Order Ophiurida Müller and Troschel, 1840
Suborder Ophiurina Müller and Troschel, 1840
Family Ophiopyrgidae Perrier, 1893
Genus Stegophiura H. Matsumoto, 1915
Stegophiura miyazakii sp. nov. (Figs. 2, 3)
Diagnosis Species of Stegophiura with tiny spines fringing the edges of dorsal and ventral disc sides (Figs. 2b, c, 3a–c) and long primary arm spines that correspond in length to arm segments (Figs. 2d, 3c).
Derivation of name Named in honour of Hayao Miyazaki, co-founder of Studio Ghibli, an animation studio in Japan, to pay tribute to his anime lifework, in particular the masterpiece “Ponyo” that celebrates marine biodiversity. In addition, Hayao Miyazaki`s favourite novelist, Soseki Natsume, lived in Kumamoto Prefecture, where the new ophiuroid fossils were found.
Holotype MDM 15017 (Mifune Dinosaur Museum, Kumamoto, Japan), an articulated skeleton exposing the dorsal side.
Paratype MDM 15018 (Mifune Dinosaur Museum, Kumamoto, Japan), an articulated skeleton exposing the ventral side.
Type locality and stratum Siltstone within the “Lower formation” of the Mifune Group (middle-to-upper Cenomanian, Upper Cretaceous) at Kawauchida (Mashiki Town, Kumamoto Prefecture), southern Japan (Fig. 1) (Matsumoto 1939).
Measurements Holotype: disc diameter, 6.1 mm, maximum preserved arm length, 10.0 mm, basal arm width, 1.5 mm. Paratype: disc diameter, 5.0 mm, maximum preserved arm length, 8.7 mm, and basal arm width, 1.1 mm.
Description
Of the holotype, the dorsal side is exposed (Figs. 2a, b, d, e, 3c, d). The disc is circular in outline, being covered by moderately large, oval or elongate, and thick scales, that either abut or overlap, with a row of tiny, contiguous papilla-like spines fringing the disc edge (Figs. 2b, 3c). Radial shields are quadrangular with rounded angles, being separated from the disc edge by very large abradial genital plates and in contact along their straight perradial edge. Abradial genital plates are fully exposed on the dorsal side of the disc and are conspicuous, large, oval, and longer than wide, being separated from each other by a row of two or three plates that are assumed to correspond to dorsal arm plates (Figs. 2b, 3c). The distal edge of the abradial genital plate bears a row of tiny, squarish papillae, in contact with each other and forming an arm comb. Arms are robust, taller than wide at the base. Dorsal arm plates are trapezoidal, wider than long in proximal segments and of equal width and length in median segments, widely separating the lateral arm plates on all observable arm segments (Fig. 2d, e). Lateral arm plates are swollen, with moderately coarse outer surface stereom and with at least four small, well-defined, prominent spurs grouped in the centre of the outer proximal edge (Fig. 2d). The spine articulations are integrated into the stereom at the distal edge of the lateral arm plates and consist of large muscle openings separated from a much smaller nerve opening by a low vertical ridge (Figs. 2e, 3d). The lateral arm plates carry a single, large, slender arm spine that is of equal length to or a little shorter than an arm segment; the spine is located in the lower part of the lateral arm plates, under a small angle to the arm (Figs. 2d, 3c). In addition to the single arm spine, lateral arm plates carry at least seven very short, squarish, contiguous, addressed papillae (Fig. 3c).
The paratype exposes the ventral disc side (Figs. 2a, c, 3a, b). The outline of the disc is circular, being covered by moderately large oval or elongate scales, abutting or overlapping, with a row of tiny, contiguous, papilla-like spines fringing the disc edge (Figs. 2c, 3a). The oral plates are slender and carry at least four squarish, contiguous lateral oral papillae. There is at least one short, conical apical papilla (Figs. 2c, 3a). Adoral shields are elongate, slender proximally, wider distally and in contact proximally (Figs. 2c, 3a). Oral shields are small, with a right to slightly obtuse proximal angle, slightly notched lateral edges and a rounded distal edge (Figs. 2c, 3a). Genital scales are small, squarish, and contiguous. Genital slits are narrow, extending from the oral shield to the disc edge. Second oral tentacle pores open outside the mouth slit and are bordered by small, squarish, and contiguous papillae similar to the lateral oral papillae, but forming a separate row. Successive tentacle pores are large, obliquely elongate, almost completely encompassed by consecutive lateral arm plates, with at least seven small, contiguous, and squarish tentacle scales on both edges of the pores in proximal arm segments (Figs. 2c, 3a). Lateral arm plates are narrow (Figs. 2c, 3a, b). Ventral arm plates are trapezoidal/pentagonal, widest distally, in contact on all observable arm segments, wider than long in proximal arm segments and as long as wide in median ones; the first ventral arm plates are trapezoidal, of comparable length and width (Figs. 2c, 3a, b).