Quaternariella obscura Thomsen 1980d

Thomsen, Helge A. & Østergaard, Jette B., 2015, Coccolithophorids in Polar Waters: Quaternariella and Porsildia gen. nov. Mercedesia gen. nov., Ericiolus,, Acta Protozoologica 54 (3), pp. 155-169 : 161-164

Quaternariella obscura Thomsen 1980d ( Figs 21–33 )

This taxon was described based on material from West Greenland (Arctic Station). The main diagnostic feature is packages of rhombohedral crystallites (typically in groups of four) evenly distributed over the entire cell surface. The coccolith base plate scale is circular and rimmed and has a diameter of approx. 0.6 µm ( Thomsen 1980d). Evidence is also available documenting the presence of small non mineralized under layer scales ( Thomsen 1980d; loc. cit. Fig. 6). Most probably there is a cluster of four crystallites in the centre of each scale. Here we illustrate additional material from the type locality ( Figs 21–22 ) and also document the occurrence of this taxon in NE Greenland (NEW / Figs 23–27 ) as well as from Antarctica (EPOS / Figs 28–29 ). Irrespective of the origin of the material the general appearance is very similar. Numerical details have been accounted for in Table 3.

We provide here for the first time evidence for the existence of different life cycle phases in Q. obscura ( Figs 30–33 ). We found three combination cells in the Arctic that display typical Q. obscura holococcoliths in one half of the cell whereas the other half carries heterococcoliths that bear resemblance to e.g. Papposphaera spp. ( Figs 30, 32 ). The contours of the individual heterococcolith can be traced and we can additionally account for at least some details of the rim calcification. In the combination cells ( Figs 30, 32 ) the heterococcolith rims have fallen apart into the individual elements. This probably is due to the very delicate structure of the rim. The heterococcolith central area is oval and measures 0.47–0.61 × 0.82–0.96 µm which is within the size range typical for species of Papposphaera . Unfortunately we were not able to resolve the exact structure of the central area calcification, although Fig. 33 shows that it seems to consist of a heap of irregularly placed elements. The rim consists of a proximal/inner cycle of rod shaped elements aligned end-to-end ( Figs 30, 32–33 ). The distal/outer cycle is built of differently sized pentagonal and square elements (height: 0.27– 0.38 µm). In an undisturbed coccosphere these element most likely will closely adjoin to form a somewhat widening tube encircling the central area. The overall appearance and the dimensions of the rim thus coincide with those described from species of Papposphaera (see e.g. Thomsen et al. 1981, loc. cit. Fig. 5).

The cell illustrated in Figs 34–35 is interpreted as a complete specimen of the heterococcolithophorid stage within the Quaternariella obscura life cycle. The absence of muroliths with a central process must be noted although more material obviously needs to be examined to verify whether this is a persistent feature of this species. An enlargement of selected coccoliths in Fig. 35 shows, in support of the morphological interpretation of the coccolith shown in Fig. 33 , that the central area calcification comprises a flat layer of irregular aggregations of elements of different size and shape. It is also evident that this cell shares the irregular appearance of the outer edge of the coccolith rim with the cells illustrated in Figs 30, 32 .

Although the rim of the heterococcoliths in the combination cells resembles that of Papposphaera coccoliths, the absence of a central process and the aberrant structure of the central area makes it is difficult to assign these heterococcoliths to Papposphaera or any other known heterococcolithophorid genus. However, from a taxonomic point of view it is in the future sufficient to refer to the heterococcolith phase as Q. obscura HET.

Figures

Figs 21–27. Quaternarielle obscura. TEM whole mounts from the Arctic, viz. West Greenland/Arctic Station (Figs 21, 22) and NEW (Figs 23–27). 21 – groups of crystallites; 22 – complete cell with appendages; 23 – complete cell with curled up haptonema and nicely organized packages of crystallites; 24 – detail from Fig. 23 showing the boxed organization of crystallites; dissolution phenomena are obvious; 25, 26 – complete cells showing an even spread of crystallite packages; 27 – high magnification of single rhombohedral crystallites.

Figs 28–35. Quaternariella obscura. TEM whole mounts from Antarctica (EPOS) (Figs 28–29) and (Figs 30–35) West Greenland (Arctic Station). 28 – complete cell from the southern hemisphere; 29 – detail of the periplast from Fig. 28; 30 – Q. obscura combination cell (Arctic Station 1990) showing Papposphaera-like coccoliths along one side of the cell; arrows point to square rim elements and arrowheads to pentagonal elements; 31– a second combination cell (Arctic Station 1990) where heterococcoliths are visible as dark, oval structures; 32 – combination cell collected near Arctic Station during 1988; arrow points to square rim elements and arrowhead to pentagonal elements; 33 – Detail from Fig. 32; 34 – complete coccosphere with heterococcoliths that are similar to those shown in Figs 30, 32; 35 – detail from Fig. 34. Arrows point to square rim elements and arrowheads to pentagonal elements; notice also the irregular elements in the central area of the coccolith.