Sarcophagodes duodecima Naya, 2021

Naya, Tomonori & Mizuno, Kiyohide, 2021, Sarcophagodes duodecima sp. nov., a new small araphid fossil diatom (Bacillariophyceae) from Lower to Middle Pleistocene sediments of Japan, Phytotaxa 505 (1), pp. 85-96 : 88-91

Sarcophagodes duodecima Naya sp. nov. (LM: Figs 3–18 , SEM: Figs 19–35 )

Description:—In LM, frustule rectangular in girdle view with round corners ( Figs 17, 18 ), attached at valve face by interlocking spines ( Fig.17 ). The larger valves are heteroplar clavate ( Figs 4, 5, 7, 9, 10 ) or isopolar elliptic ( Figs 3, 6, 8 ) and the smaller valves isopolar elliptic ( Figs 11–16 ). Sternum widely lanceolate ( Figs 12, 19 ). Striae alternate and robust ( Fig. 12 ). Length 3–16 µm, width 2.5–3 µm, and striae density 8–10 in 10 µm. In SEM, striae uniseriate, composed of up to three areolae on the valve face and two to four on the mantle ( Figs 19–22 , 25–30 ). Areolae apically reniform at the ends of each stria, otherwise rectangular with rounded corners ( Figs 20 , 25–27, 29 ), occluded by one or multiple profusely branched volae with stalk-like rod extending from vimines ( Figs 21 , 26, 27, 30 , 35 ). Marginal spines solid, spatulate ( Fig. 19 ) or conical ( Fig. 22 ), interrupting the striae on the thickened vimen at valve face-mantle junction ( Figs 19 , 25–30 ). Apical pore field present at both poles, consisting of multiple round pores present at both poles ( Figs 30 , 34, 35 ). Cingulum formed of up to eight wide open copulae ( Figs 20 , 29, 30 ). Valvocopula wider than other copulae. Edge of pars interior of valvocopula undulate ( Fig. 23 ). Fringed structure observed at the end of copula ( Fig. 21 ). Mantle plaques present ( Figs 20 , 29 ). No rimoportulae.

Type:— JAPAN. Higashiomi, Oita Prefecture: Himeshima village, from siliceous silt of the Lower to Middle Pleistocene Karato Formation , sample 180327–2 D3, 33°43′30.0″ N, 131°40′20.1″ E, T. Naya and Y. Horiuchi 27 March 2018 (holotype: slide MPC-44141= Fig. 12 , Micropaleontology Collection, National Museum of Nature and Science, Tsukuba, Japan, England Finder M 35/0)

Etymology:—Duodecim, Latin = twelfth, after a myth of Himeshima Island, where the type locality is situated. According to the Kojiki, the oldest extant chronicle in Japan ( Kurano 1963), Himeshima Island was the twelfth island in the Japanese archipelago to be created.

Observation:—The isopolar valves are observed in the entire size range of the species, whereas the heteropolar valves are recognized in only approximately half of the larger valves (>10 µm). However, due to the small proportion of the larger valves, few heteropolar valves occurred in the analyzed samples ( Figs 4, 5, 7, 9, 10, 23 , 30 ). Vimines are thickened at the valve-face mantle junction ( Figs 26–27 ). Each thickened vimen can be observed as a distinct bar crossing the middle of the stria in internal view under SEM ( Figs 24 , 32, 33 ); the vimines are also visible vaguely under LM ( Figs 5, 6, 11 ). A reduced apical pore field is observed at both poles, consisting of multiple round pores. In most specimens, the apical pore field could not be observed in external view, and could be seen only at the foot pole of the heteropolar valve ( Fig. 30 ). Mantle plaques are arranged along the external rim of the mantle ( Figs. 20 , 29 ).

Distribution and geologic age:—Thus far, S. duodecima has been found at two localities in Japan. The type locality of the species is in the Karato Formation on Himeshima Island, western Japan. The age of these sediments is inferred to be ca. 0.78 Ma. The other locality is in the Lower Pleistocene Bushi Formation in Iruma City, Saitama Prefecture, central Japan ( Fig. 1 ). S. duodecima occurs in the M1 and M2 marine beds in the Bushi Formation ( Figs 36–45 ); the age of these beds is estimated to be 2.5–2.4 Ma ( Naya and Mizuno 2020).

Figures

FIGURE 1. Location of the Karato Formation in Himeshima Island, Oita Prefecture, western Japan (the type locality of Sarcophagodes duodecima sp. nov), the Bushi Formation in Saitama Prefecture, central Japan, and the Osaka Group in the Kinki district, central Japan.

FIGURES 3–24. Sarcophagodes duodecima sp. nov. from the type locality. 3–18. LM images. Figs 3, 4, 7–9, 11–18 sample 180327-2 D3 (type material, Fig. 12 corresponds to the holotype); Figs 5, 6, 10, sample 13HMO3. 19–24. SEM images, sample 180327-2 D3 (type material). 19. An entire valve showing valve outline, valve face, spatulate marginal spines at the valve face–mantle junction (indicated by arrow), mantle, and parts of the girdle band. 20. Girdle view showing mantle areolae, open copulae, and mantle plaques (indicated by arrow). 21. Close-up of external valve of Fig. 20 showing fringed structure (indicated by arrow) observed at the end of copula. 22. Close-up of girdle view showing conical marginal spines. 23. Entire girdle view showing valvocopula. Undulate edge of the pars interior (indicated by arrows). 24. Internal view showing large internal openings of striae, with thickened vimines at the valve-face mantle junction (indicated by arrows) observed as distinct bars crossing the middle of the stria. Scale bars = 10 µm (Figs 3–18), 0.5 µm (Figs 21, 22), 1 µm (Figs 19, 20, 24), 2 µm (Fig. 23).

FIGURES 25–30. SEM external views of Sarcophagodes duodecima sp. nov. from the type locality. Figs 25–28, sample 180327-2 D2; Figs 29, 30, sample 13HMO1. 25. Valve outline and widely lanceolate axial area. 26, 27. Close-up of the specimen in Fig. 25 showing each apex, apically reniform areolae at the ends of striae, rectangular areolae with rounded corners at the midway of the striae, and profusely branched volae extending from vimines. 28. Larger valve. 29. Girdle view showing mantle areolae, marginal spines at the valve face– mantle junction, mantle plaques, and open copulae. 30. Oblique view showing the apical pore field at the foot pole composed of multiple round pores (indicated by arrow). Scale bars = 0.5 µm (Figs 26, 27), 1 µm (Figs 25, 29, 30), 2 µm (Fig. 28).

FIGURES 31–35. SEM internal views of Sarcophagodes duodecima sp. nov. from the type locality, sample 180327-2 D2. 31–33. Valves of different sizes and large internal openings of striae are visible. Note that rimoportulae are not present. 34, 35. Close-up of both apices of the specimen illustrated in Fig. 33, showing the apical pore fields composed of small round pores, uniseriate row of areolae, and volae. Scale bars = 0.5 µm (Figs 34, 35), 1 µm (Figs 31–33).

FIGURES 36–45. Sarcophagodes duodecima sp. nov. from the Lower Pleistocene Bushi Formation. Figs 36–42. LM images; Figs 43–45. SEM images. 43. Internal view showing valve outline, with large openings of striae. 44. External view showing valve outline, uniseriate row of areolae. 45. Close-up of Fig. 44 showing apical part of the valve, areolae occluded by branched volae with stalk-like rod extending from vimines. Scale bars = 10 μm (Figs 36–42), 0.5 µm (Fig. 45), 1 µm (Figs 43, 44).