Acanthoceras cf. zachariasii (Brun) Simonsen, 1979

Liu, Yan, Kociolek, J. P. & Fan, Yawen, 2016, Urosolenia and Acanthoceras species from Hainan Province, China, Phytotaxa 244 (2), pp. 161-173 : 169

Acanthoceras cf. zachariasii (Brun) Simonsen ( Figs 52–74 )

Description:— LM: frustules solitary, subcylindrical. Valve with two conical apices, connected by a thin bridge, with a long tubular extension forming from the conical part of the valve. Whole frustule 67–83 μm long, 6.0–27.5 μm wide, pervalvar length: 28–42 μm, extension length: 15–27 μm, girdle band density: 6–7 per 10 μm. SEM: the areolae in the valve are small and irregular, extending to the base of the extension ( Figs 63, 67 , 70, 73, 74 ). Poroids or rib-like structures were not observed on the valve. Girdle band width is 1.3–2.6 μm, areolae are small, open and irregular, 6–13 rows per band, 6–8 per 1 μm. Imbrication line is visible. The tip of the extension is sharply pointed with 5–7 teeth ( Figs 71, 72 ).

Remarks:— Acanthoceras zachariasii ( Brun 1894: 53) Simonsen (1979: 55) is the most commonly reported taxon within this genus. Tremarin et al. (2015) observed this species from Brazil and presented a comparison of morphological data for this species ( Tremarin et al. 2015, Table 3). Based on this summary, we can conclude A. zachariasii has a total frustule length of 12–100 μm, a frustule width of 6–25 μm, a process length of 14.9–70 μm, girdle bands numbering 4–14/10 μm, poroids of the bands 7/1 μm and four teeth at the tip of the calyptra. Since Tremarin et al. (2015) did not examine the type material, we are without this basic knowledge of the species. Our specimens do share features with A. zachariasii based on the documentation assembled by Tremarin et al. (2015), with the exception that our species is slightly wider and has 5–7 teeth at the tip of the extension. Subsequent examination of the type material may make it easier to decide whether the variation seen in the Hainan Island specimens warrant separation from the type species of the genus.

Figures

FIGURES 52–61: Acanthoceras cf. zachariasii, LM. Specimen from Yalong vally forest park. Scale bar = 10 μm.

FIGURES 62–67: Acanthoceras cf. zachariasii, SEM. 62. External view of valve and girdle bands, scale bar = 5 μm. 63. External view of areolae on valve and bands, scale bar = 4 μm. 64. External view of areolae on bands and imbrication lines, scale bar = 4 μm. 65. External view of the whole frustule, scale bar = 5 μm. 66. External view of valve and bands without areolae, scale bar = 5 μm. 67. Magnification of the base of the extension, showing the areolae on the valve, scale bar = 2 μm.

FIGURES 68–74: Acanthoceras cf. zachariasii, SEM. 68, 69, 74. External view of valve and girdle bands, scale bar = 5 μm. 70, 73. Magnification of the base of extension, showing the areolae on the valve, Fig. 70: scale bar = 2 μm, Fig. 73: scale bar = 4 μm. 71, 72. Magnification of the extension ends with 5–7 teeth, Fig. 71: scale bar = 400 nm, Fig. 72: scale bar = 500 nm.