Denisea magna Varol, 2022
In honour of Denise Varol, Senior Geologist, Total Energies, Qatar.
Circular to elliptical placoliths which contain shields of high inclination angle (> 55° with horizontal axis), birefringent distal and proximal shields and a prominent birefringent tube cycle.
Denisea is a circular to elliptical placolith formed by a tube cycle surrounding a featureless central area and double shields (proximal and distal). The tube cycle is triangular in the side view, curved distally, and its apex is directed to the proximal side.
The high inclination angle (> 55° with the horizontal axis) shields are approximately equal in length. The inclination angle between the tube cycle and the inner end of the shields is between 20° and 35°. To distinguish the tube cycle from shields, it is necessary to focus carefully , as their birefringences are slightly different; the smaller the species size, the greater the difference in birefringence. Denisea species are determined by their general shape, size and ratio of the central area to pelaga – defined by Varol & Bowman (2019) – referring to the tube cycle and shield, excluding the central area and its structures (Fig. 4 in Varol, 2022). Three new species, Denisea atalikii, Denisea salleyi and Denisea turkmenii are distinguished by their general shape, from sub-circular to elliptical, by Varol (2022).
Small specimens (< 5.0 μm), regardless of the size of the central area, are assigned to Denisea orbis.
Medium-sized specimens (5.0–9.0 μm) are separated into three species: Denisea aliquanta, Denisea foramena and Denisea subpertusa. These three species are differentiated by the ratio of the central area to the pelaga.
The large species (> 9.0 μm) are also assigned to three new species: Denisea constricta, Denisea magna , and Denise polymegalo.
After years of tracking their size and stratigraphic distribution around the world, size-dependent species have been established. The separation of large-sized species leads to the establishment of significant events (Fig. 3 in Varol, 2022).
Optical properties: In cross-polarised light, in the plan view, the more birefringent tube cycle shows whiteish-yellowish interference colours, while shields produce whitish interference colours. The extinction lines are often difficult to define. Denisea shows dextrogyre extinction lines on the distal side and laevogyre extinction lines on the proximal side. When using the
gypsum plate, the distal side exhibits alignment of the horizontal axis and blue interference colours. However, the vertical axis is associated with the blue interference colours appearing on the proximal side. The entire species is birefringent in the side view.
Large (> 9.0 μm) Denisea species (i.e. Denisea magna), along with Tonromenia species, were erroneously assigned to Ericsonia robusta by Garzarella & Raffi (2018; Ericsonia robusta morphotype B and morphotype Aa in fig. 4 and fig. 5.14–5.18); Bown (2005; pl. 4, figs. 26–29) and Bown (2010; Ericsonia robusta large in pl. 2, figs. 15, 16).
Medium-sized (5.0–9.0 μm) species of Denisea (i.e. Denisea subpertusa) were erroneously assigned to Coccolithus (van Heck & Prins 1987: p.189; Varol 1989c, pl. 12.4, fig. 25; Varol 1998, pl. 7.3, fig. 6) or its synonym Ericsonia (Hay & Mohler 1967, pl. 198, figs. 11, 15, 18; pl. 199, figs. 1–3; Roth 1973, pl. 12, fig. 5; Haq & Lohmann 1976, pl. 1, figs. 7, 8; Perch-Nielsen 1977, pl. 16, fig. 3; Bown 2005, pl. 4, figs. 22–25).
Denisea differs from Coccolithus (syn. Ericsonia) by having birefringent shields due to their high inclination angle (> 55° with the horizontal axis). The latter genus has a distal shield that is not birefringent, making an inclination angle of about 30–40° with the horizontal axis. From the side view, the tube cycle is a well-developed triangular shape in Denisea, but it is lath-like in Coccolithus. Denisea varies from Mauriceblackia for similar reasons by having birefringent shields due to their high inclination angle (> 55° with the horizontal axis). The latter genus has a non-birefringent distal shield that makes an angle of about 30–40° with the horizontal axis. In Mauriceblackia, the proximal shield is nearly flat (< 25° with the horizontal axis), and the tube cycle (with a serrated periphery) protrudes well above the distal shield.
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Roth, P. H. 1973. Calcareous nannofossils: Leg 17 of the Deep Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project. 17: 695-795.
van Heck, S. E., Prins, B., 1987. A refined nannoplankton zonation for the Danian of the Central North Sea. Abhandlungen der Geologischen Bundesanstalt 39, 285-303.
Varol, O. 1989c. Palaeocene calcareous nannofossil biostratigraphy. In: Crux, J. A. & van Heck, S. E. (Eds), Nannofossils and their applications. Proceedings of the 2nd INA Conference, London 1987. British Micropalaeontological Society Publication Series 265-310.
Varol, O. 1998. Palaeogene. In: Bown, P. R. (Ed.), Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series pp. 200-224
Varol, O. 2022. Denisea, a new genus of Paleocene calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 304/2 (2022): 159-186.