Stephanodiscus neoaegypticus var. fekrii Flower, 2013

Flower, R. J., Keatings, K., Hamdan, M. A. & Hassan, F. A., 2013, Stephanodiscus Ehr. species from Holocene sediments in the Faiyum Depression (Middle Egypt), Phytotaxa 127 (1), pp. 66-80 : 73-75

Stephanodiscus neoaegypticus var. fekrii Flower var. nov.

Cell diameters ca. 20–40 µm. Fascicles radial, uniseriate striae occupy ca. 50% of the valve radius. Interfascicles often divide beyond the valve face-mantle junction and slightly widen in the mid-valve region. Central area fultoportulae 2–3. Rimoportulae sub-marginal, 2–3.

Type:— EGYPT. Gharak: Faiyum Depression, 29.14840° N, 30.60976° E, Holocene lake sediment core ( GARK1 ) material, sediment depth 450 cm, collected April–May 2008, (circled specimen ( Figs 8 , 9 ) on slide BM 101660, holotype, designated here) .

Sediment cores QARU 9 and QARU 10: In the thinly laminated sections of both QARU 9 & 10 (see Fig. 2 ), a third common Stephanodiscus species was present that was not S. neoeagyptiacus but much more similar to S. galileensis Håkansson & Ehrlich (1987: 20) . The type locality for this species is sub-surface (fossil) sediment in Lake Kinneret ( Israel). They reported a heterovalvic condition where the concave and convex valves differed with regard to fascicle arrangement and to location of fultoportulae. Since some of their material was evidently rather poorly preserved (see their Figs 3 , 14 and 16 ), the species is re-examined here.

In LM, Stephanodiscus galileensis valves show strong concentric undulations according to the plane of focus either on the marginal or central areas ( Figs 19 & 20 ). Valves range from 20–48 µm in diameter. The marginal area ( Fig. 19 ) also displays a characteristic sub-marginal hyaline silica ring and broad interfascicles. Fascicles are c. 5 in 10 µm, proximally uniseriate with large blocky areolae becoming finer and bi- or triseriate in the distal third portion of the valve. Interfascicles are distinct and about 1.5 µm broad in the midradius area. Concave valves show a similarly strong undulation ( Figs 21 & 22 ) with 5–6 hyaline areas (marking fultoportulae, see below) in the mid-radius position on the valve face ( Fig 21 , arrowed) and these are particularly obvious in well silicified specimens ( Fig. 23 ). Fascicles on these valves (morphotype ‘B’ of Håkansson & Ehrlich 1987) appear to contain finer areolae and are biseriate for about half the valve distal radius. In this region, interfascicles are often about 1 µm broad but in strongly silicified specimens the width is similar to those in valves with hyaline sub-marginal rings.

In SEM, the external view of convex valves of S. galileensis clearly shows the marked central undulation with irregularly spaced spines subtended from the sub-marginal silica ring ( Fig. 24 , note the similarity of this specimen with S. aegyptiacus as depicted in Fig. 63, Håkansson & Locker 1981). The fascicles are uniseriate becoming biseriate or triseriate near the valve face-mantle junction ( VFMJ) with small areolae (12–13 in 10 µm). External areolar openings are generally <1 µm in the mid valve face area and, consequently, the fascicles are narrower than the interfascicles. Internally, these convex valves ( Fig. 25 ) show marginal fultoportulae on each interfascicle at the VFMJ, one or two sub-marginal rimoportulae (arrowed) and a ring of central area fultoportulae ( Fig. 26 ) with either two or three satellite pores. The mantle areolae, cribra and fultoportulae are generally as in Håkansson & Ehrlich (1987) except in this material the number of central area processes is lower. The concave valves confirm heterovalvy ( Fig. 27 ) with raised interfascicles each subtended by a spine, distinct external openings of central area fultoportulae (arrowed) and a greater degree of areolation (with distally extended bi- and triseriate fascicles) compared to convex valves. Internally, the concave valves display a ring of sub-marginal fultoportulae ( Fig. 28 , arrowed) that initiate distally an usually biseriate fascicle ( Fig. 29 ). There are typically three oblique sub-marginal rimoportulae ( Figs 28–29 ). Central area fultoportulae possess two ( Fig. 29 ) or three ( Fig. 30 ) satellite pores and occur in depressions near finely porate domed cribra ( Fig. 30 ).

Size distributions:—To help substantiate the two varieties of S. neoeagypticus , diameters of 70 random valves ( S. neoaegypticus sensu lato) were assigned to size classes and the distribution plotted ( Fig. 31a ). These data show a bimodal size distribution with two modal size classes, 30–40 µm and 50–60 µm; the larger class was always composed of S. neoaegypticus valves and the smaller class was mainly S. aegypticus var. fekrii . For S. galileensis , diameters of 60 random valves were assigned to size classes and the distribution plotted ( Fig. 31b ). There were approximately equal numbers of convex and concave valves resulting in a unimodal size class distribution with 35% of valves in the 30–35 µm size class.

Morphological comparisons:—The morphological characteristics of S. neoaegypticus and S. neoeagypticus var. fekrii are summarized in Table 1 where valve size, extent of uniseriate areolae serve best to discriminate the latter in routine LM examination. Another small form, similar to S. neoagypticus var. fekrii but with distally wider (4/5 areolae) fascicles, was recognised in GARK 1 and tentatively named as cf. asteriodes but this taxon requires further study. Comparable data for S. aegyptiacus from Håkansson & Locker 1981 are included in Table 1 to demonstrate differences between this species and the two newly described S. neoaegypticus taxa. The most discriminating features of the latter are a larger valve diameter range and differences in valve face areolar structure. Strongly concentric valve undulations a distinct marginal hyaline ring and marginal spines are all lacking in S. neoaegypticus sensu lato. The morphological characteristics of S. galileensis obtained from the Qarun core material compare favourably with the published data for this species ( Håkansson & Ehrlich 1987) shown in Table 2. The original description and the existence of heterovalvy is generally substantiated and only very minor differences were found in this study. The latter extends slightly the original description by increasing the valve size range and including more variation in the number of rimoportulae and in the arrangement of satellite pores around fultoportulae.

Figures

FIGURE 2. Gross sediment stratigraphies of the three cores, QARU9, QARU10 and GARK1 (see text for sediment descriptions). Note the basal sands below finely laminated diatom marl sediment in QARU9 & 10.

FIGURE 31. Size distributions for diatom valve diameters (see text). Fig. 31a: Stephanodiscus neoaegypticus sensu lato from material at 430 cm depth in core GARK1. Fig. 31b. Stephanodiscus galileensis Hakansson & Ehrlich from material at 1930 cm depth in core QARU9.

FIGURES 3–8. Stephanodiscus neoaegypticus, LM. Fig. 3. Valve showing an indistinct sub-marginal hyaline ring and broad interfascicular ribs. Fig. 4. Valve focused to show the valve face-mantle zone with occasional bifurcating interfascicular ribs (arrowed). Fig. 5. A strongly silicified valve. Fig. 6. A part dissolved valve with eroded margin and diminished interfascicular ribs. Figs 7–8. S. neoaegypticus v. fekrii LM. Fig. 7. Whole valve focused to show the broad interfascicular ribs and marginal bifurcations (arrowed). Fig. 8. The same whole valve (Fig. 7) focused to show the central area. Scale bars = 5 µm (Figs 3–8)

FIGURES 9–14. Stephanodiscus neoaegypticus, SEM. Fig. 9. Iconotype, external valve showing small concentric convex undulation with out spines. Fig. 10. External valve showing small concentric concave undulation and external rimoportula opening (arrowed). Fig. 11. Internal valve showing sub-marginal rimoportulae (arrowed). Fig. 12. Detail of the same valve showing two sub-marginal rimoportulae and marginal ring fultoportulae. Fig. 13. Detail of the central ring of fultoportulae (each with two satellite pores). Stephanodiscus neoaegypticus v. fekrii SEM. Fig. 14. Iconotype, external valve with a concave central area, marginal ring of fultoportula apertures and external opening of a rimoportula (arrowed). Scale bars = 10 µm (Figs 9–11, 14), 1 µm (Fig. 12), and 2 µm (Fig. 13).

FIGURES 15–18. Stephanodiscus neoaegypticus v. fekrii, SEM. Fig. 15. External valve with a convex central area. Fig. 16. Internal valve showing two rimoportulae (arrowed). Fig. 17. Detail of internal marginal zone showing fultoportulae with either three or four satellite pores. Fig. 18. Internal central area fultoportula with two satellite pores. Scale bars = 2 µm (Fig. 15), 5 µm (Fig. 16), and 1 µm (Figs 17, 18).

FIGURES 19–23. Stephanodiscus galileensis, LM. Fig. 19. Convex valve showing a distinct sub-marginal hyaline ring in focus. Fig. 20. Similar showing marginal ring of spines and valve central area in focus. Fig. 21. Concave valve showing finer fascicle structure (see S. galileensis morphotype B, Hakansson & Locker 1987) and valve face fultoportulae (arrowed). Fig. 22. Similar showing valve central area in focus. Fig. 23. Similar, but valve is more strongly silicified. Scale bars = 5 µm (Figs 19–23).

FIGURES 24–30. Stephanodiscus galileensis, LM. Fig. 25. External convex valve note irregular spines and sub-marginal silica ring. Fig. 25. Internal convex valve with rimoportula arrowed. Fig. 26. Detail of same valve showing central area ring of fultoportulae with two or three satellite pores. Fig. 27. External concave valve with ring of spines and raised interfascicular ribs. Fig. 28. Internal concave valve showing ring of sub-marginal fultoportulae and rimoportulae (arrowed). Fig. 29. Similar showing detail of a rimoportula, a submarginal fultoportula with two satellite pores and marginal fultoportulae. Fig. 30. Detail of a sub-marginal fultoportula with three satellite pores and surrounding domed cribra. Scale bars = 10 µm (Figs 24, 25, 27, 28), 1 µm (Figs 26, 29), and 0.5 µm (Fig. 30).