Tonromeinia

Tonromeinia deniseae Bowman & Varol, 2021

In honour of renowned Dutch nannofossil specialist Dr A. J. T. Romein, seasoned nannofossil taxonomist and biostratigrapher.

Circular to elliptical heliolith constructed of two discs and a column that exhibits a diverse set of characteristics shared with both helioliths and placoliths, suggesting that Tonromeinia likely represent intermediate or transitional heliolith/placolith species. The discs and low column are all of equal diameter and similar height/thickness. In plan-view, the discs/a column appear striated, and the large circular to oval central opening lacks a tube cycle and any other features. The dominant settling position of this species is typically restricted to the plan-view, as the limited height of the column prevents sufficient mass distribution along the height axis. The terminology of Tonromeinia is presented in Fig. 13 in Bowman & Varol (2021).

Optical properties: In cross-polarised light, Tonromeinia show dextrogyre extinction lines on the distal side and laevogyre extinction lines on the proximal side. The distal side exhibits an equal configuration of the horizontal axis and the blue sector. Still, the vertical axis correlates with the blue sector appearing on the proximal side when observed with the gypsum plate. The discs/a column are strongly birefringent in plan and side views. They display yellowish-whiteish birefringence colours in the plan view and the reddish-blueish birefringence colours in the side view.

The species of Tonromeinia are identified based on the overall shape, size, and ratios of the central opening and the discs/a column. The species of Tonromeinia typically settle in plan view because they possess a low column that limits their height-to-thickness ratio.

Throughout the literature, the species of this genus are often erroneously identified as or placed into Cyclococcolithus robusta by Müller (1974b) or Ericsonia robusta by Perch-Nielsen (1977a), Wei (1998), Bown (2010), and Garzarella & Raffi (2018). Wise & Wind (1977) interpreted these species as Heliolithus sp. based on their appearance in SEM photos, but they also misassigned the light-microscope photos of these forms to Heliolithus universus. Problematic species concepts of the above authors include Denisea species in Ericsonia robusta [e.g. Ericsonia robusta morphotype B and morphotype A in Figs. 4 and 5, photos 14–18 by Garzarella & Raffi (2018); Pl. 4, figs. 26, 27 by Bown (2005d) and Pl. 2, fig. 15 by Bown (2010)]. None of the forms listed above exhibits features consistent with the holotype of Cyclolithus? robustus Bramlette and Sullivan (1961). Bramlette and Sullivan (1961) utilised a mobile mount method to observe the different profiles of various species.

For the following reasons, Bowman & Varol (2021) established a new genus, Tonromeinia.

• The side-view photo of Cyclolithus? robustus shows two cycles of equal thickness and a fragile third cycle. However, in their study, Bowman & Varol (2021) have demonstrated that the forms subsequently assigned to Ericsonia robusta and the new genus Tonromeinia have cycles in similar height/thickness (e.g. Pl. 92, figs. 5–20 Bowman & Varol, 2021).
• The holotype of Cyclolithus? robustus possesses a distinct tube cycle, but the species incorrectly assigned to Ericsonia robusta and the new genus Tonromeinia lack a tube cycle.
• The holotype of Cyclolithus? robustus contains a photo in phase contrast and a hand-drawn illustration without a polarised light photo. Both images characterise the species as appearing dimmer than all other placolith and discolith species in the original publication. This subtle evidence indicates that Cyclolithus? robustus is weakly birefringent (Varol, 2022), whereas the species assigned to Tonromeinia are strongly birefringent.

Bowman, A. R. & Varol, O. 2021. A Taxonomic Revision of Heliolithaceae - Applications in Resolving the Problematic Calcareous Nannofossil Biostratigraphy of the Paleocene. In: M. Montenary, M. (Ed.). Calcareous nannofossil biostratigraphy of the Stratigraphy and timescales. 6: 43-223.

Bown, P. R. 2005d. Palaeogene calcareous nannofossils from the Kilwa and Lindi areas of coastal Tanzania (Tanzania Drilling Project 2003-4). Journal of Nannoplankton Research 27(1): 21-95.

Bown, P. R. 2010. Calcareous nannofossils from the Paleocene/Eocene Thermal Maximum interval of southern Tanzania (TDP Site 14). Journal of Nannoplankton Research 31(1): 11-38.

Bramlette, M. N. & Sullivan, F. R. 1961, Coccolithophorids and related nannoplankton of the Early Tertiary in California. Micropaleontology 7(2): 129-188.

Garzarella, A. & Raffi, I. 2018. Taxonomy and evolutionary relationships within the calcareous nannofossil genus Ericsonia in the Upper Paleocene. Rivista Italiana di Paleontologia e Stratigrafia. 124(1): 105-126.

Müller, C., 1974b. Calcareous nannoplankton, Leg 25 (Western Indian Ocean). Initial Reports of the Deep Sea Drilling Project. 25, 579-633.

Perch-Nielsen, K.1977a. Albian to Pleistocene calcareous nannofossils from the Western South Atlantic, DSDP Leg 39. Initial Reports of the Deep Sea Drilling Project. 39: 699-823.

Varol, O. 2022. Denisea, a new genus of Paleocene calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 304/2 (2022): 159-186.

Wei, W. 1998. New calcareous nannofossil species and stratigraphic markers from the Upper Paleocene. Journal of Nannoplankton Research. 20(2): 107-115

Wise, S. W. & Wind, F. H. 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP Leg 36 drilling on the Falkland Plateau, south-west Atlantic sector of the Southern Ocean. Initial Reports of the Deep Sea Drilling Project 36: 269-491