Staurosirella tigris M.A.Harper, E.Morales & Van de Vijver, 2022

Harper, Margaret A., Morales, Eduardo A. & Vijver, Bart Van De, 2022, An unusual freshwater diatom with bilobate ends from the Mid-Miocene of East Antarctica: Staurosirella tigris sp. nov. (Fragilariaceae, Bacillariophyta), Phytotaxa 541 (2), pp. 201-208 : 203-205

Staurosirella tigris M.A.Harper, E.Morales & Van de Vijver , sp. nov. ( Figs 1–20 )

Description:— LM ( Figs 1–15 ): Frustules rectangular in girdle view ( Fig. 1 ), occasionally linked valve face to valve face, form short filaments (not shown). Valves linear, dog-bone-shaped with ends having two lateral lobes, each terminating in broadly rounded apices. Longitudinal axis longer than transapical axis. Smaller valves almost quadri- to tri-radiate ( Figs 14, 15 ). Valve dimensions between the lobes (n=30): central length 22–34 µm, central width 7.5–14.0 µm. Dimensions measuring from apices of lobes (n=30): length 24–43 µm, width 16.5 – 24.0 µm. Mantle depth (n=6): 6.0–7.5 µm. Axial sternum linear, moderately broad, approximately 1/4–1/3 of the total valve width (measured in the center), occasionally slightly eccentric due to unequal shortening of central striae (e.g. Figs 4, 8 ). Sternum continuing but gradually narrowing into the terminal lateral lobes. Striae present on all four margins, 6–7 in 10 µm, gradually shortening from the middle towards the apices, parallel in the center along the valve face longitudinal axis, more radiate towards the apices. Between apical lobes, striae typically radiate. Areolae not or only very weakly discernible in LM.

SEM ( Figs 16–20 ): Valve face slightly undulate with virgae slightly raised above the striae ( Figs 16, 17 &18 ). Virgae not wider or as wide as the striae. Vimines apparently continuing as low raised ridges on the virgae ( Fig. 17 , arrows). Striae uniseriate, composed of lineolate areolae, ca. 42–45 in 10 µm (n=7). Striae continuing without interruption onto the mantle. Small, hollow conical spines present on the virgae between the striae ( Fig. 17 ). Apical porefields of ocellulimbus type, large, composed of many parallel rows of small pores, present at each apex ( Fig. 18 ). Internally, apical porefields obscured by thickening of the valve margin and virgae thickened with striae sunken in between them ( Fig. 20 ). Rimoportulae not observed. Girdle composed of several bands ( Fig. 19 ). Valvocopula clearly fimbriate ( Fig. 19 ).

Type:— ANTARCTICA, Mid-Miocene glacial-lacustrine sediments in the Friis Hills , sample WD 19 (Friis Hills Drilling Project site 1, drill hole 1B core 3 preliminary sample at 16.295 m drillers’ depth) (161°26’57.646” E / 77°45’2.681”S, alt 1259 m, coll. date 25th May 2017, leg. Warren Dickinson), (holotype BR-4689 != Fig. 6 , isotype PLP-393 , University of Antwerp) .

Etymology:— The specific epithet “ tigris ” meaning “tiger” in Latin, refers to the general outlook of the species resembling a tiger-skin rug.

Age:— Early to Middle Miocene, younger than 19.76 Ma based on occurrence in Friis Drift II above a 19.76 Ma tephra bed. Older than ~14 Ma as fossils of Nothofagus Blume (Southern beech) and Isoetes L. (Cantrill et a l. 2016) indicate a moist warm climate before the mid-Miocene climate transition of ~13.9 Ma ( Lewis & Ashworth 2016). More precise dating based on 40 Ar/ 39 Ar dating of the tephra deposits in the Friis Hills Drillholes constrains the age of these sediments to between 15–14 Ma ( Verret et al. 2020, Chorley 2021).

Distribution and Ecology:— The new species was found in several samples, together with large populations of another Staurosirella species ( Pinseel et al. 2016a, see two largest Staurosirella valves). The observed diatom flora, co-dominated by these Staurosirella species together with (at present unidentified) Aulacoseira taxa most likely indicates the presence of shallow, open water as these tychoplanktonic taxa often thrive in waters prone to regular mixing. Staurosirella tigris was absent from more peaty samples co-dominated by Eunotia and Brachysira indicating it preferred more oligotrophic conditions than the latter two genera. The deposits contain macrofossils that indicate tundra vegetation ( Lewis & Ashworth 2016).

Figures

FIGURES 1–15. Staurosirella tigris sp. nov., LM pictures taken from the holotype population (sample WD 19, BR-4689). 1. Frustule in girdle view. 2–15. Several valves in valve face view in decreasing valve length. Fig. 15 most likely represents a teratological valve. Scale bar = 10 µm.

FIGURES 16–20. Staurosirella tigris sp. nov., SEM pictures taken from the holotype population (sample WD 19, BR-4689). 16. External view of an entire valve. 17. External detail of the valve margin showing the conical spines with hollow bases and the linear areolae. 18. External detail of the apical pore fields. 19. Internal detail showing part of the valvocopula and the copulae. 20. Internal detail of a valve apex showing the apical pore fields and some vimines with small or remnant volae. Scale bars = 10 µm (Figs 16, 19), 5 µm (Figs 18, 20), 1 µm (Fig. 17).