Tertiarius minutulus, Zaova & Cvetkoska & Mitic-Kopanja & Jovanovska & Ognjanova-Rumenova & Levkov, 2020

Zaova, Dušica, Cvetkoska, Aleksandra, Mitic-Kopanja, Danijela, Jovanovska, Elena, Ognjanova-Rumenova, Nadja & Levkov, Zlatko, 2020, Tertiarius minutulus sp. nov. (Stephanodiscaceae, Bacillariophyta) - a new fossil diatom species from Lake Ohrid, European Journal of Taxonomy 670 (670), pp. 1-14 : 3-7

Tertiarius minutulus sp. nov.

Figs 1–3

Etymology

The specific epithet ʻ minutulus ʼ refers to its small size.

Type material

Holotype

REPUBLIC OF NORTH MACEDONIA • Lake Ohrid , core DEEP-5045-1; 41.049167° N, 20.715000° E; sediment sample from 451.92 m depth of the composite core; Mar.–Jun. 2013; MKNDC 010093 .

Isotype

REPUBLIC OF NORTH MACEDONIA • 1 spec.; same collection data as for holotype; BM 81918 (slide) .

Type locality

Lake Ohrid core 5045-1, site DEEP, at a depth of 451.92 m of the composite sequence.

Description

Light microscope (LM)

Valves circular, 3.5–8.0 µm in diameter, central area 3.0– 6.5 µm in diameter ( Fig. 1 ). Valve face uneven with two parts of distinctly different morphology. Marginal area with short, radiating striae exceeding ¹/8–¼ of valve diameter, with ca 24–30 striae in 10 µm. Central area uneven and colliculate with scattered central areolae that are not radially arranged. Frustules rectangular in connective view.

SEM external view ( Fig. 2 )

Valve face uneven and colliculate ( Fig. 2 A–C), with small granules in central area ( Fig. 2 D–E). Areolae present in central area with simple round openings. Marginal striae short, occluded by cribra, perforated with pores of variable size. Line of bigger pores bordering each costa ( Fig. 2 D–E). Marginal fultoportulae situated on costae, close to margin edge ( Fig. 2 A–B), situated on every 6 th– 7 th costa. Externally marginal fultoportulae with simple, slightly elevated and round opening ( Fig. 2C, E ). One to three valve face fultoportulae present, with external small openings, round ( Fig. 2 A–B). Single solid ligulate girdle band present ( Fig. 2A ).

SEM internal view ( Fig. 3 )

Internal valve face flat with several central areolae occluded with domed cribra (arrow in Fig. 3B ). Marginal areolae loculate, crossed with inwardly raised circumferential silica trabeculae (arrowhead in Fig. 3B ), while central areolae occluded with domed cribra ( Fig. 3 B–C). Costae strongly thickened, ca 30 in 10 µm ( Fig. 3A , E–F). Marginal fultoportulae with short tube located on short and narrow costae ( Fig. 3A ). Each marginal fultoportula with two satellite pores in lateral position (arrow in Fig. 3A ). One to three valve face fultoportulae present. Valve face fultoportulae with short central tube surrounded with 2–3 satellite pores (arrow in Fig. 3E ). One rimoportula present, located at valve face/mantle junction in the middle of costa (arrow in Fig. 3F ) or on its side (arrow in Fig 3D ). Rimoportula consists of short and narrow labium with oblique slit ( Fig. 3 D–F).

Distribution

Freshwater fossil species observed only at its type locality, Lake Ohrid.

Differential diagnosis

The main differential features of T. minutulus sp. nov. that can be observed in LM are its small valve size (diameter = 3.5–8.0 µm) and the presence of central scattered areolae that are not radially arranged. Tertiarius pygmaeus (Pantocsek) Håkansson & Khursevich (1997: 22) is characterized by a valve diameter ranging from 8.0–16.0 µm, and occasionally the areolae are not arranged in radial striae (e.g., Pantocsek 1892: figs 2: 22, 4: 59; Houk et al. 2010: fig. 296: 1, 4). Important differences between T. minutulus sp. nov. and T. pygmaeus can be observed with SEM. The marginal striae in T. pygmaeus do not have a complex alveolar structure and are composed of regular rows of fine pori. Externally, distinct spines are present on every costa. Internally, the alveolus is round and small in T. pygmaeus whereas it is larger and more elongated in T. minutulus sp. nov. In T. pygmaeus , the marginal fultoportulae are present on each costa, the number of valve face fultoportulae is one to seven scattered in the central area, and the rimoportula is situated inside of the alvelolar opening on a side of costa. Additionally, the costae are much thicker and with a lower density (8–10 in 10 µm) in T. pygmaeus than in T. minutulus sp. nov.

Another species with small-sized valves is T. mariovensis Ognjanova-Rumenova, Jovanovska, Cvetkoska & Levkov (diameter 4.0–19.0 µm), recently described from a diatomite near the village Manastir, Mariovo Neogene Basin, Republic of North Macedonia ( Ognjanova-Rumenova et al. 2015: 56, figs 36–105). This species was also observed in several sediment samples from Lake Ohrid DEEP- 5045-1 core. However, both species can easily be differentiated by the structure of the central area. In T. mariovensis , the areolae in the central area form radially arranged striae. The number of valve face fultoportulae in T. mariovensis is higher (2–9) and the tube of the central fultoportula is always surrounded by three satellite pores. Additionally, the marginal fultoportulae are positioned on every costa, while the rimoportula is located inside and in the middle of the alveolus, but not connected to a costa as in T. minutulus sp. nov. Another small-sized species, Tertiarius distinctus Khursevich & Kociolek (2002: 333 , figs 1–5, 12–22), has a comparable valve size as T. minutulus sp. nov. (diameter 4.5–14.0 µm), but coarser areolae arranged in short or long radial striae composed of large areolae, while the marginal striae are composed of 4–6 rows of small pori. Marginal fultoportulae are present on every costa, while in the central area there are 1–5 valve face fultoportulae. The rimoportula is located near the base of a costa within the alveolus. Differences between T. distinctus and T. minutulus sp. nov. can be noticed in the structure of the marginal and central striae, the number of valve face and marginal fultoportulae, and the position of the rimoportula. Smaller specimens of T. indigenus Khursevich & Kociolek (2002: 336 , figs 6–11, 23–32) have a valve size comparable to T. minutulus sp. nov. (diameter 5.0– 27.5 µm). However, differences between these two species can be noticed in the striae structure (longer marginal striae composed of 4–5 regular rows or pori), central area (colliculate central area with 3–5 large areolae and several ʻbumpsʼ), presence of fultoportulae on each costa and one to several rimportulae located on thinner costae within the alveolar chamber.

Tertiarius baicalensis Khursevich & Fedenya (in Khursevich et al. 2003: 306, figs 1: 1, 2, 4, 5, 14) is characterized by circular valves with flat face and diameter of 6.4–16.6 µm, three to 17 valve face fultoportulae arranged in radial rows and one to four sessile rimoportulae located in the submarginal zone of the valve face. Differences between T. minutulus sp. nov. and T. baicalensis can be noticed in the ornamentation of the central area (areolae organized in radial striae in T. baicalensis ), number and position of valve face fultoportulae, number of satellite pores on marginal fultoportulae (three satellite pores in T. baicalensis ), and position of rimoportula(e) that are located in the submarginal zone of the valve face (not connected with costa or within the alveolar chamber).

Another small-celled species, Cribrionella ohridana , was recently described from the same core from Lake Ohrid ( Jovanovska et al. 2016b). Both species share several characters such as small size (2.0– 7.5 µm in diameter), the presence of areolae in the central area and a single rimoportula situated on a costa, marginal fultoportulae located on each 4 th– 5 th costa surrounded by two satellite pores and the presence of inwardly raised circumferential silica trabeculae. However, differences between these two species can be noticed in the absence of valve face fultoportula in C. ohridana and areolae internally not occluded with domed cribra as in T. minutulus sp. nov.

One of the dominant and very frequent species in fossil diatom assemblages of Lake Ohrid is Cyclotella minuscula (Jurilj) Cvetkoska. Cyclotella minuscula was recently transferred to Lindavia (Schütt) De Toni & Forti as L. minuscula (Jurilj) Nakov, Guillory, Julius, Theriot & Alverson ( Nakov et al. 2015) and later to the genus Pantocsekiella Kiss & Ács as P. minuscula (Jurilj) Kiss & Ács ( Ács et al. 2016). Both species, Tertiarius minutulus sp. nov. and Cyclotella minuscula , have a similar valve size (diameter 3–7 µm in C. minuscula ), a colliculate central area, a small marginal zone, a single valve face central fultoportula, a marginal fultoportula with two satellite pores located on each 4 th– 10 th costa and a single rimoportula positioned at a rib, beneath the ring of marginal fultoportulae. Difference between these two species can be observed only in a presence/absence of areolae in the central area: in C. minuscula areolae are absent, while in T. minutulus sp. nov. they are present.

Figures

Fig. 1. Tertiarius minutulus sp. nov., LM valve views, Lake Ohrid, North Macedonia. The asterisk indicates the holotype. Scale bar = 10 µm.

Fig. 2. Tertiarius minutulus sp. nov., SEM external valve views. A. Valve view showing the marginal openings of the fultoportulae (see white arrow). B–D. View of the whole frustule. E. Close view of marginal openings of the fultoportula (see white arrow). F. Close view of the external openings of the valve face fultoportulae (see white arrows). Scale bars: A–D = 2.0 µm; E–F = 1.0 µm.

Fig. 3. Tertiarius minutulus sp. nov., SEM internal valve views. A. View of the whole frustule and the marginal fultoportulae surrounded by two satellite pores, cowlings absent (see white arrow). B. Close view of marginal areolae with circumferential silica trabeculae and central areolae occluded by domed cribra (see white arrow). C. View of the whole frustule. D. Close view of rimoportula (see white arrow) located on costa at junction between valve face/ mantle. E. Internal valve view showing the central fultoportulae with short central tube surrounded with 2–3 satellite pores (see white arrows). F. Valve view. Scale bars: A, C, E–F = 2.0 µm; B, D = 1.0 µm.