Thalassiosira dolmatovae Oreshkina ex A. Gladenkov, 2013

Gladenkov, Andrey, 2013, Morphological features and new description of the fossil species Thalassiosira dolmatovae (Bacillariophyceae), Phytotaxa 127 (1), pp. 100-112 : 104

Thalassiosira dolmatovae Oreshkina ex A. Gladenkov sp. nov. ( Figs. 4–29 )

Synonyms:— Thalassiosira sp. 9 Dolmatova 1992b (p. 86, pl. 30, figs. 1–5); Thalassiosira dolmatovae Oreshkina 1992 (p. 129, pl. XL, figs. 7, 8), invalid species name.

Valves circular, slightly convex to almost flat, diameter 12–37 µm. On valve face size of the areolae gradually decreases from the centre (3–5 areolae in 10 µm) towards the margin, in irregular, concentric wavy arrangement. Abrapt change in areolae pattern at marginal zone, up to 19–25 areolae in 10 µm near the margin, arranged in sublinear to tangental rows. Margin is hyaline, 0.7–1.5 µm wide. Simple external (occluded) processes are scattered on the valve face between areolae. One single strutted process occurs near the valve center between two areolae, one of which has a smaller size than other central areolae. 4–6 strutted processes in 10 µm are located along the margin zone near the boundary between valve face and low mantle. Internal openings of central and marginal strutted processes are surrounded by 4 satellite pores. External opening of central process as a simple rounded opening, and marginal processes have external extensions. One single labiate process is located at the margin and enclosed by two strutted processes. External opening of labiate process is simple.

Type:— USA. Alaska: Sandy Ridge stratigraphic section of the Alaska Peninsula, Milky River Formation, 56° 70' N, 159° 92' W, material from 28 m above an angular unconformity with the underlying Bear Lake Formation , ( Circled specimen ( Fig. 4a–c ) on slide Slide 608382/1, Sample # CAS 608382 , ( Geological Institute , Russian Academy of Sciences , Moscow, Russia), holotype, designated here; circled specimen ( Fig. 5 ) on Slide #608382/2, Sample # CAS 608382 , ( Geological Institute , Russian Academy of Sciences , Moscow, Russia), isotype, designated here; material from 139.9 m above an angular unconformity with the underlying Bear Lake Formation , circled specimen ( Fig. 8a– b ) on Slide #608400/1, Sample # CAS 608400 (Geological Institute, Russian Academy of Sciences, Moscow, Russia), paratype, designated here ).

Occurrence: —Marine neritic species. Marine deposits of the latest Miocene to early Pliocene (the North Pacific Neodenticula kamtschatica diatom Zone) of the Alaska Peninsula, southwestern Alaska; the latest Miocene to late Pliocene (the Neodenticula kamtschatica Zone to Neodenticula koizumii Zone ) of Karaginsky Island, northeastern Kamchatka.

Remarks: —This species is distinguished from Thalassiosira baldaufii Bodén (1993: 68) by the presence of one single central strutted process on the valve face, by the absence of prominent marginal occluded processes, and by areolae pattern. It is similar to Thalassiosira labimarginata Bodén (1993: 68) but differs by larger valve diameter, wider hyaline margin, and size of areolae and their arrangement.

Etymology: —The species is named in honour of Dr. Lora M. Dolmatova who studied fossil diatoms from Russian Far East during many years.

Figures

FIGURES 4–10: Thalassiosira dolmatovae, LM (Figs. 4–8), SEM (Figs. 9–10), external views of whole valves. Arrows show central fultoportula. Black line segment is scale bar = 10 µm for Figs. 4–8. a–c—valve in different focus. Fig. 4. Holotype. Fig. 5. Isotype. Figs. 4–6, 9–10, sample #CAS 608382. Fig. 7, #CAS 60269-s. Fig. 8. Paratype, Sample #CAS 608400.

FIGURES 18–23: Thalassiosira dolmatovae. SEM Fig. 18. Internal view of the valve with arrow showing occluded (?) areolae (OA). Fig. 20. Internal view of the valve showing labiate process (LP), copula (C), central fultoportula (CF) and marginal strutted processes (short white arrows) surrounded by 4 satellite pores. Fig. 19, 21, 23. External views of valves center showing opening of central strutted process (long arrows) and occluded (?) areolae (short arrows). Fig. 22. Enlarged fragment of Fig. 18 showing central fultoportula (CF), occluded (?) areolae (OA), and marginal strutted processes (short arrows). Figs. 18–22, #CAS 608382. Fig. 23, #CAS 60269.

FIGURES 24–29: Thalassiosira dolmatovae, SEM. Figs. 24, 25 (enlarged fragment of Fig. 18), 27. Internal views of the valves showing central fultoportula (CF) and marginal strutted processes (MF) surrounded by 4 satellite pores, labiate processes (LP), and copula (C). Fig. 26. Internal view of the valve showing broken labiate processes (LP), central fultoportula (CF), and marginal strutted processes (black arrows). Fig. 28 (enlarged fragment of Fig. 17). External view of the valve with white arrows showing openings of simple occluded processes scattered on the valve face, and black arrow showing rounded opening of marginal strutted process. Fig. 29 (enlarged fragment of Fig. 15). External view of the valve with arrows showing outward extensions of marginal strutted processes with small rounded openings on their tops. Fig. 24, #CAS 60269-s. Figs. 25–26, 28–29, #CAS 608382. Fig. 27, #CAS 608400.