2026 年 30 巻 p. 109-121
Well-preserved external molds of an articulated brittle star from the upper lower Maastrichtian (Upper Cretaceous) strata of the Futakawa Formation (Sotoizumi Group) of Wakayama, southwestern Japan, are described as a new ophiosphalmid genus and species, Enakosphalma nagashimai. It differs from other members of the Ophiosphalmidae in having lateral arm plates with a smooth outer surface stereom, a beveled outer proximal edge, very small spine articulations grouped ventrally in a shallow depression, small dorsal and ventral contact surfaces with the opposite lateral arm plates, oval radial shields, small disc scales, a large oval plate in the ventral interradius, small and rounded pentagonal oral shields, and tentacle pores developed as between-plate openings until the sixth arm segment. The new species is morphologically intermediate between Enakomusium from the Jurassic of Europe and the extant Ophiosphalma. It therefore contributes to a better understanding of the fossil record and evolutionary history of the important deep-sea family Ophiosphalmidae. It furthermore demonstrates that the Ophiosphalmidae have inhabited deep-sea environments since at least the Late Cretaceous.
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Ophiuroids, also called brittle stars, are an abundant and widespread component of modern marine benthic communities (Stöhr et al., 2012). The ophiuroid skeleton is composed of high-Mg calcite ossicles that readily fossilize, but due to the rapid post-mortem disintegration of the skeleton, intact ophiuroid fossils are rare (Ausich et al., 2001). Furthermore, research on ophiuroid fossils has long been biased towards Europe, with only a minority of reported occurrences originating from other parts of the world. This is particularly clear in the Late Cretaceous fossil record of the Ophiuroidea (e.g. Thuy et al., 2018). Whereas the study of Late Cretaceous ophiuroids from Europe has a long history (Spencer, 1905–1908; Valette, 1915; Müller, 1950; Rasmussen, 1950, 1952, 1972; Hess, 1960; Jagt, 1991, 1999, 2000, 2001; Helm et al., 1999; Stuwe, 2000; Jagt and Odin, 2001; Štorc and Žítt, 2008; Kutscher, 2011; Thuy et al., 2020; Thuy and Numberger-Thuy, 2023), few reports have been made from other parts of the world.
Recently, considerable efforts have been made to expand the fossil record of the Ophiuroidea from eastern Asia, particularly in Japan (Ishida et al., 2015, 2018, 2023, 2024; Okanishi et al., 2019, 2021, 2022, 2024; Gale et al., 2024; Numberger-Thuy et al., 2024). Numerous records have been described from Mesozoic and Cenozoic strata, but ophiuroids from the Late Cretaceous remain rare. The most notable exception is Stegophiura miyazakii Ishida, Thuy, Fujita, Kadokawa, Ikegami and Numberger-Thuy, 2018 from the Cenomanian of Kumamoto in southern Japan (Ishida et al., 2018). In order to achieve a more thorough assessment of the Late Cretaceous fossil record of the Ophiuroidea and enable comparison between faunas from various parts of the world, new records from outside Europe are needed.
Here, we describe an exceptional assemblage of new ophiuroid fossils comprising an articulated individual and disarticulated lateral arm plates and vertebrae from the Maastrichtian (Upper Cretaceous) Futakawa Formation in Wakayama, southwestern Japan (Figure 1).
The Futakawa Formation of the Sotoizumi Group is widely distributed in the Aridagawa area of central Wakayama (Hirayama and Tanaka, 1956a, b) and has been dated to the Santonian–Maastrichtian based on ammonoids, inoceramids, radiolarians, and similarities in lithofacies with contemporaneous strata in other regions of Japan (Hirayama and Tanaka, 1956a, b; Kamon and Nakazawa, 1989; Tashiro and Kawamura, 1995; Tashiro et al., 1995; Misaki and Ohara, 2011; Shigeta et al., 2012; Kurishima et al., 2013; Misaki, 2016). The ophiuroid specimens described here were collected from dark gray mudstones of the Futakawa Formation exposed in a small tributary of the river Aridagawa in the western area of Kunohara, Aridagawa Town, Wakayama (34°05′45.92″N, 135°26′38.52″E: see Figure 1).
The strata at the fossil locality were initially dated to the early late Maastrichtian based on the occurrence of the ammonoid Gaudryceras tombetsense Matsumoto, 1984 (Shigeta et al., 2012). Shigeta and Tsutsumi (2019) subsequently revised the Maastrichtian stratigraphic framework of the northwestern Pacific region and correlated the G. tombetsense-bearing beds with the upper lower Maastrichtian based on bio- and magnetostratigraphic evidence and zircon U–Pb geochronologic studies in this region (Kodama, 1990; Nifuku et al., 2009; Shigeta et al., 2015, 2017). Here, we follow the stratigraphic correlation of Shigeta and Tsutsumi (2019).
Shigeta and Tsutsumi (2019) recognized four Maastrichtian ammonoid zones: Nostoceras hetonaiense Matsumoto, 1977; Gaudryceras izumiense Matsumoto and Morozumi, 1980; Pachydiscus flexuosus Matsumoto, 1979; and P. subcompressus Matsumoto, 1954, in ascending order. They suggested that the G. tombetsense fossil-bearing beds are located at the lower part of the P. flexuosus Zone. Based on the magnetostratigraphic correlation of the N. hetonaiense Zone (C32.2n–C32.1n; Kodama, 1990) and the upper part of the P. flexuosus Zone (C31n; Nifuku et al., 2009; Shigeta et al., 2015) and U–Pb zircon ages of tuff beds overlying the G. izumiense Zone (70.5 ± 1.1 Ma, 95% confidence; Shigeta et al., 2017) and in the middle part of the P. flexuosus Zone (69.8 ± 0.8 Ma, 95% confidence; Shigeta and Tsutsumi, 2019), the G. izumiense Zone and the lower to middle part of the P. flexuosus Zone were correlated to polarity chron C31r (71.45–69.27 Ma; Gradstein et al., 2020) (Shigeta and Tsutsumi, 2019; Shigeta and Maeda, 2023). The results of those studies suggest that the G. tombetsense fossil-bearing beds were deposited about 70 Ma.
The new ophiuroid specimens described herein are preserved within a mudstone block (80 × 60 × 40 mm). In the articulated specimen, only parts of the dorsal and ventral sides of the disc and the ventral and lateral sides of three arms are exposed, with half of the ventral side of the disc, the ventral side of one additional arm, nine dissociated lateral arm plates, and three dissociated vertebrae still embedded in matrix. All specimens are preserved as dissolution molds of the original calcitic skeletons. The new specimens are reposited in the collections of the Kitakyushu Museum of Natural History and Human History (KMNH) in Kitakyushu, Fukuoka, Japan under catalog numbers KMNH IvP 907012–907024. Recent specimens of Ophiosphalma laqueatum (Lyman, 1878) and Ophiomusa granosa (Lyman, 1878) from the collections of the National Museum of Nature and Science (NSMT) in Tsukuba, Ibaraki, Japan (NSMT E-2879, 13622 and NSMT E-2989, respectively) were used for morphological comparison with the fossils. To enable a detailed morphological assessment, a synthetic resin cast of KMNH IvP 907012 was made, painted black, whitened with ammonium chloride, and photographed. Line drawings were produced using a camera lucida device. Scanning electron microscope (SEM) images (JEOL, JSM-6380LV) were used to document isolated ossicles of the recent specimen. X-ray micro-computed tomography (micro-CT; Shimadzu Corporation, inspeXio SMX-225CT FPDHR Plus) was used to visualize the internal and external morphologies of recent specimens and the mode of occurrence and morphologies of fossil specimens still embedded in matrix. Recent specimens were scanned at a voltage of 115 kV and a current of 70 μA for 20 minutes, and scan data were manually segmented and rendered using VGSTUDIO MAX 3.2 (Volume Graphics Company). Fossil specimens were first scanned at a voltage of 225 kV and a current of 70 μA for 60 minutes. Regions of interest were then rescanned at higher spatial resolution at a voltage of 199 kV and a current of 70 μA for four hours. Scan data of the fossil specimens were manually segmented and rendered using the software Molcer Plus (White Rabbit). All micro-CT data were illustrated using isosurface renderings.
We follow the terminology of Stöhr et al. (2012), Stöhr (2024), Thuy and Stöhr (2011, 2016), and Hendler (2018) and the classification proposed by O’Hara et al. (2018).
Class Ophiuroidea Gray, 1840
Superorder Euryophiurida O’Hara, Hugall, Thuy, Stöhr and Martynov, 2017
Order Ophiurida Müller and Troschel, 1840
Suborder Ophiomusina O’Hara, Hugall, Thuy, Stöhr and Martynov, 2017
Family Ophiosphalmidae O’Hara, Stöhr, Hugall, Thuy and Martynov, 2018
Enakosphalma gen. nov.
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Diagnosis.—Ophiosphalmid genus with a round disc covered dorsally by a continuous to near-continuous ring of large, thick, oval radial shields accounting for slightly more than half of the disc radius; disc scales very small, thin, elongate; ventral interradii with a very large, thick, oval plate surrounded by smaller round, thick scales and proximally bordered by a smaller, slightly elongate, rounded-pentagonal oral shield; adoral shields and abradial genital plates very large and wide; lateral arm plates thick, devoid of outer surface ornamentation, outer proximal edge of lateral arm plates beveled, with two to three poorly defined spurs; very small spine articulations grouped ventrally in a shallow depression; inner side of lateral arm plates with large tongue-shaped central vertebral articular ridge and small, trapezoidal dorsal and ventral contact surfaces with opposite lateral arm plate; arm spines unknown; tentacle pores developed as between-plate openings in the first five or six arm segments.
Type species.—Enakosphalma nagashimai sp. nov., by present designation.
Etymology.—The genus name (neuter noun) is a combination of Enakomusium Thuy, 2015 and Ophiosphalma Clark, 1941. Enakomusium means “yet another” Ophiomusium-like genus (Thuy, 2015). The name Enakosphalma refers to the similarity to and assumed close phylogenetic ties with Enakomusium.
Enakosphalma nagashimai gen. et sp. nov.
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Diagnosis.—As for genus.
Material.—The material comprises one fully articulated specimen, nine dissociated lateral arm plates, and three dissociated vertebrae, probably belonging to the same individual. All are preserved as external molds without original skeletal calcite.
The holotype is KMNH IvP 907012, dorsal and ventral sides (Figures 2, 3, 4), and only ventral side (Figure 5A, B).
The paratypes are KMNH IvP 907013 (dissociated lateral arm plate; Figures 5A-1, 6A), KMNH IvP 907014 (dissociated lateral arm plate; Figures 5A-2, 6B), and KMNH IvP 907015 (dissociated vertebra; Figures 5A-10, 6C).
Additional materials include seven dissociated lateral arm plates, KMNH IvP 907016 (Figure 5A-3), KMNH IvP 907017 (Figure 5A-4), KMNH IvP 907018 (Figure 5A-5), KMNH IvP 907019 (Figure 5A-6), KMNH IvP 907020 (Figure 5A-7), KMNH IvP 907021 (Figure 5A-8), and KMNH IvP 907022 (Figure 5A-9); two dissociated vertebrae, KMNH IvP 907023 (Figure 5A-11) and KMNH IvP 907024 (Figure 5A-12).
Etymology.—The specific epithet honors the late Tsugito Nagashima, who collected the new fossil-bearing mudstone block.
Locality and horizon.—A small tributary of the river Aridagawa in the western area of Kunohara, Aridagawa Town, Wakayama. Gaudryceras tombetsense-bearing beds of the Futakawa Formation (Sotoizumi Group), upper lower Maastrichtian (Shigeta et al., 2012; Shigeta and Tsutsumi, 2019).
Description of holotype.—KMNH IvP 907012 (Figures 2, 3, 4; 5A, B) is an articulated skeleton showing part of the dorsal and whole of the ventral disc (18.7 mm in diameter), round, covered dorsally by large, thick, smooth, oval radial shields accounting for slightly more than half of the disc radius; radial shields forming a near-continuous ring, with those of the same radius in contact, those of neighboring radii separated by a very narrow jumble of very small, thin, elongate scales (Figures 3A, 4A); center of disc with no discernible plates or scales; no granules or disc spines discernible; ventral interradii with a very large, thick, oval plate surrounded by smaller round, thick scales and proximally bordered by a smaller, slightly elongate, rounded-pentagonal oral shield (Figures 3A; 4A, B; 5B); adoral shields very large, wide, larger than oral shield and meeting on their proximal tips; oral plates relatively small, slender, bearing at least three small, conical oral papillae s.l. (Figures 3A, B; 4A, B; 5B); adradial genital plates long and slender (Figure 5B), and abradial genital plates very large and wide, as long as half the ventral interradius, no granules or scales discernible, lining a relatively long genital slit (Figures 3A, 4A, 5B).
Four arms preserved, all showing the ventral and lateral sides, the longest arm preserving at least 39 segments and measuring over 36.3 mm in length, 2.8 mm in basal arm width; arms large, massive, incorporating the first four or five arm segments in the disc, rapidly tapering upon emerging from disc, and small elongated arm segments distally (Figure 5A); lateral arm plates very large, thick, bulky, rounded rectangular, devoid of outer surface ornamentation or constriction; outer proximal edge beveled, presence of spurs not discernible due to overlap of lateral arm plates in anatomical connection; small, shallow, ventrally oriented tentacle pores on the ventrodistal edges of the first six lateral arm plates (Figures 3A, D; 4A, D; 5B); following lateral arm plates with very small within-plate tentacle perforations; distal edge of lateral arm plates slightly pinnacled, with two to three very small spine articulations, poorly prominent, oblique, sunken in a shallow depression close to the ventrodistal corner of the outer surface, composed of small muscle opening proximally encompassed by a slender ridge and distally separated from a smaller nerve opening by a large, vertical ridge (Figures 3C, 4C); no arm spines discernible; ventral arm plates thick, roughly pentagonal, with an obtuse proximal angle, weakly concave latero-proximal edges and a very wide distal portion with a weakly convex distal edge; first ventral arm plate nearly as long as wide, the following four wider than long; ventral arm plates beyond the fifth arm segment much smaller, triangular, as long as wide; all ventral arm plates separated by lateral arm plates (Figures 3A, D; 4A, D), no distalmost ventral arm plates discernible (Figure 5A); dorsal arm plates unknown.
Description of paratypes.—KMNH IvP 907013 (Figures 5A-1, 6A) is a dissociated median lateral arm plate, almost as tall as long, robust, rounded squarish outline; dorsal edge straight, distal edge weakly convex, ventral edge slightly convex, proximal edge beveled, with three small, poorly defined, weakly prominent and protruding spurs; three very small spine articulations sunken in a shallow depression close to the ventrodistal corner of the plate; within-plate tentacle perforation comparatively large at ventrodistal edge of lateral arm plate.
Inner side of lateral arm plate with a large, wide and sharply-defined, tongue-shaped central vertebral articular ridge, with a wide dorsoproximally oriented dorsal tip; small, rounded trapezoidal dorsal contact surface with opposite lateral arm plate; ventral contact surface more slender than dorsal one; tentacle perforation large, close to ventrodistal tip of central vertebral articular ridge; spur absent on inner distal edge.
KMNH IvP 907014 (Figures 5A-2, 6B) is a dissociated median lateral arm plate, very similar to paratype KMNH IvP 907013 but outer proximal edge with two very poorly defined spurs; spine articulations with large muscle opening and small nerve opening (Figure 6B3); inner side as paratype KMNH IvP 907013 but central vertebral articular ridge convex proximally.
KMNH IvP 907015 (Figures 5A-10, 6C) is a dissociated median to distal vertebra, slender, with zygospondylous articulation type, and with a short zygosphene, large proximal muscle fossae, narrow podial basins.
The specimens described herein possess a combination of characters that are typical of the extant ophiomusin family Ophiosphalmidae (Figure 7), including large and thick radial shields; small, thin disc scales; relatively long genital slits bordered by large and wide abradial genital plates; conical oral papillae; relatively large ventral arm plates present on most arm segments; rounded rectangular and relatively thin lateral arm plates without a constriction, with small and oblique spine articulations composed of muscle and nerve openings separated by a relatively thick vertical ridge, and with a large vertebral articular ridge and a small dorsal contact surface with the opposite lateral arm plate on the inner side; and tentacle pores developed as between-plate openings beyond the first two segments (Figures 3, 4, 5, 6). While some of these characters also occur in the sister family Ophiomusaidae (e.g. large radial shields and small, thin disc scales), the combination of characters strongly favors assignment to the Ophiosphalmidae. It should be noted that spurs on the outer proximal edge of the lateral arm plates were proposed as a diagnostic feature of the Ophiomusaidae, whereas ophiosphalmids generally lack such spurs (O’Hara et al., 2018). However, a more exhaustive survey of lateral arm plate morphologies in members of both families has revealed ophiosphalmid species with well-developed spurs and ophiomusaid species without spurs, in particular Ophiosphalma laqueatum (Lyman, 1878) (Figure 7E1) and Ophiomusa granosa (Lyman, 1878) (Figure 8C, D), respectively. These previously unpublished observations challenge the diagnostic value of the presence or absence of spurs on the outer proximal edge of the lateral arm plates. In contrast, the presence of a constriction and/or an oblique dorsal edge seem to have a previously underestimated diagnostic value in identifying members of the Ophiomusaidae (Thuy and Numberger-Thuy, 2021) (Figure 8). The absence of such a constriction and the non-oblique dorsal edge further corroborate the ophiosphalmid affinities of the material described herein.
Within the Ophiosphalmidae, the specimens described herein are morphologically intermediate between the extinct Enakomusium (Thurmann, 1851; Damon, 1880; Wright, 1880; Boehm, 1889; Hess, 1966; Kutscher, 1992; Thuy, 2015; Numberger-Thuy and Thuy, 2015; Ewin and Thuy, 2015, 2017; Numberger-Thuy et al., 2024) and the extant Ophiosphalma (Clark, 1941; Ishida et al., 2015; Baker, 2016). It is similar to Enakomusium in possessing tentacle pores developed as between-plate openings beyond the first three arm segments (Figures 3A, D; 4A, D; 5A, B) but differs from that genus in lacking a furrow along the outer proximal edge of the lateral arm plates and in having much smaller spine articulations grouped ventrally in a shallow depression and much smaller contact surfaces with the opposite lateral arm plate (Figure 6A, B). With respect to lateral arm plate morphology, the specimens described here are strikingly similar to members of the extant genus Ophiosphalma (Figure 7E). However, the new Japanese specimens cannot be assigned to Ophiosphalma given the higher number of arm segments with between-plate tentacle openings as well as the conspicuously large oval plate in the ventral interradius distally bordering, small rounded pentagonal and slightly elongate oral shield, larger abradial genital plates, and large central vertebral articular ridges on the inner sides of the lateral arm plates (Figures 3A, B; 4A, B; 5A, B; 7). We therefore describe the specimens as the new genus and species Enakosphalma nagashimai. The new taxon is most similar to the recent Ophiosphalma laqueatum (Figure 7), especially with respect to the rounded-rectangular outline of the lateral arm plates, the presence of spurs on the outer proximal edge of the lateral arm plates, and the small dorsal contact surface with the opposite lateral arm plate.
Enakosphalma nagashimai gen. et sp. nov. is the first Maastrichtian ophiuroid from Japan and thus represents a significant addition to the Late Cretaceous fossil record of the Ophiuroidea from eastern Asia. Furthermore, it is the first record of the family Ophiosphalmidae from the Cretaceous. Within the morphological spectrum of the family, Enakosphalma nagashimai gen. et sp. nov. lies between Enakomusium and Ophiosphalma, having a general Ophiosphalma-like appearance, particularly regarding the lateral arm plates, but showing a tentacle pore development more similar to the older Mesozoic Enakomusium. This is particularly interesting considering that Enakosphalma nagashimai gen. et sp. nov. also occurs stratigraphically between the predominantly Jurassic Enakomusium and the Cenozoic to recent Ophiosphalma. With the morphological details provided here, Enakosphalma nagashimai gen. et sp. nov. can be included in future morphology-based phylogenetic analyses to further explore the poorly known evolutionary histories of both the family Ophiosphalmidae and the suborder Ophiomusina.
Jurassic Enakomusium species are interpreted to have inhabited shallow-water environments (Thuy, 2015; Ewin and Thuy, 2017). In contrast, both Cenozoic and recent species of the extant Ophiosphalma predominantly occur at bathyal depths (Matsumoto, 1917; Clark, 1941; Okanishi et al., 2025), as evidenced by the occurrence of the extant Ophiosphalma cancellatum in early Miocene-aged upper bathyal deposits (Ishida et al., 2015). The horizons of the Futakawa Formation that yielded Enakosphalma nagashimai gen. et sp. nov. contain frequent turbidite deposits (Tanaka, 1974; Tashiro and Kawamura, 1995; Shigeta et al., 2012), and neither tidal nor wave-induced sedimentary structures are observed in this formation (Hirayama and Tanaka, 1956a, b; Tanaka, 1974). This suggests that it was deposited in a relatively deep marine environment below fair-weather and storm wave bases. The holotype of Enakosphalma nagashimai gen. et sp. nov. from the Futakawa Formation is articulated and preserves the fragile tips of the arms, although most arm spines and tentacle scales are dissociated. This suggests that the animal was subject to incipient post-mortem decay but suffered little or no damage due to the mechanical strains of pre-burial transport. The dissociated ossicles are extremely well preserved, lacking signs of fragmentation or abrasion, again suggesting little to no pre-burial transport. We conclude from these observations that the Enakosphalma nagashimai gen. et sp. nov. holotype was buried at or very close to its original habitat rather than washed in by currents or turbiditic flows. The discovery of a new ophiosphalmid from deep marine sediments of the Futakawa Formation shows that members of the family Ophiosphalmidae have lived in the deep sea since at least the Late Cretaceous.
We wish to thank the late fossil collector Tsugito Nagashima for collecting the new specimen-bearing block; Takayoshi Harada for introducing the main authors to the late Tsugito Nagashima; Makoto Nagashima and Nagako Nagashima for donating the new specimen-bearing block; Yasunari Shigeta (National Museum of Nature and Science, Tsukuba; NMNS) for useful comments on the age of the fossil locality; Takuma Haga (NMNS) for his kind help in taking photographs; and Takahiko Kutsuna, Shuhei Nomura, Yasunari Shigeta, and Gento Shinohara (NMNS) for their help in using micro-CT and 3D visualization software. We are grateful to the reviewers Sabine Stöhr (Swedish Museum of Natural History) and Masanori Okanishi (Hiroshima Shudo University) for pertinent comments on an earlier version of the manuscript. This study was supported in part by JSPS KAKENHI Grant Number JP25800290 (to AM).
CT data and 3D surface models of all specimens used in this study are available in MorphoSource (Project ID: 000794931): https://www.morphosource.org/projects/000794931.
We declare that none of the material in this manuscript has been published or is under consideration for publication elsewhere. Y. I., L. N-T., and B. T. identified the fossil. T. F. commented on comparisons with extant brittle star species. T. M. performed micro-CT scanning and constructed images. A. M. and M. O. surveyed the site. All authors contributed to the writing of the manuscript.
