Disarticulated fossil euryalid ophiuroid vertebrae from the Middle Pleistocene Miyata Formation, Miura, Kanagawa Prefecture, eastern Japan, are described. The vertebrae are assigned to the family Gorgonocephalidae on the basis of arm branching and the presence of an open oral groove along the entire arm. This is the first record of fossil euryalids from the Indo-Pacific region.
Introduction
The order Euryalida includes the basket stars and snake stars, in the class Ophiuroidea (Echinodermata). Ophiuroid arms are segmented and each segment has a central arm ossicle, which is referred to as a “vertebra” in ophiuroid systematics (e.g. Okanishi, 2016). The vertebrae have matching articulations on their distal and proximal surfaces, and euryalid ophiuroids have hourglass-shaped articulations, referred to as “streptospondylous”. This form of articulation is also shared by Ophiomyxidae and some species of Ophiacanthidae in the superorder Ophin-tegrida (e.g. Stöhr et al., 2012; O'Hara et al., 2017). The “branching vertebrae” of basket stars have two articulations on the distal surface, confirming the presence of branching arms; therefore, dissociated fossil vertebrae can be regarded as coming from euryalid basket stars (e.g. Kroh, 2002; Kroh and Jagt, 2006). Fossil records of basket stars are very poor, and only four records are currently known; these are from the upper Miocene–Pliocene of Algeria (Pomel, 1885–1887), lower Miocene of Austria (Kroh, 2002), middle Miocene of the Central Mediterranean (Kroh, 2004), and Pliocene strata of eastern Netherlands (Kroh and Jagt, 2006). Apart from these records, other streptospondylous vertebrae have been recorded from the Miocene of Hungary (Vadász, 1915) and of France (Valette, 1928), Oligocene of New Zealand (Spencer and Wright, 1966), upper Pliocene of Jamaica (Donovan and Paul, 1998), Early to Middle Jurassic of Germany (Thuy, 2015) and Middle Triassic to Lower Jurassic of Germany (Thuy and Stöhr, 2018), but these are not unequivocally assignable to those of basket stars. There is also no record of unquestionable body fossils of basket stars to date (e.g. Kroh, 2002; Thuy, 2015).
Ophiuroid fossils from Japan have been recorded mainly based on associated body fossils, and more than 50 species are currently listed (e.g. Fujita, 1992; Ishida et al., 2011, 2015). Although records of disarticulated ossicles are poor in Japan (Ishida, 2004), those ossicles occur from Japanese strata indeed (Ishida, unpublished), and generic or even specific identifications based on vertebrae and oral plates (e.g. Kroh, 2002), and lateral arm plates (e.g. Thuy, 2015), are possible.
Recently, the fifth record of unquestionable fossil basket stars was confirmed in the Pleistocene Miyata Formation in Kanagawa Prefecture, eastern Japan (Okumura et al., 2005). This is also the first record of branching vertebrae of basket stars from the Indo-Pacific region and the first record of euryalid brittle stars from Japan.
Material and methods
Fossil ossicles were collected from the Sha'ana Tuffaceous Sand Member (corresponding to the Kamimiyata Tuffaceous Sand Member in Kanie and Ohkoshi, 1981) of Miyata Formation, exposed at Sha'anadai, Minamishi-taura Town, Miura City, Kanagawa Prefecture, about 1 km northwest of the Miura-Kaigan railway station of the Keikyu Line on 7 March 2012, 13 October, 2013 and 29 December 2017 (Figure 1).
The Miyata Formation (Cenozoic, Quaternary, Middle Pleistocene), originally described by Aoki (1925), is distributed on the Miyada plateau, southern Miura Peninsula, Kanagawa Prefecture, central Japan. The stratigraphy of this formation has been examined historically (e.g. Fujita, 1951; Okumura et al., 1977; Kanie and Ohkoshi, 1981). Okumura et al. (1977) considered that the Miyata Formation is lithologically subdivided in ascending order into five members; Sugaruya Sand Member, Tsukuihama Sandy Gravel Member, Koenbo Sand Member, Sha'ana Tuffaceous Sand Member and Itchoda Sand Member. Kanie and Ohkoshi (1981) considered that the “Miyata Formation” should be subdivided into the Miyata Formation and the Tsukui Formation, the latter of which was previously described as the Tsukuihama Sandy Gravel Member in Okumura et al. (1977), and in turn the Miyata Formation could be subdivided into three members (Kamimiyata Sandy Tuffaceous Member, Sugaruya Sand Member and Ohkine Pumiceous Sand Member). Although a consensus has not been reached about the geological structure of this formation, Okumura et al.'s (1977) stratigraphy has been supported by recent studies (e.g. Shibata et al., 2014; Ishihama et al., 2017).
The geological age of the Miyata Formation was estimated to be 0.44 to 1.22 Ma by Yamaguchi et al. (1983) based on the calcareous nannofossils, 0.46 to 1.02 Ma by Kanie et al. (2000) based also on the calcareous nannofossils, or 0.325 Ma, using the Electron Spin Resonance method (Toyota and Okumura, 2000).
Fossil mollusks (Yokoyama, 1920; Arai et al., 1971; Horikoshi and Kosuge, 1971; Kanie, 1971; Okumura et al., 1977, 1979; Kanie and Ohkoshi, 1981; Yamaguchi et al., 1983; Kanie et al., 2000; Okumura et al., 2005; Shibata et al., 2014; Ishihama et al., 2017), ahermatypic solitary corals (Omura et al., 1991), brachiopods (Kuramochi and Katsuzawa, 1999), barnacles (Yamaguchi, 1971), foraminifers (Higuchi, 1954; Eto, 1971, 1972), elas-mobranchs (Taru and Matsushima, 1998) and elephants (Hasegawa and Kanie, 1971) were previously recorded from the Miyata Formation.
The sediment is semiconsolidated, poorly sorted massive fine-grained tuffaceous sand, which contains scoria and pumice (1 to 3 mm diameter), and fine gravel (2 to 10 mm diameter). Well preserved fossil mollusks (e.g. Mizuhopecten tokyoensis, Cyclocardia ferruginea, Acta (Truncacila) insignis), bryozoans, brachiopods, decapods, barnacles, echinoids and foraminifers co-occurred with fossil ophiuroids. Most bivalves occurred as single valves, and were buried horizontally, with their convex sides upward. Other fossils, such as decapods, barnacles and echinoids, were represented as disarticulated cheli-peds, shells, plates and spines, respectively, suggesting that the fossil assemblages from the outcrop could be allochthonous.
To collect ophiuroid ossicles, sediment samples were air-dried and then disintegrated in water. Then samples were washed using a sieve of 0.063 mm mesh. Ossicles were handpicked from the residues under a stereo microscope and cleaned with hydrogen peroxide (30% solution). Photographs of Figures 2 and 3 were focus-stacked using the software CombineZM1 v.1.0.0 (Hadley, 2008).
Materials are deposited in the National Museum of Nature and Science, Tsukuba (NMNS). Morphological terminology and systematics follow Stöhr et al. (2012) and O'Hara et al. (2017), respectively.
Systematic description
Superorder Euryophiurida O'Hara et al., 2017
Order Euryalida Lamarck, 1816
Family Gorgonocephalidae Ljungman, 1867
Gorgonocephalidae gen. et sp. indet.
Figures 2, 3
Material examined.—Eighty-four vertebrae (NMNS PA 18548–18631), all originating from bulk samples collected from the Middle Pleistocene Miyata Formation, Miura (Kanagawa, Japan). All materials are preserved in the form of dissociated ossicles. Although it cannot be confirmed, all vertebrae are considered to be conspecific.
Description of “normal” vertebrae.—In total 69 vertebrae range in size from 0.6 to 5.2 mm in width, having an hourglass-shaped, streptospondylous articulation. In lateral view, they are approximately two-thirds times wider than high. Most vertebrae lack any ornament in lateral view, probably due to abrasion (e.g. Figure 2A-3, B-3), but some well preserved specimens show some remaining ornament (Figure 2C-3). In these well preserved vertebrae, vertical furrows are visible on the oral-lateral side (Figure 2C-3). The aboral (Figure 2A-1, A-2, B-1) and oral (Figure 2C-1) grooves are shallow and U-shaped. No oral grooves were covered (Figure 2A-1, A-2, A-5, B-1, B-2, B-5, C-1, C-2, C-5). A pair of probably radial water canals open on the oral groove and can be observed in well preserved specimens (Figure 2C-5).
Description of branching vertebrae.—Fifteen branching vertebrae were picked from bulk samples. These are slightly wider than “normal” vertebrae but of the same height, approximately 1.1 to 2.6 mm in width, having two inclined articulation surfaces on the distal face (Figure 3A-2, B-2, C-2, D-2, E-2). Both symmetrical (Figure 3B-2, C-2, D-2) and asymmetrical (Figure 3A-2, E-2) branching angles were found in examined materials. No ornamentation was observed in examined materials due to the abrasion. Aboral and oral furrows branching on center, and two furrows extend to distal portion (Figure 3A-4, A-5, B-4, B-5, C-4, C-5, D-4, D-5, E-4, E-5). A pair of probably radial water canals lie on the proximal portion of the oral furrow and another pair of radial canals lie on one of two branched distal furrows (Figure 3A-5).
Figure 1.
Position of the sampling locality (marked by ×) of euryalid vertebrae from the Miyata Formation, Miura City, Kanagawa Prefecture. 1:50000 topographic map by the Geospatial Information Authority of Japan.
Figure 2.
Gorgonocephalidae from the Miyata Formation, unbranching “normal” vertebrae. A, proximal vertebra (NMNS PA 18548); B, distal vertebra (NMNS PA 18549); C, middle vertebra (NMNS PA 18550). Numbers show orientations, proximal (1), distal (2), lateral (proximal to left) (3), aboral (4) and oral (5). Scale bars, 1 mm. Arrows indicate oral furrows. Arrowheads indicate lateral furrows. Abbreviation: rwc, radial water canal.
Discussion
The examined vertebrae fall within Gorgonocephalidae, for they possess: open oral furrows throughout the arms; streptospondylous-type articulations; no projecting area in distal side; and branching arms. Distal side of vertebrae in the middle to distal portion of the arms of Euryalidae are projecting, (e.g. see Mortensen, 1933, pl. 4, fig. 2), but these projections are not observed in the present materials. Branching vertebrae are also found in the family Euryalidae, but their oral furrow is closed by an “oral bridge” in the middle to distal portion of the arms (e.g. Kroh, 2004; Okanishi et al., 2013). There is a possibility that these oral bridges are heavily abraded. However, the oral bridges of Euryalidae, especially in arm tips, are very thick (e.g. see Mortensen, 1933, pl. 4, fig. 2) and it is unlikely that they are all abraded in our examined materials.
Figure 3.
Gorgonocephalidae from the Miyata Formation, “branching” vertebrae. A, proximal “asymmetrical” vertebra (NMNS PA 18551); B, middle “asymmetrical” vertebra (NMNS PA 18552); C, middle “symmetrical” vertebra (NMNS PA 18553); D, distal “symmetrical” vertebra (NMNS PA 18554); E, distal “asymmetrical” vertebra (NMNS PA 18555). Numbers show orientations, proximal (1), distal (2), lateral (proximal to left) (3), aboral (4) and oral (5). Scale bars, 1 mm. Arrows indicate oral furrows. Abbreviations: drwc, distal radial water canal; daf, distal aboral furrow; dof, distal oral furrow; paf, proximal aboral furrow; drwc, distal radial water canal; prwc, proximal radial water canal.
The paleoenvironment of the Sha'ana Tuffaceous Sand Member was reconstructed as shallow water or continental shelf in depth, under the influence of cold currents (Kuramochi and Katsuzawa, 1999; Kanie et al., 2000). Additionally, Higuchi (1954) estimated the paleotem-perature of the Miyata Formation to be similar to, or somewhat colder than, the current water temperature of the adjacent waters of the Miura Peninsula, based upon the fossil foraminifer fauna. The gorgonocephalid basket stars are highly specialized suspension feeders, trapping plankton in their network of branching arms, which extend into water currents. In general, they are found at depths of 40 to 2000 m, with strong currents, attached primarily to rocks, hard corals, soft octocorals, and black corals (Okanishi, 2016). Therefore, although the fossils possibly show signs of allochthonous occurrence because of abrasion and dispersion of dissociated ossicles, the discovery of many ossicles probably supports the known paleoenvironment of the Miyata Formation and, as well, indicates the possible existence of a paleoenvironment characterized by strong current and hard substrates near the formation.
This fifth undoubted record of fossil basket stars of the family Gorgonocephalidae is the youngest in the previously known records of basket stars (Pomel, 1885–1887; Kroh, 2002, 2004; Kroh and Jagt, 2006). The previous undoubted records of basket stars are known from Europe (Kroh, 2002, 2004; Kroh and Jagt, 2006) and North Africa (Pomel, 1885–1887). Therefore, this is the first record of basket stars from the Indo-Pacific region. Kroh (2002) suggested that the lack of previous euryalid fossil records from modern diversity hotspots (the Indo-Malayan region) may be the result of sampling bias. The present discovery from the Japanese area indicates that the scarce fossil records of euryalid vertebrae may indeed be due to sampling bias. Further investigations focusing on these ossicles in the Indo-Malayan region as well as Japan, will surely increase the discoveries of euryalid vertebrae.
Acknowledgements
We are most grateful to David L. Pawson of the Smithsonian Institution, National Museum of Natural History for his careful and critical reading of the manuscript and for providing constructive comments. Thanks are also extended to Kiminori Taguchi of Kanagawa Prefectural Museum of Natural History for his comments on the geological structure of the Miyata Formation. Thanks are due to Tatsuo Oji of the Nagoya University Museum and Ben Thuy of the National Museum of Natural History, Luxembourg for greatly improving an earlier draft of this manuscript.
