Diploneis praeclara (Pantocsek) Cleve-Euler, 1934

Jovanovska, Elena, Levkov, Zlatko & Edlund, Mark B., 2015, The genus Diploneis Ehrenberg ex Cleve (Bacillariophyta) from Lake Hövsgöl, Mongolia, Phytotaxa 217 (3), pp. 201-248 : 214-217

Diploneis praeclara (Pantocsek) Cleve-Euler ( Figs 41–53 )

Valves are elliptical-lanceolate to elliptic with convex margins and bluntly rounded ends ( Figs 41–49 ). The valve length is 39.5–69.5 μm, and the valve breadth is 18.0–26.0 μm. The axial area is narrow, linear to lanceolate, expanding toward the central area. The central area is transapically expanded, 4.0–6.0 μm wide. The longitudinal canal is lanceolate, expanded in the middle with two rows of areolae, that coalescence into one at the valve apices ( Figs 41–49 ). Externally, the areolae of the longitudinal canal open in depressions slightly lower then the rest of the non-porous valve surface ( Figs 49, 51 ). Externally, the areolae of the canal are each covered with a cribrum similar to those of the striae ( Figs 49, 50, 53 ). Externally, the raphe is straight with expanded proximal ends, which are positioned within a small depression ( Figs 49, 53 ). Distally, the raphe branches terminate with short terminal fissures that are bent to the same side of the valve ( Figs 49, 51, 52 ). The stria are radiate, 8–9 in 10 μm, composed of round to rectangular areolae, 8–9 in 10 μm. Externally the alveolate striae are covered with cribrate occlusions that are placed in depressions slightly lower then the rest of the non-porous silica ( Fig. 50 ).

Observations:— The measurements of Lake Hövsgöl’s Diploneis praeclara (Pantocsek) Cleve-Euler (1934 : pl. 5, fig. 151), fully fit the original description given by Pantocsek (1892, 1905). Diploneis praeclara is described as a fossil from Köpecz, the Neogene fossil deposits in Romania ( Pantocsek 1892, 1905). In the modern flora it was reported from Lake Ohrid, reassigning its status from fossil to relic ( Jurilj 1954). Recently, Jovanovska et al. (2013a) documented a population with a smaller size range in both modern and fossil sediments in Lake Ohrid (length: 31.0–39.7 μm; breadth: 16.3–18.3 μm). The illustrations given by Jovanovska et al. (2013a) show differences in the valve outline between Lake Ohrid and Köpecz populations (elliptical with acute apices vs. elliptical with bluntly rounded apices). The areola density is slightly different between lakes Hövsgöl and Ohrid populations (8–9 in 10 μm vs. 10–12 in 10 μm). In the original description Pantocsek (1905) gives a stria density of 10 in 10 μm, which fully fits the Lake Ohrid population. On the other hand, Jurilj (1954) in his description gives a stria density of 7–8 in 10 μm, similar to Lake Hövsgöl population. Due to this, using the stria density as a distinguishing feature is unclear.

Followed by the recent typification of D. praeclara ( Jovanovska et al. 2013b) it seems like the population from Lake Hövsgöl more closely resembles the type population rather then the one from Lake Ohrid. However, the presence of the biseriate pattern in the middle of the longitudinal canal makes the separation of Lake Ohrid’s population rather difficult, and we currently consider this morphological variation to be within the concept of D. praeclara .

This study expands the distribution of D. praeclara , previously reported only from the Neogene fossil deposits in Romania ( Pantocsek 1892, 1905), Scandinavian fossil deposits ( Cleve-Euler 1934, 1953) and Lake Ohrid ( Jurilj 1954, Jovanovska et al. 2013).

Ecology and Distribution: —M274A; M247A; M248A; M284A; M288A; M329A; M330A: found in shallow to deep habitats in northern and central Lake Hövsgöl on sediments and marl deposits .

Figures

FIGURES 41–48. Diploneis praeclara, LM valve views. Scale bar = 10 μm.

FIGURES 49–53. Diploneis praeclara, SEM external valve views. Fig. 49. View of the entire valve. Fig. 50. Close view of the cribrate occlusions. Fig. 51. Distal raphe endings. Fig. 52. One row of areolae of the longitudinal canal at the valve apices. Fig. 53. Central area with proximal raphe endings. Scale bars = 10 μm (Fig. 49); 5 μm (Figs 51–53); 2 μm (Fig. 50).