Staurosirella rhombus (Ehrenb.) Ognjanova-Rumenova, Buczkó, Wojtal & R. Jahn 2015

Ognjanova-Rumenova, Nadja, Buczkó, Krisztina, Wojtal, Agata Z. & Jahn, Regine, 2015, Staurosirella rhombus (Ehrenberg), Ognjanova-Rumenova, Buczkó, Wojtal & R. Jahn, comb. nov. - Typification, morphology and biostratigraphic significance, Phytotaxa 218 (3), pp. 279-288 : 282-283

Staurosirella rhombus (Ehrenb.) Ognjanova-Rumenova, Buczkó, Wojtal & R. Jahn , comb. nov.

Basionym: ‘ Fragilaria? rhombus ’ Ehrenberg, Mikrogeologie, Atlas : 13, Pl. VIII: I: Fig. 16 , 1854.

Valves are heavily silicified, rhomboid with broadly rounded apices, more or less protruding, 20–37 μm long and 12–15 μm wide, with stria density 4–5 in 10 μm. Externally, the valve is flat or weakly undulate, due to the raised costae. Striae are composed of pronounced lineolate areolae. The striae are of similar width as costae, alternate, parallel to slightly radial towards apices. The areolae, slit-like at the volae, are similar to those in Staurosirella leptostauron (Ehrenb.) Williams & Round (1987: 276) . They are located continuously from near the sternum to midway into the valve mantle ( Figs. 21 , 23, 24 ). The sternum is lanceolate to linear. The diffusely branched spines are located between the striae, on the costae at the junction between valve face and valve mantle ( Figs. 22 , 24, 25 ). The spines have rudimental character. Striae extend onto the valve mantle. In girdle view, frustules are rectangular, joined by linking spines. The frustules form ribbon-like colonies. In the internal valve view the striae are narrower than the costae. Each stria occurring on a transapical depression between raised costae. The apical pore fields are composed of several rows of round poroids and are observed on both valve apices ( Figs. 18, 19 ). In internal view, the apical pore fields lie within a circular depression, no rimoportulae are present. There are several girdle bands in each individual and the valvocopulae are larger than the rest of the elements ( Figs. 21 , 23, 24 ). The girdle bands are open, vary in number 2 and 5, and lack perforations.

Type:— SLOVAK REPUBLIC. Jastrabá: Dichtes Tripel, diatomaceous earth. (lectotype designated here: BHUMP!, Ehrenberg Collection, Preparation ‘440109 d.red’, here illustrated in Fig. 4 . Isolectotypes designated here: slide ‘2847 #2/ Ehrenberg’ in Coll. Nadja Ognjanova-Rumenova , Institute of Geology , Bulgarian Academy of Sciences, here illustrated in Fig. 13 ; and slide DW 15 in Institute of Nature Conservation, Polish Academy of Sciences) .

Age range: —Middle Miocene.

Differential diagnosis: —The type material of Fragilaria rhombus corresponds to Ehrenberg’s published drawings ( Ehrenberg 1854). The material possesses all morphological features ascribed to the genus Staurosirella ( Williams & Round 1987, Morales 2003, Morales & Manoylov 2006). The strongest support for this placement comes from the detailed analysis of the areolae, the structure of the apical pore fields, the lack of rimoportulae, and the girdle band characteristics. The main differences between Staurosirella rhombus and S. leptostauron are the shape of the valve: cruciform ( ‘ leptostauron ’) vs. rhomboid ( ‘ rhombus ’), the stria density 5–11 in 10 μm in S. leptostauron vs. 4–5 in 10 μm in S. rhombus , and the presence of ribbon like colonies, but there are also stellate/zig-zag colonies (‘ leptostauron’); only ribbon like colonies (‘ rhombus’) ( Morales & Manoylov 2006).

Figures

FIGURES 16–22. Staurosirella rhombus, SEM. 16, 17. External view of valves showing features of valve outline, sternum, striae and spines. 18, 19. Apical pore fields. 20. Striae structures. 21. Ribbon like colony. Note the number and the width of the girdle bands. 22. The diffusely branched spines. Scale bars = 1μm (Figs. 18, 19, 20, 22), 5 μm (Figs. 16, 17), 10 μm (Fig. 21).

FIGURES 23–25. Staurosirella rhombus, SEM. Ribbon like colonies of valves in a girdle view. 23. Two frustules joined by the spines. Note the several girdle bands in each individual and the larger valvocopulae than the rest of the elements. Scale bars = 5 μm (Fig. 25), 10 μm (Figs. 23, 24).

FIGURES 4–15. Fragilaria rhombus. 4–7. Drawings and light microscopy documentation of F. rhombus in the original Ehrenberg’s slides. 8–15. Staurosirella rhombus (Ehrenb.) Ognjanova-Rumenova, Buczkó, Wojtal & R. Jahn from materials #2847, Jastrabá Kieselgur (present study), LM. Figs. 8, 9 and 11–13 represent the same valvae at different focus, showing details of their ornamentation.