Introduction

The leperditicopid ‘ostracod’ Paraeoleperditia fukujiensis Adachi and Igo, 1980 is one of the most well known Japanese fossils. This ‘Ostracod’, crowned as the oldest fossil record from Japan, was discovered as a rolling stone in the vicinity of the ‘Ordovician’ Yoshiki Formation of the Ichino-tani Valley, Takayama City, Gifu Prefecture, Japan (Adachi and Igo, 1980). Kurihara (2003) identified Early Devonian radiolarians from the Yoshiki Formation, indicating that the taxonomic attribution of this oldest fossil record of Japan is problematic. Wang (1994) assigned P. fukujiensis to the Sinoleperditiini as an Early Devonian representative, while Hamada (1959) described a leperditicopid ‘ostracod’, Leperditia japonica, from the Devonian strata in Ichino-tani Valley. Leperditicopid arthropods have been previously classified as Ostracoda. They are large (maximum length ca. 5 mm — over 40 mm) and well calcified bivalved arthropods reported worldwide from the Lower Ordovician to the Upper Devonian (Benson et al., 1961). They usually occur in sediments from lagoons and shallow sea environments and are, therefore, an important fossil taxon that can be used as a palaeoenvironmental indicator (Vannier et al., 2001). However, their higher taxonomic position is problematic. Vannier et al. (2001) classified the leperditicopids as a group within the Arthropoda, while Wang and McKenzie (2000) and Wang (2009) classified them as Ostracoda. This is the third report of a new leperditicopid arthropod species from Japan.

The new species was recovered from a small outcrop on the right side of the mountain path near the west part of Fossil Garden, Okuhida-onsen-gou, Takayama City, Gifu Prefecture, Central Japan (Figure 1). It was found in muddy shale with other small fossils (1–5 mm in length), such as bivalves, gastropods, ostracods, and large fossils such as Orthoceras and trilobites, together with some plant fragments. This locality is the same as the sampling point no. 15 of Kuwano (1987), who identified it as a middle shaley member of the Lower Devonian Fukuji Formation.

Systematic Palaeontology

The morphological terms used are based on Moore (1961), Scott (1961), Berdan (1984) and Wang and McKenzie (2000), while the higher taxonomy follows that of Vannier et al. (2001) and Williams and Siveter (2008). The numbered specimens have been deposited in the Micropalaeontology Collection, National Museum of Nature and Science, Japan (MPC-number).

Phylum Arthropoda Latreille, 1829
Class Unknown
Order Leperditicopida Scott in Benson et al., 1961
(in Benson et al., 1961)
Family Leperditiidae Jones, 1856
Genus Sinoleperditia Wang, 1989
Sinoleperditia hamadai sp. nov.
Figures 2.1–2.4

Etymology.—The species is named after the Japanese palaeontologist, the late Emeritus Professor Takashi Hamada (University of Tokyo). He first discovered this fossil locality with one of the authors (T.O.).

Type specimens.—Holotype: MPC-3205 (Adult left valve; length = 8.29 mm, height = 4.87 mm). Paratype: MPC-3206 (Adult left valve).

Type locality.—An outcrop along the left side of a mountain path near the west part of the Fossil Garden (36°13′39.0″N, 137°31′35.4″E, altitude 1035 m), Okuhida-onsen-gou, Takayama City, Gifu Prefecture, central Japan; middle part of Fukuji Formation, Lochkovian (Lower Devonian) (Figure 1).

Materials.—Adult left valve, (Holotype: MPC-3205), adult left valve (Paratype: MPC-3206).

Diagnosis.—In lateral view, highest at one fourth from the posterior end. Posterior margin broadly curved and protruding postero-dorsally, straight dorsal margin; ventral margin broadly curved, most protruded at one third from posterior end. Surface smooth, slightly inflated except for the ventral area of anterior cardinal angle where the valve is flat.

Description.—Carapace broad-bean-shaped in lateral view in adult left valve (Figures 2.1, 2.2). Highest at one fourth from posterior end. Anterior and posterior cardinal angles well defined and greater than 90°. Posterior cardinal angle distinct and about 130°. In lateral view, anterior margin parabolic curved and protruding at midheight; dorsal margin straight; ventral margin broadly curved, protruding most at one third from posterior end. Same height at anterior and posterior ends. Surface smooth, slightly inflated except for ventral area of anterior cardinal angle where valve is flat. Prominent round-shaped adductor muscle scar developed at antero-median to median area. Trailing chevron muscle scar observed at anterodorsal area from adductor muscle scar. Distinct eye tubercle dorsal to the area of the trailing chevron muscle scars (Figures 2.1, 2.3). Eye tubercle oblong, long axis inclined from dorsal to anterior. Straight hinge with crenulations (Figure 2.4).

Remarks.—This species differs from Sinoleperditia (Pseudobriartina) guilinensis Wang, 1994 from the Devonian strata of South China by its broadly curved and postero-dorsally protruding posterior margin that is straight for one ninth from the postero-dorsal margin and distinctly connects with the dorsal margin, and broadly curved boomerang-shaped ventral margin. Sinoleperditia hamadai sp. nov. is distinguished from S. (Pseudobriartina) parallela Wang, 1994 from Middle Devonian strata from South China by its broadly curved and postero-dorsally protruding posterior margin and flat area below the anterior cardinal angle. This species is distinguished from Leperditia japonica Hamada, 1959 from the Devonian Fukuji Formation, Ichino-tani Valley, Okuhida-onsen-gou, Takayama City, Gifu Prefecture, central Japan by its longer lateral outline, prominent eye tubercle and posterior margin that is straight for one ninth from the postero-dorsal margin and distinctly connects with the dorsal margin. The present species is distinguishable from Paraeoleperditia fukujiensis Adachi and Igo, 1980 by its broadly curved and postero-dorsally protruding posterior margin and round-shaped adductor muscle scar.

Figure 1.

Map showing the fossil locality. A, location of Gifu Prefecture (gray area); B, detail map of the fossil locality.

Figure 2.

Optical micrographs of a new leperditid arthropod (1–4) from the middle part of the Lower Devonian (Lochkovian) Fukuji Formation exposed in an outcrop along the left side of a mountain path near the west part of Fossil Garden (36°13′39.07Prime;N, 137°31′35.4″E, altitude 1035 m), Okuhida-onsen-gou, Takayama City, Gifu Prefecture, central Japan. 1–2, Sinoleperditia hamadai sp. nov., adult left valve. (holotype: MPC-3205); 3–4, adult left valve (paratype: MPC-3206). 3, magnification of anterodorsal area; 4, magnification of dorsal area showing crenulated straight hinge.

Discussion

Wang and McKenzie (2000) reported that the tribe Sinoleperditiini (leperditicopid) was probably benthic and endemic, and inhabited a nearshore shallow-water zone comprising intertidal and lagoonal environments. Furthermore, Vannier et al. (2001) mentioned that Ieperditicopids probably inhabited lagoons where environmental conditions were largely controlled by the degree of connection with the open sea, the climate (arid or more humid) and also the freshwater input (phreatic, surficial, meteoric). Wang and McKenzie (2000) mentioned that Paraeoleperditia fukujiensis Adachi and Igo, 1980 is regarded as an Early Devonian representative of the Sinoleperditiini, but the lack of any further information about the type specimen of P. fukujiensis leads to uncertainties concerning the biogeographic distribution of the genus Sinoleperditia. The first discovery of Sinoleperditia from Japan strongly suggests that the Early Devonian Japanese leperditicopid arthropods were part of the Early Devonian leperditicopid arthropod biogeographic province of South China, as Wang and McKenzie (2000) mentioned. However, the leperditicopid arthropods are different at the species level, indicating that Japan was isolated from the continent of South China by sea or ocean. This hypothesis will be supported in the near future by comparing the benthonic ostracod assemblages from Japan with those from China.