tomikawa, Setana, Sawane, Junicho, Omma, Kitaura, Wakimoto, Shibikawa,, and Sasaoka Formations. Material. 347 specimens.
Baffinicythere? sp. A (PI. 2, fig. 18) Remarks. This large, thick-shelled species was referred to as Amhostrucon sp. 1 by    1, fig. 13) Remarks.  reported this species from the Daishaka Formation. Japanese specimens have a less conspicuous anterior marginal rim than North Atlantic specimens. W. M. Briggs (pers. comm., 1986) Finmarchinella Swain, 1963 Finmarchinella angulata (Sars, 1865) (Pl. 1, fig. 9) Remarks. Comparison of specimens of Finmarchinella angulata from off Greenland, Norton Sound and the Gulf of Alaska with fossil specimens of F. japonica (Ishizaki, 1966) indicate the two are conspecific. Okada (1979) also identified F. angulata from late Cenozoic deposits of the Oga Peninsula, Akita Prefecture, Japan and  lists it from the Daishaka Formation.

Explanation of
Hemicythere quadrinodosa Schornikov, 1974 (PI. 2, fig. 1) Remarks. This species is distinguished from others by its large, thick carapace and the conspicuous nodes in the dorsal and posterior parts of the carapace surface. Distribution. Setana Formation. Material. 12 specimens.
Hemicythere sp. C (Pl. 2, fig. 2) Remarks. This species bears some resemblance to H. emarginata but it is smaller, has a rounded outline and a different ridge pattern. Distribution. Setana and Sawane Formations. Material. 30 specimens.
Normanicythere sp. A (Pl. 1, fig. 8) Remarks. This species is characterised by a variably reticulated surface. The fossae are arranged in subhorizontal rows in the posterior half of the carapace; the anterior half has a few scattered pits. Distribution. Tomikawa, common in the Omma Formation. Material. 53 specimens.              Patagonacythere dubia (Brady, 1868) (Pl. 2, fig. 10) Remarks. Japanese specimens are very similar to North Atlantic specimens (Cronin, in press). Two other related, undescribed species of Patagonacythere also occur in the Omma-Manganji fauna.