Pseudostaurosira ellipticolanceolata André, Sabbe & Van de Vijver, sp. nov. (Figs 1–21 LM, 22–28 SEM) Valves lanceolate to linear-lanceolate in larger specimens, becoming elliptic-lanceolate to elliptic in smallest valves (Figs 1–21). Valve margins parallel in longer valves to distinctly convex in the smaller range of the cell diminution series. Apices very weakly protracted (Figs 1–3), subrostrate, to non-protracted, broadly rounded in smaller specimens. Valve dimensions (n=40): valve length 4–20 µm, valve width 3.5–4.0 µm. Sternum broad in the largest specimens, becoming moderately broad in smaller valves, lanceolate. Striae uniseriate, parallel to weakly radiate throughout the entire valve length, 16–18 in 10 µm, composed of 1–2, rarely 3 areolae, on valve face (Figs 22, 25), and 1–2 areolae on valve mantle. Areolae at valve face/ mantle junction markedly larger, becoming smaller towards sternum (Figs 22, 24). Mantle areolae separated from those on valve face by short linking spines (Figs 22, 25). Interdigitating spines originating from vimines, solid, spatulate with broad tip (but probably broken and eroded in all observed specimens) (Fig. 23, white arrows). Areola occlusions eroded, vestiges partly visible in some areolae, suggesting volate occlusion type (Figs 27, 28, black arrows). Apical pore field reduced to a handful of very small pores (Figs 27, 28, white arrows). Internally, sternum flat with raised virgae between striae (Fig. 26). Girdle structure not observed due to severe erosion of the frustules. Mantle plaques not observed (but unclear whether being a result of valve erosion). Type:— BELGIUM. Burchtstraat, Aardenburg, sediment core, sample 85, C. André, 1 st March 2019 (holotype slide BR-4776= Fig. 6. Isotype slide 421, University of Antwerp, Belgium). PhycoBank registration:— http://phycobank.org/103600 Ecology and associated diatom flora:— Pseudostaurosira ellipticolanceolata has a relative abundance of almost 10% in the type sample. This sample is part of a clay layer from a sediment core taken at the edge of a ditch around the Roman castellum at Aardenburg, which most likely received marine input via a tidal channel and freshwater input from an inland river, possibly the river Ee. The clay layer dates back to the Holocene, with the first indications pointing to a possible age between the 4 th and 10 th century AD (André, unpubl. data). Other dominant species include Epithemia adnata (Kützing 1833: 544) Brébisson (1838: 16), Staurosira cf. venter (Ehrenberg 1854: 13, pl. XIV) Cleve & J.D.Möller (1879: no. 242) sensu Lange-Bertalot et al. (2017), Eunotia minor (Kützing 1844: 39) Grunow (in Van Heurck 1881: pl. 33: figs 20, 21) and Cymatosira belgica Grunow (in Van Heurck 1881: pl. 45: figs 38–41). This diatom flora points to a brackish, eutrophic environment (Lange-Bertalot et al. 2017). Epithemia adnata is typically epiphytic and may point to the presence of submerged vegetation, but is also salinity-tolerant and often occurs in aquatic conditions with higher electrolyte contents (Lange-Bertalot et al. 2017). Cymatosira belgica is considered a marine tychoplanktonic species, typically observed in tidal inlets and large tidal channels (Vos & de Wolf, 1988). The presence of Eunotia minor is more difficult to explain but most likely the result of the influx of foreign material. Unfortunately, the taxonomic identity of S. cf. venter is too unclear to draw ecological conclusions from their presence in the sample. Taxonomic comments:— Morales et al. (2021) discuss a large number of Pseudostaurosira species that are known worldwide when they described several new species from Bolivia. Based on valve outline and valve dimensions, the new P. ellipticolanceolata shows some similarity with the group of Pseudostaurosira subsalina (Hustedt) E. Morales (2005: 115) and P. polonica (M.Witak & Lange-Bertalot in Witkowski et al. 1995: 736) E.Morales & M.B.Edlund (2003: 235). Pseudostaurosira occulta E.Morales, C.E.Wetzel & Ector (2021: 40) shows comparable valve dimensions (length 7–35 µm, width 3.5–4.0 µm) and, at least in the longer range of the size diminution series, a comparable valve outline with almost parallel margins. However, differences can be noted in the shape of the apices (squarish in P. occulta, more broadly rounded in P. ellipticolanceolata), stria density (14–16 in 10 µm in P. occulta versus 16–18 in 10 µm in P. ellipticolanceolata) and stria structure. In P. occulta, striae are usually composed of 3–4 areolae whereas striae with 3 areolae are rather rare in P. ellipticolanceolata. Due to the high degree of erosion, it was not possible to comment on the structure and shape of the apical pore field (covered by an external flap in P. occulta) and the linking spines. Pseudostaurosira subsalina has broader valves (valve width 4.0–5.5 µm), a lower stria density (13–14 in 10 µm), a higher number of areolae per stria, a larger apical pore field (compared to the reduced pore field in P. ellipticolanceolata), and a broader transition step between valve face and mantle, excluding possible conspecificity (Cejudo-Figueiras et al. 2011). Pseudostaurosira zolitschkae M.L.García et al. (in Garcia et al. 2021: 265) differs in having acutely protracted valve apices and a much lower stria density (11–14 in µm) (García et al. 2021). Other taxa in this group differ by the shape of the areolae (P. polonica has only one, transapically elongated areola per striae), valve dimensions or valve outline. Pseudostaurosira oliveraiana Grana et al. (2018: 63) has longer valves (19–39 µm) and typically capitate to rostrate apices throughout its entire cell diminution series (Grana et al. 2018, figs 2–15). Smaller taxa such as P. alvareziae Cejudo-Figueiras, E.Morales & Ector (in Cejudo-Figueiras et al. 2011: 69) and P. oblonga E.Morales, C.E.Wetzel & Ector (2021: 41) have a more elliptic to elliptic-lanceolate valve outline with strictly convex margins, with elongated, more linear-lanceolate valves so far not observed for these species, in contrast to P. ellipticolanceolata that shows a more linear outline with parallel margins in the longer specimens., Published as part of André, Coralie, Sabbe, Koen & Vijver, Bart Van De, 2023, Pseudostaurosira ellipticolanceolata, a new araphid diatom (Bacillariophyta) from Flanders, Belgium, pp. 83-86 in Phytotaxa 591 (1) on pages 83-85, DOI: 10.11646/phytotaxa.591.1.10, http://zenodo.org/record/7784131, {"references":["Kutzing, F. T. (1833) Synopsis diatomearum oder Versuch einer systematischen Zusammenstellung der Diatomeen. Linnaea 8: 529 - 620. https: // doi. org / 10.5962 / bhl. title. 65634","Brebisson, A. de (1838) Considerations sur les diatomees et essai d'une classification des genres et des especes appartenant a cette famille, par A. de Brebisson, auteur de la Flore de Normandie. Falaise & Paris: Bree l'Ainee Imprimeur-Libraire, Meilhac. pp. 1 - 20. https: // doi. org / 10.5962 / bhl. title. 64353","Ehrenberg, C. G. (1854) Mikrogeologie. Einundvierzig Tafeln mit uber viertausend grossentheils colorirten Figuren, Gezeichnet vom Verfasser. Leipzig: Verlag von Leopold Voss. pp. 1 - 31, 40 pls [Taf. I - XXXX].","Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017) Freshwater benthic diatoms of Central Europe: over 800 common species used in ecological assessment. English edition with updated taxonomy and added species. Koeltz Botanical Books, Schmitten- Oberreifenberg, 942 pp. [Cantonati, M., Keely, M. & Lange-Bertalot, H. (eds.)]","Kutzing, F. T. (1844) Die Kieselschaligen Bacillarien oder Diatomeen. Nordhausen. pp. 1 - 152. https: // doi. org / 10.5962 / bhl. title. 64360","Van Heurck, H. (1881) Synopsis des Diatomees de Belgique Atlas. Anvers: Ducaju et Cie. pls XXXI - LXXVII [31 - 77].","Vos, P. C. & de Wolf, H. (1988) Methodological aspects of paleo-ecological diatom research in coastal areas of the Netherlands. Geologie en Mijnbouw 67: 31 - 40.","Morales, E. A., Wetzel, Z. E. & Ector, L. (2021) New and poorly known´araphid \" diatom species (Bacillariophyta) from regions near Lake Titicaca, South America and a discussion on the continued use of morphological characters in´araphid \" diatom taxonomy. PhytoKeys 187: 23 - 70. https: // doi. org / 10.3897 / phytokeys. 187.73338","Morales, E. A. (2005) Observations of the morphology of some known and new fragilarioid diatoms (Bacillariophyceae) from rivers in the USA. Phycological Research 53 (2): 113 - 133. https: // doi. org / 10.1111 / j. 1440 - 1835.2005. tb 00363. x","Witkowski, A., Lange-Bertalot, H. & Witak, M. (1995) Diatom taxa of unusual frustule structure belonging to the genus Fragilaria. Fragmenta Floristica et Geobotanica 40 (2): 729 - 741.","Cejudo-Figueiras, C., Morales, E. A., Wetzel, C. E., Blanco, S., Hoffmann, L. & Ector, L. (2011) Analysis of the type of Fragilaria construens var. subsalina (Bacillariophyceae) and description of two morphologically related taxa from Europe and the United States. Phycologia 50 (1): 67 - 77. https: // doi. org / 10.2216 / 09 - 40.1","Garcia, M. L., Bustos, S., Villacis, L. A., Laprida, C., Mayr, C., Moreno, P. I., Maidana, N. I. & Morales, E. A. (2021) New araphid species of the genus Pseudostaurosira (Bacillariophyceae) from southern Patagonia. European Journal of Phycology 56 (3): 255 - 272. https: // doi. org / 10.1080 / 09670262.2020.1813810","Grana, L., Morales, E. A., Maidana, N. I. & Ector, L. (2018) Two new species of Staurosira and Pseudostaurosira (Bacillariophyta) from the highlands of Argentina (south-central Andes) and two new nomenclatural combinations. Phytotaxa 365 (1): 60 - 72. https: // doi. org / 10.11646 / phytotaxa. 365.1.2"]}