The Early Permian organic-rich black coloured limestone from Mugi County, Gifu Prefecture, central Japan, was sampled for ostracods. Eleven species belonging to six genera were identifıed This ostracod assemblage is the first report of Palaeozoic species from central Japan. Nine species are new: Amphissites gifuensis Tanaka and Yuan sp. nov., Aurikirkbya miyakei Tanaka and Ono sp. nov., Glyptopleura mugiensis Tanaka and Yuan sp. nov., Bairdia fujisan Tanaka and Maeda sp. nov., Bairdia ikeyai Tanaka sp. nov., Bairdia spindlica Tanaka and Yuan sp. nov., Bairdia minoensis Tanaka and Ono sp. nov., Microcheilinella subovata Tanaka and Maeda sp. nov., and Microcheilinella tsubogawensis Tanaka and Ichida sp. nov.
Introduction
The study of Japanese Palaeozoic Ostracoda was inspired by Ishizaki (1963), who described 14 species from the Carboniferous Nagaiwa Formation of northeastern Japan. He subsequently (1964a) reported anothor 22 species, including 15 new ones from the Middle Permian Iwaizaki Limestone from northeastern Japan, and 9 new species (Ishizaki, 1964b) from the Carboniferous strata around Japan. Three years later (Ishizaki, 1967) he reported 12 new species from the Carboniferous Tassobe Formation, northeastern Japan, and then in 1968 described 2 new species from the Carboniferous Takezawa Formation, northeastern Japan (Ishizaki, 1968). Adachi and Igo (1980) reported a new genus and species, Palaeoleperditia fukujiensis, from the Devonian (see Kurihara, 2003) Yoshiki Formation of Fukuji, Gifu Prefecture, central Japan. However, according to Vannier et al. (2001) and Williams and Siveter (2008), the Leperditicopida are not Ostracoda, rather they classify this taxon as an extinct group of bivalved Arthropoda. In the present study, we have followed their opinion and excluded the Leperditicopida from the Ostracoda.
Some sixty species of Palaeozoic ostracods have now been reported from Japan. The number of described species from Palaeozoic strata from Japan is extremely low compared with those in other countries. The number of Cenozoic to Recent ostracod species described from Japan and its adjacent areas amounts to over 900 species (Ikeya et al., 2003). Our Japanese Palaeozoic Ostracoda Research Team has investigated Palaeozoic strata around Japan and found many fossil localities containing ostracod specimens. The aim of the team is to find and describe Palaeozoic ostracods from Japan. The present study is the first report of Early Permian ostracods from central Japan.
Geological Setting
Middle Jurassic to earliest Cretaceous sedimentary rocks and associated Permian bedded cherts, limestones and basalts are widely distributed in the Mino district, central area of Gifu Prefecture, central Japan (Figure 1). Based on a region enclosed by faults, four geological units have been recognised in the Mino district: Nabi Unit, Funafuseyama Unit, Kanayama Unit, and Kamiaso Unit (Wakita, 1995). This study area is a part of the Kamiaso Unit, which is characterised by an assembly of tectonic slices composed of ‘toishi (which means whetstone in Japanese)’ type siliceous claystone, bedded chert, siliceous mudstone, alternating sandstone and mudstone, and massive sandstone, in ascending order. The brecciated conglomerates are dispersed in the Tominoho area located in the northern part of Mugi County (Figure 1B, C). Most of the gravels in the conglomerate are cherts. Gravels of siliceous mudstone, mudstone, sandstone, shale, limestone and basalt also occur.
Figure 1.
A, B, index map and route map of the study area in the Mugi County, Gifu Prefecture, central Japan (B, after a 1:25,000 scale topographic map Kami-asou published by the Geospatial Information Authority of Japan, Ministry of Land, Infrastructure, Transport and Tourism, Japan); C, geological map around the study area (modifıed from Mizutani and Koido, 1992).
Figure 2.
Characteristic fusulinoidean foraminifers from an organic-rich black limestone slab from the Mugi County, Gifu Prefecture, central Japan. 1, Pseudofusulina paratschernyschewi Kanuma; 2, Parafusulina sp; 3, Misellina cf. parvicostata (Deprat); 4, Mesoschubertella sp. A. Scale A for 1, 2; B for 3, 4.
Fusulinoidean foraminifers were recovered from a drifted organic-rich black limestone slab (Figure 2). The species identified include Pseudofusulina paratschernyschewi Kanuma, Pseudofusulina sp., Parafusulina sp., Misellina cf. parvicostata (Deprat), Schubertellidae? gen. et sp. indet., Mesoschubertella sp. A, and Mesoschubertella sp. B. According to Ueno (1991, 1996), M. cf. parvicostata (Deprat) occurs in the middle part of the Misellina (Brevaxina) dyhrenfurthi otai (Sakaguchi and Sugano, 1966) Zone which indicates a middle Bolorian age (late Early Permian).
Materials and methods
A total of about 10 kg of organic-rich black limestone slabs was collected from the Tani-gawa River, a branch of the Tsubo-gawa River, Tominoho, Mugi County, Gifu Prefecture, central Japan. The Permian black limestones are exposed in the northwestern part of this locality (Figure 1C). The slabs used in this study may be derived from that region. Each large slab was split into approximately 1–2 cubic centimetre(s), and dried in an oven at 60°C. Dried rock samples, of 80 g each, from a total of 12 samples were placed on a 16 mesh (1 mm) sieve attached below to a 235 mesh (63 µm) sieve, and arranged in a bucket where the sieves were filled with 5% acetic acid. The bucket remained standing for about two weeks at room temperature. After that, the sieves were picked up and rinsed in water inside a big bucket to remove the acetic acid from the residue that contained the ostracod fossils. The residues were collected into a beaker and dried in an oven at 60° for three days. The specimens were picked under a binocular microscope (Nikon SMZ-10A) at X 30 magnification and stored in faunal slides.
Systematic Palaeontology
The morphological terms used are based on Moore (1961) and Scott (1961). The following abbreviations are used in the descriptions: L = length, H = height, W = width, N = number of specimen.
A total of 308 specimens belonging to 11 species were found from the residues (Table 1). The leading species is Bairdia minoensis sp. nov., followed by Bairdia ikeyai sp. nov., Bairdia spindlica sp. nov., Aurikirkbya miyakei sp. nov. and Glyptopleura mugiensis sp. nov. Thus, the assemblage is characterized by the genus Bairdia. All illustrated specimens have been deposited in the Micropaleontology Collection, National Museum of Nature and Science, Japan (MPC-number).
Order Palaeocopida Henningsmoen, 1953
Suborder Beyrichicopina Scott, 1961
Superfamily Kirkbyoidea Ulrich and Bassler, 1906
Family Amphissitidae Knight, 1928
Genus
Amphissites
Girty, 1910
Amphissites gifuensis
Tanaka and Yuan, sp. nov.
Figure 3.1 and 3.2
Etymology.—Named after Gifu Prefecture from which the type specimens were recovered.
Type specimens.—Holotype: MPC-03636 (carapace; L = 941 µm, H = 686 µm, W = 539 µm). Paratype: MPC-03637 (carapace; L= 843 µm, H = 490 µm, W = 520 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 9), A-1 stage carapaces (N = 7), A-2 stage carapace (N = 1) (Figure 4).
Diagnosis.—Carapace reticulated except for anterodorsal and posterodorsal areas, one prominent concentric velate ridge parallel to free margins. One dorsal ridge on the left valve and one dorsal ridge on the right valve run parallel to the hinge line.
Description.—Carapace suboblong in lateral view. Anterior and posterior cardinal angles a little greater than 90°. In lateral view, posterior margin narrowly rounded with infracurvature; dorsal margin straight to slightly convex in posterior due to the presence of a hump; ventral margin broadly convex; posterior margin almost straight. Carapace reticulated except for anterodorsal and posterodorsal areas, one prominent concentric velate ridge developed between anterior and posterior margins. Prominent rounded small pit (possibly outer side of adductor muscle scar; S2 of Scott, 1961) situated ventromedianly. Eye spot observed anterodorsally. In dorsal view, carapace ovate, widest at a third of length away from posterior end, pointed at anterior end. A pair of dorsal ridges runs parallel to the hinge line. In ventral view, carapace subovate, broadest at point near mid-height; one prominent velate ridge runs parallel to carapace outline. In anterior and posterior views, velate ridges of right and left valves merge at antero- and posterodorsal areas showing a reverse V-shape.
Remarks.—This species differs from Amphissites nodosus Scott and Borger, 1941 from the Pennsylvanian McLeansboro Group, Illinois, USA in its developed dorsal ridge and hump, and rounded small pit situated ventromedianly. A. gifuensis sp. nov. is distinguished from Croftsendella giffordensis Bless, 1974 reported from the Upper Carboniferous of the Bristol District, UK (Becker, 1997), in the straight dorsal outline, prominent velate ridge and fine reticulation. The present new species somewhat resembles Kirkbya sp. 3 from the uppermost Permian of Anhui Province, China (Yuan et al., 2009), however, it has no carina along the ventral margin.
Table 1.
Fossil ostracods from the Middle Permian limestone from the Mugi County, Gifu Prefecture, central Japan.
Family Kirkbyidae Ulrich and Bassler, 1906
Genus
Aurikirkbya
Sohn, 1950
Aurikirkbya miyakei
Tanaka and Ono, sp. nov.
Figure 3.3 and 3.4
Etymology.—In honour of Yukio Miyake (Gifu Prefecture, Japan), who discovered many ostracods by etching limestone from the type locality.
Type specimens.—Holotype: MPC-03638 (left valve; L = 1275 µm, H = 745 µm, W = 451 µm). Paratype: MPC-03639 (left valve; L = 1196 µm, H = 686 µm, W = 448 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N,137°02'E).
Material.—Adult left valves (N = 5), A-1 stage left valves (N = 4), A-2 stage left valve (N = 1)(Figure 5).
Diagnosis.—In lateral view, anterior margin curved like the bow of a boat. Carapace reticulated except for the anterodorsal and posterodorsal areas. Surface reticulation tends to be finer at anterodorsal, posterodorsal, and middorsal to median areas. Posterodorsal lobe slightly more conspicuous than anterodorsal one.
Description.—Carapace subsemicircular in lateral view. Anterior and posterior cardinal angles a little greater than 90°. In lateral view, anterior margin curved like the bow of a boat; posterior margin rounded with infracurvature; dorsal margin slightly sinuous because of the protuberance of anterior and posterior lobes; ventral margin broadly convex. Carapace reticulate except for the anterodorsal and posterodorsal areas, and the reticulation tends to be finer at the anterodorsal, posterodorsal, and mid-dorsal to median areas. Two lobes developed anterodorsally and posterodorsally, respectively. Posterodorsal lobe slightly more conspicuous than anterodorsal one. Prominent rounded small pit (possible outer side of adductor muscle scar; S2 of Scott, 1961) situated at median area. Eye spot not observed. Denticles well developed along the free margins. A line with hollow spines developed along free margins. In dorsal view left valve widest at middle, pointed at anterior end. In internal view, straight hinge line runs parallel to dorsal margin. Adont hinge. No duplicature. Sexual dimorphism not identifıed.
Remarks.—This species differs from Aurikirkbya ventrocallosa Becker and Wang, 1992 from the Upper Permian strata of Sichuan Province, China by its more elongated lateral outline, anterior margin curved like the bow of a boat and surface reticulation which tends to be finer in the anterodorsal, posterodorsal, and mid-dorsal to median areas. A. miyakei sp. nov. is distinguished from Coronakirkbya binodosa Hao, 1992 from the Upper Permian Changhsing Formation of Guizhou province, China, by its more rounded lateral outline, lack of protuberances at anterior and posterior cardinal angles, and coarser surface reticulation. The present new species differs from Kirkbya atolla Ishizaki, 1964 from the Middle Permian Iwaizaki Limestone, northeastern Japan by having a less inflated valve and coarser reticulation.
Figure 3.
SEM micrographs of two new palaeocopid ostracod species from Mugi County, Gifu Prefecture, central Japan. 1, 2, Amphissites gifuensis Tanaka and Yuan sp. nov.; 1, holotype MPC-03636; 1a, left lateral view; 1b, right lateral view; 1c, anterior view; 1d, posterior view; 1e, dorsal view; 1f, ventral view; 2, paratype MPC-03637; 2a, left lateral view; 2b, right lateral view; 2c, anterior view; 2d, posterior view; 2e, dorsal view; 2f, ventral view; 3, 4, left valves of Aurikirkbya miyakei Tanaka and Ono sp. nov.; 3, holotype MPC-03638; 3a, left lateral view; 3b, dorsal view; 3c, internal view; 4, paratype MPC-03639.
Figure 4.
Length and height values of ontogenetic stages of Amphissites gifuensis Tanaka and Yuan sp. nov.
Figure 5.
Length and height values of ontogenetic stages of Aurikirkbya miyakei Tanaka and Ono sp. nov.
Suborder Kloedenellocopina Scott, 1961
Family Glyptopleuridae Girty, 1910
Genus
Glyptopleura
Girty, 1910
Glyptopleura mugiensis
Tanaka and Yuan sp. nov.
Figure 6.1 and 6.2
Etymology.—Named after Mugi County from which the type specimens were recovered.
Type specimens.—Holotype: MPC-03640 (carapace; L = 930 µm, H = 484 µm, W = 426 µm). Paratype: MPC-03641 (carapace; L = 969 µm, H = 523 µm, W = 426 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 13), A-1 stage carapaces (N = 2), A-2 stage carapace (N = 1)(Figure 7).
Diagnosis.—In lateral view, anterior margin evenly rounded with infracurvature; dorsal margin sinuous and slightly concave at the middle; ventral margin broadly convex; posterior margin truncated dorsally and rounded ventrally In dorsal view, carapace with arrowhead outline. Three strong and nearly parallel ridges run from mid-anterior area to posterodorsal and mid-posterior area. One dorsal ridge on left valve and one dorsal ridge on right valve run parallel to hinge line.
Description.—Carapace subquadrate in lateral view. Anterior and posterior cardinal angles obtuse and greater than 90°. In lateral view, anterior margin evenly rounded with infracurvature; dorsal margin sinuous and slightly concave at the middle; ventral margin broadly convex; posterior margin truncated dorsally and rounded ventrally. Three strong ridges run from mid-anterior area to posterodorsal and mid-posterior area, nearly parallel to each other: most of the dorsal ridge is prominent and runs from the mid-anterior area to the front of the posterior cardinal angle; middle ridge runs from mid-anterior area to median area, from posteromedian area to posterior of the posterior cardinal angle; most of the ventral ridge runs from the mid-anterior area to mid-posterior area. Prominent rounded small pit (possibly outer side of adductor muscle scar; S2 of Scott, 1961) situated on the median area. Eye spot not observed. In dorsal view, carapace arrowhead-shaped, widest at a fifth of length from posterior end, pointed at anterior and posterior ends. Right valve overlaps left valve at anterior cardinal angle and along the free margins. One dorsal ridge on left valve and one dorsal ridge on right valve run parallel to the hinge line. Outline of carapace arrowhead-shaped in ventral view. Velate ridge runs parallel to each valve outline. In anterior and posterior views, five pairs of ridges are visible, creating a round, chestnut-like outline with 10 protruding points.
Remarks.—This species differs from Glyptopleura inopinata (Coryell and Johnson, 1939) from the Upper Mississippian of the USA in its three prominent ridges and arrowhead outline in dorsal and ventral views. G. mugiensis sp. nov. is distinguished from G. adunca Croneis and Thurman, 1939 reported from the Upper Mississippian Kinkaid Formation, USA, in the overlap by the right valve of the left valve at the anterior cardinal angle and along the free margins.
Figure 6.
SEM micrographs of two new ostracod species from Mugi County, Gifu Prefecture, central Japan. 1, 2, Glyptopleura mugiensis Tanaka and Yuan sp. nov.; 1, paratype MPC-03640; 1a, left lateral view; 1b, right lateral view; 1c, anterior view; 1d, posterior view; 1e, dorsal view; 1f ventral view; 2, holotype MPC-03641; 2a, left lateral view; 2b, right lateral view; 2c, anterior view; 2d, posterior view; 2e, dorsal view; 2f, ventral view; 3, 4, Bairdia fujisan Tanaka and Maeda sp. nov.; 3, holotype MPC-03642; 3a, left lateral view; 3b, right lateral view; 3c, posterior view; 3d, dorsal view; 3e, ventral view.
Figure 7.
Length and height values of ontogenetic stages of Glyptopleura mugiensis Tanaka and Yuan sp. nov.
Order Podocopida Sars, 1866
Suborder Podocopina Sars, 1866
Superfamily Bairdioidea Sars, 1887
Family Bairdiidae Sars, 1887
Genus
Bairdia
McCoy, 1844
Bairdia fujisan
Tanaka and Maeda sp. nov.
Figure 6.3
Etymology.—Named after Mt. Fuji, which is similar in shape to the dorsal outline of the new species.
Type specimen.—Holotype: MPC-03642 (L = 1298 µm, H = 833 µm, W = 659 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 5), A-1 stage carapaces (N = 6)(Figure 8).
Diagnosis.—In lateral view, anterior margin protrudes at mid-height, upper half of margin straight and connects with dorsal margin at a blunt angle, lower half of margin curved like the bow of a boat. Anterior end higher and more rounded than posterior. Left valve with weakly developed bow-shaped projection. Dorsal and ventral margins straight.
Description.—Carapace subtrapezoidal in lateral view. Left valve larger than right valve and overlaps it along dorsal, anterior and ventral margins. In lateral view, anterior margin protrudes at mid-height, upper half of margin straight and connects with dorsal margin in a blunt angle, lower half of margin curved like the bow of a boat; dorsal and ventral margins straight; posterior margin truncated dorsally and rounded ventrally Anterior end higher and more rounded than posterior. A weak ridge develops midventrally along the ventral margin. Surface of carapace smooth. In dorsal view, lateral outlines symmetrically convex, spindle-shaped, widest at the middle of the carapace, pointed at anterior and posterior ends, anterior end more acute than posterior. In ventral view, carapace spindle-shaped, contact margin sinuous at the middle. Left valve with weakly developed bowshaped projection. In posterior view, carapace has the outline of an acute triangle with a flat ventral area.
Remarks.—This species differs from Ceratobairdia? ambigua Ishizaki (1964) from the Middle Permian Iwaizaki Limestone, northeastern Japan in its straight dorsal and ventral margins, upwardly directed anterior end in lateral view and extremely large left valve in posterior view. This species is distinguished from “?Bairdia anbeedei” Belousova (1965) in Yuan et al. (2009) from the uppermost Permian of Guangxi, China, by its straight dorsal margin, higher carapace and outline of the anterior and posterior ends. The present new species differs from Bairdia menardensis Harlton (1929) of Chen (1958) reported from the Lower Permian Chihsia Limestone, Nanking, China in its straight dorsal margin and higher anterior than posterior end. B. fujisan sp. nov. is distinguished from Bairdia hassi Sohn (1960) by its parallel dorsal and ventral margins. The new species differs from Bairdia rhomboidalis Hamilton (1942) from the Upper Permian of Texas in its shorter dorsal margin and taller lateral outline.
Figure 8.
Length and height values of ontogenetic stages of Bairdia fujisan Tanaka and Maeda sp. nov.
Bairdia ikeyai
Tanaka sp. nov.
Figure 9.1 and 9.2
Etymology.—In honour of the late emeritus professor Noriyuki Ikeya (Shizuoka University, Japan), who was a specialist in the taxonomy of podocopid ostracods around Japan.
Type specimens.—Holotype: MPC-03643 (carapace; L = 1333 µm, H = 824 µm, W = 748 µm). Paratype: MPC-03644 (carapace; L = 1314 µm, H = 833 µm, W = 706 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 4), A-1 stage carapace (N = 1), A-2 stage carapaces (N = 5)(Figure 10).
Diagnosis.—Left valve overlaps right valve along dorsal and ventral margins. Upper half of anterior margin sinuate, connects with dorsal margin at a blunt angle, lower half rounded with infracurvature.
Description.—Carapace subtrapezoidal in lateral view. Left valve larger than right valve and overlaps latter along dorsal and ventral margins. Anterior and posterior cardinal angles obtuse and greater than 90°. In lateral view, anterior margin protrudes at mid-height, upper half of margin sinuous and connects with dorsal margin at a blunt angle, lower half of margin rounded with infracurvature; dorsal margin broadly arched; ventral margin straight in left valve and sinuous in right valve; posterior margin truncated dorsally and rounded ventrally. Anterior end higher and more rounded than posterior. Surface of carapace smooth. In dorsal view, lateral outline asymmetrically convex, spindle-shaped, widest at the middle, anterior and posterior ends pointed, anterior end more acute than posterior. Left valve overlaps right valve. In ventral view, carapace spindle-shaped, contact margin undulated at posterior third. Carapace ornate in anterior view.
Remarks.—This species differs from Bairdia menardensis Harlton (1929) of Chen (1958) from the Lower Permian Chihsia Limestone of Lungtan, Nanking, China by its less overlapped left valve along the anterior margin, more elevated anterior end and less truncated posterior end. This species is distinguished from Bairdia pecosensis Delo (1930) from the Upper Carboniferous strata of West Texas by its greater height and valve length in lateral view, more broadly arched posteroventral margin and arched dorsal margin in the right valve. The present new species differs from Bairdia (Rectobairdia) sandulescui Crasquin-Soleau and Gradinaru (1996) reported from the early Anisian strata of Romania by its shorter dorsal margin, sinuous anterodorsal margin, and narrower, rounded anteroventral margin.
Figure 9.
SEM micrographs of two new ostracod species from Mugi County, Gifu Prefecture, central Japan. 1, 2, Bairdia ikeyai Tanaka sp. nov.; 1, holotype MPC-03643; 1a, left lateral view; 1b, right lateral view; 1c, ventral view; 1d, anterior view; 2, paratype MPC-03644; 2a, left lateral view; 2b, anterior view; 2c, dorsal view; 2d, ventral view; 3, Bairdia spindlica Tanaka and Yuan sp. nov., holotype MPC-03645; 3a, left lateral view; 3b, right lateral view; 3c, anterior view; 3d, posterior view; 3e, dorsal view; 3f, ventral view.
Bairdia spindlica
Tanaka and Yuan sp. nov.
Figure 9.3
Etymology.—Named after its outline in dorsal and ventral views.
Type specimen.—Holotype: MPC-03645 (L = 1010 µm, H = 520 µm, W = 412 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 5), A-1 stage carapaces (N = 2), A-2 stage carapaces (N = 2), A-3 stage carapace(Figure 11).
Diagnosis.—Carapace lips-shaped in lateral view. Anterodorsal margin concave, forming a blunt angle with dorsal margin, lower half of margin curved like the bow of a boat and meets ventral margin at a blunt angle. Weak velate ridge developed at mid-ventral area parallel to dorsal margin. In dorsal view, posterior end more acute than anterior.
Description.—Carapace lips-shaped in lateral view. Left valve larger than right valve and overlaps it all around. Anterior and posterior cardinal angles obtuse and greater than 90°. In lateral view, anterior margin protrudes at a third of the maximum height below the dorsal margin, anterodorsal margin concave and forms a blunt angle with dorsal margin, lower half of margin curved like the bow of a boat and meets ventral margin at a blunt angle; dorsal margin slightly convex in left valve and straight in right valve; ventral margin straight in right valve and widely arched in left valve; posterior end truncated and meets dorsal margin concavely and ventral margin straight. Anterior end higher and better rounded than posterior. Weak ridge developed at mid-ventral area parallel with dorsal margin. Surface of carapace smooth. In dorsal view, lateral outline symmetrically convex, spindle-shaped, widest at the middle, anterior and posterior ends pointed, posterior end more acute than anterior. Left valve overlaps right valve. In ventral view, carapace spindle-shaped, contact margin rather undulated at the middle. An adventral ridge parallel to the mid-ventral margin. In anterior and posterior views, carapace ovate.
Remarks.—This species differs from Bairdia pseudoemaciata Ishizaki (1963) from the Carboniferous Nagaiwa Formation, northeastern Japan in its concave posterodorsal outline, upwardly directed dorsal outline and broadly arched anteroventral margin. This species is distinguished from Bairdia cristata Chen (1958) from the Lower Permian Chihsia Limestone, Nanking, China in its straight dorsal outline, stronger overlap along the anterodorsal margin, steeper posterodorsal margin and blunter anterior end. The present new species differs from Bairdia girtyi Sohn (1960) in Chen and Bao (1986) reported from the Lower Permian Chihsia Formation of China by its rounded lateral outline and longer posterior end. B. spindlica sp. nov. is distinguished from Bairdia cf. austriaca Kollmann in Crasquin-Soleau et al. (2008a) from the Lercara Formation (Middle Triassic to Carnian?) of Italy by its strongly concave anterodosal margin, tapered anterior end and adventral ridge.
Figure 11.
Length and height values of ontogenetic stages of Bairdia spindlica Tanaka and Yuan sp. nov.
Bairdia minoensis
Tanaka and Ono sp. nov.
Figure 12.1
Etymology.—Named after old provincial name Gifu region ‘Mino’ from which the type specimens were discovered.
Type specimen.—Holotype: MPC-03646 (L = 1191 µm, H = 544 µm, W = 287 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapace (N = 4), A-1 stage carapace (N = 1), A-2 stage carapace (N = 5)(Figure 10).
Diagnosis.- In dorsal view, carapace lentoid, widest at a third of the length away from posterior end, pointed at anterior and posterior end, anterior end more acute than posterior.
Description.—Carapace elongated ellipsoid in lateral view. In lateral view, anterior margin protrudes at middle height, anterodorsal margin straight in left valve, slightly concave in right valve and forms a blunt angle with the dorsal margin, lower half of margin rounded with small curvature in left valve, curved like the bow of a boat in right valve and meets ventral margin forming a blunt angle; dorsal margin straight; ventral margin concave at the middle; posterior margin truncated with triangular shape, and meets in a straight line dorsal and ventral margins. Anterior and posterior ends about the same height. Surface of carapace smooth. In dorsal view, carapace lentoid, widest at a third of the length away from posterior end, pointed at anterior and posterior ends, anterior end more acute than posterior. Left valve overlaps right valve all around, most strongly along posteroventral margin. In ventral view, contact margin rather sinuous one third of the length away from anterior end. In posterior view, carapace oval with protruded venter of left valve.
Remarks.—This species differs from Bairdia cf. trianguliformis Chen (in Shi and Chen, 1987) from the Upper Permian Changhsing Formation of China by the strong posteroventral overlap of the right valve by the left valve, maximum width a third of the length away from the posterior and dorsal margin more gently sloping toward the posterior end. This species is distinguished from Bairdia piscariformis Chen (1958) from the Lower Permian Chihsia Limestone, Nanking, China by its elongated outline, maximum width a third of the length away from the posterior end and less overlap at the dorsal margin. The present new species differs from Orthobairdia sp. Chen and Bao (1986) reported from the Lower Permian Chihsia Formation in not having compressed anterior and posteroventral areas, triangular posterior outline and less overlap at tne dorsal margin.
Figure 12.
SEM micrographs of two new podocopid ostracod species from the Mugi County, Gifu Prefecture, central Japan. 1, Bairdia minoensis Tanaka and Ono sp. nov., holotype MPC-03646; 1a, left lateral view; 1b, right lateral view; 1c, posterior view; 1d, dorsal view; 1e, ventral view; 2, Microcheilinella subovata Tanaka and Maeda sp. nov., holotype MPC-03647; 2a, left lateral view; 2b, right lateral view; 2c, posterior view; 2d, ventral view; 3, Microcheilinella tsubogawensis Tanaka and Ichida sp. nov., holotype MPC-03648; 3a, left lateral view; 3b, right lateral view; 3c, posterior view; 3d, ventral view.
Figure 13.
Length and height values of ontogenetic stages of Bairdia minoensis Tanaka and Ono sp. nov.
Family Microcheilinellidae Gramm, 1975
Genus
Microcheilinella
Geis, 1933
Microcheilinella subovata
Tanaka and Maeda sp. nov.
Figure 12.2
Etymology.—Named after its lateral outline.
Type specimen.—Holotype: MPC-03647 (L = 485 µm, H = 316 µm, W = 346 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 3), A-1 stage carapaces (N =2), A-2 stage carapaces (N = 2)(Figure 14).
Diagnosis.—Carapace subovate in lateral view. Dorsal margin protruded at midlength in left valve and straight in right valve. In ventral view, carapace ovate, maximum width at a point slightly posterior from the middle, posterior end more pointed than anterior. Circular outline in posterior view.
Description.—Carapace subovate in lateral view. Left valve much larger than right valve and overlaps it all around. In lateral view, anterior margin evenly rounded in left valve and protruding at midheight in right valve, anterodorsal margin broadly rounded and connects with dorsal margin, lower half of margin rounded and forming a blunt angle with the ventral margin; dorsal margin protruding at middle of length in left valve and straight in right valve; ventral margin broadly arched in right valve, rather narrowly arched in left valve; posterior margin evenly rounded with smaller curvature than anterior margin. Anterior and posterior ends about the same height. Surface of carapace smooth. In ventral view, carapace ovate, maximum width slightly posterior from middle, posterior end more pointed than anterior. Left valve strongly overlaps right valve at anterior, ventral and posterior margin. Carapace circular in posterior view. The contact margin between the left and right valves is curved to about 90° in posterior view.
Remarks.—This species differs from Microcheilinella venusta Chen (1958) from the Lower Permian Chihsia Limestone of Nanking, China in its evenly rounded posterior margin, more broadly convex ventral margin in left valve and dorsal margin protruding at its midlength on the left valve. This species is distinguished from Microcheilinella speciosa Chen (1958) from the Lower Permian Chihsia Limestone in its evenly rounded posterior margin in the left valve, rounded dorsal margin in the right valve and ovate carapace in ventral view. The present new species differs from Microcheilinella sp. Crasquin-Soleau et al. (2004) reported from the Permian-Triassic boundary in Turkey by its more acutely rounded anterior and posterior margins in the right valve, convex dorsal outline and less overlapped left valve at the dorsal and ventral margins.
Figure 14.
Length and height values of ontogenetic stages of Microcheilinella subovata Tanaka and Maeda sp. nov.
Microcheilinella tsubogawensis
Tanaka and Ichida sp. nov.
Figure 12.3
Etymology.—Named after the Tsubo-gawa River from which the type specimens were discovered.
Type specimen.—Holotype: MPC-03648 (L = 588 µm, H = 360 µm, W = 368 µm).
Type locality.—A drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan (35°58'N, 137°02'E).
Material.—Adult carapaces (N = 2), A-1 stage carapace (N = 1), A-2 stage carapaces (N = 3)(Figure 15).
Diagnosis.—Carapace ellipsoid in lateral view. In posterior view, lateral outline of right valve is straight. In lateral view, dorsal margin convex and inclined towards posterior in right valve and ventral margin broadly arched in left valve.
Description.—Carapace ellipsoid in lateral view. Left valve considerably larger than right valve and overlaps it all around. In lateral view, anterior margin rounded and protruded antero-ventrally, posterior margin rounded and protruding postero-dorsally in left valve. In left valve, anterodorsal margin broadly rounded and connects with dorsal margin, lower half of margin rounded and forms a blunt angle with ventral margin; dorsal margin straight and parallel to ventral margin in left valve, convex and inclined towards posterior in right valve; ventral margin broadly arched in left valve, straight in right valve. Anterior end lower than posterior in left valve. Surface of carapace smooth. In ventral view, carapace ovate, widest at a third of the length from posterior end, anterior end more pointed than posterior. Left valve strongly overlaps right valve at anterior, ventral and posterior margins. Carapace circular in posterior view. Contact margin between left and right valves curved about 90° in posterior view. Lateral outline of right valve straight.
Remarks.—This species differs from Microcheilinella sp. Olempska (1979) from the Devonian of Poland by its convex dorsal margin in the right valve, stronger overlap at the ventral margin and more inflated outline in ventral view. This species is distinguished from Microcheilinella subreniformis Chen (1958) from the Lower Permian Chihsia Limestone of Nanking, China by its evenly rounded ventral margin in the left valve, ellipsoidal shape in lateral view and ovate outline in ventral view. The present new species differs from Microcheilinella lata Crasquin-Soleau (2008b) reported from the Permian-Triassic boundary in the southern Alps, Italy by having less overlap at the dorsal and posterior margins of right valve and a more convex dorsal margin in the right valve. The present new species somewhat resembles M. aff. subcorbuloides figured by Olempska (1999), however, the new species differs by its broadly rounded dorsal outline and less overlap along the dorsal margin of the right valve.
Discussion and conclusion
The plate tectonic, paleogeographical and paleoceanographic significance of the Middle Permian ostracods from Japan has not been clarifıed yet. However, the present study of the ostracod assemblage from a drifted muddy limestone slab from the Tani-gawa River, a branch of the Tsubo-gawa River, Mugi County, Gifu Prefecture, central Japan, revealed that all of the species are new and have not been described from other areas. It is well known that marine podocopid ostracods are benthonic throughout their life, based on the life style of Recent species (Athersuch et al., 1989). The palaeocopid ostracods possibly had a benthonic and/or nekto-benthonic life style throughout their life based on the shell outline, shell growth, and the functional morphology of the eye (Siveter, 1984; Tanaka et al., 2009). Thus, the ostracod assemblage is a good indicator for (paleo) biogeographic studies. According to Isozaki (2006), Japan consisted of small sea mounts and atolls in the Panthalassa Ocean during the Middle Permian (Figure 16). He mentioned the similarity between assemblages of alatoconchid bivalves, corals and fusulinacean foraminifers and those in the Tethyan Ocean at low latitude in other areas. Because bivalves, corals and foraminifers have a planktonic stage in their early life, they can migrate easily in the oceans. However, ostracods are mainly benthonic throughout their life, and consequently an ostracod assemblages are regional. Thus, the present study revealed the ostracod biogeography in the Panthalassan province. This is the first report of a typical Panthalassa fauna during the Middle Permian. Further systematic studies of ostracod assemblages from Japanese Palaeozoic strata will clarify the meio-benthos fauna in the long disappeared world of the largest ocean in the Earth's history, ‘Panthalassa’.
Figure 15.
Length and height values of ontogenetic stages of Microcheilinella tsubogawensis Tanaka and Ichida sp. nov.
Figure 16.
Paleogeograpgical map of the Middle Permian of Earth. Black star shows the location of Japan (Altered from Isozaki, 2006).
Acknowledgements
I (G.T.) dedicate this paper to the late Noriyuki Ikeya. We thank Yukio Miyake (Gifu Prefecture, Japan) for providing information about fossil localities. Thanks are also due to Dr. Alicia Moguilevsky (Translations Group) for correcting the English. Thanks must also go to two reviewers (Ewa Olempska, Polish Science Academy; Toshiaki Irizuki, Shimane University) for their critical reading of our manuscript and suggestions for improving it. We would also like to sincerely thank De-quing Chen and Shang-qi Wang (Nanjing Institute Geology and Palaeontology, Academia Sinica, China) and Sylvie Crasquin-Soleau (CNRS, Université Pierre et Marie Curie, France) for sending relevant literature to G. T. Thanks go to staff of Gunma Museum of Natural History for providing facilities. This research was partly supported by NSFC (Nos. 40839903, 40730209, 40902002), the 111 Project (B08030) to A. Y. and a Grantin-Aid for Science Research of the Ministry of Education and Science Government of Japan (No. 21740370 to G.T.)
