Eunotia petasum Siver, Oddsund & Lott, 2022

Siver, Peter A., Oddsund, Erik & Lott, Anne M., 2022, Descriptions of three new diatom species in the genus Eunotia (Eunotiaceae, Bacillariophyta) from the Eocene Arctic, Phytotaxa 567 (1), pp. 21-35 : 24-26

Eunotia petasum Siver, Oddsund & Lott sp. nov. (LM Figs 15–21 , SEM Figs 22–27 )

Description:— Valves are dorsi-ventral, elongate, widest in the middle of the valve, and becoming tapered and drawn out at the apices forming protracted ends ( Figs 15–21 ). The ventral margin is concave, becoming slightly linear at the apices. The dorsal margin is more convex relative to the concave ventral margin such that the valve becomes progressively narrower from the center to the apices ( Figs 24–27 ). Valves range in length from 19.6–57.6 µm with a mean of 38 µm, and in diameter at the valve center from 2.9–5.1 µm with a mean of 3.8 µm (n=25). At the apices, valve width ranges from 1.5–2.2 µm, with a mean of 1.8 µm. The margins of the valve face, especially along the ventral side, are slightly thickened and form right angles with the mantle ( Figs 23–24 ). Striae are parallel, widely spaced with 13.5–19 per 10 µm, a mean of 16 µm, and consist of small, circular areolae that are opened on both the external and internal valve surfaces. Striae are continuous from the valve face onto the dorsal mantle, but may be interrupted and discontinuous with those on the ventral mantle ( Figs 23–24 ). On smaller valves, the areolae on the ventral mantle may become more randomly spaced, especially below the raphe branches ( Fig. 23 ). The mantle is shallow, approximately 1.5–2 µm, and the margin slightly thickened around each apex. The proximal end of the raphe commences near the middle of the mantle approximately 1/4 to 1/3 of the valve length from the apex, rises slowly on a linear path to the valve margin, and then bends up and onto the valve face terminating close to the end of the valve ( Figs 23, 25 ). The proximal and distal raphe fissures both terminate as small round pores ( Figs 23–25 ). On most specimens, the portion of the mantle below the raphe consists of short striae composed of a few pores, but on some valves the pores are randomly spaced and not organized into distinct striae. Internally, the distal raphe fissure terminates within a small helictoglossa positioned about 1/3 the distance across the valve face ( Figs 26–27 ). A single rimoportula is found on one of the valve apices.

Type:— CANADA. Lac de Gras kimberlite field region, Northwest Territories: Rock specimen sub sampled from section 15-3-75 of the Giraffe Pipe core (holotype circled specimen on slide “ GP 15-3 - 75 D, LM 5” Canadian Museum of Nature CANA 129308 = Fig. 17 . Isotype circled specimen on slide labeled “ GP 15-3 - 75 LM4” P. Siver’s personal collection = Fig. 21 ) .

Etymology:— The name refers to a hat, whereby smaller specimens give the impression of a hat worn, for example, by Napoleon.

Figures

FIGURES 15–21. LM Micrographs of Eunotia petasum sp. nov. from the Giraffe Pipe fossil locality. Figure 17 is the type specimen circled on slide “GP 15-3-75D, LM5” (CANA 129308). Figure 21 corresponds to the isotype specimen circled on slide “GP 15-3-75 LM4” in P. Siver’s collection. Scale bar = 10 µm.

FIGURES 22–27. SEM micrographs of Eunotia petasum sp. nov. from the Giraffe Pipe fossil locality. 22. Dorsal view of a frustule showing the continuation of the striae from the valve face onto the mantle. 23. Ventral view of a valve depicting the raphe and reduced striae, often consisting of only 1–2 pores. 24. Interior view of a valve showing the continuation of the striae from the dorsal mantle onto the valve face. Striae on the ventral mantle are often reduced to random pores. 25. Exterior view of a valve depicting the narrow and protracted end and the distal raphe fissure. 26, 27. Internal views showing the striae, the open nature of the pores, the protracted apex, and the small rounded helictoglossa. Scale bars = 5 µm (Figs 22–26), 10 µm (Fig. 27).