Cyclocostis rolfii Paillès

Paillès, Christine, Sylvestre, Florence, Tonetto, Alain, Mazur, Jean-Charles & Conrod, Sandrine, 2020, New fossil genus and new extant species of diatoms (Stephanodiscaceae, Bacillariophyceae) from Pleistocene sediments in the Neotropics (Guatemala, Central America): adaptation to a changing environment?, European Journal of Taxonomy 726, pp. 1-23 : 4-8

Cyclocostis rolfii Paillès gen et sp. nov.

Figs 1 –40

Etymology

This species is named in honor of Rolf Klee for his dedicated career on Stephanodiscaceae .

Type material

Holotype

Slide PC0608731 and sediment PC0608728 deposited at the Laboratoire de Cryptogamie, Muséum national d’histoire naturelle ( MNHN) Paris, France. Specimen on slide PC0608731 ( Fig. 4 ) represents the holotype designated here.

Isotype

Slide ZU 11/30 and sediment R1284 deposited at the Friedrich Hustedt Diatom Center in Bremerhaven, Germany.

Type locality

GUATEMALA • Department of Petén, Lake Petén-Itzá ; 16º15′50″ N, 89º15′00″ W; lacustrine sediment in core PI-06; sample GLAD9 - PET06-6B-10H1 - 98– 99 cm (27.83 m below lake floor) consisting of light gray carbonated sediment; core collected in February 2006 .

Description

Light microscopy ( Figs 1–24 )

In girdle view, cells quadrangular, displaying strongly undulated valve faces ( Fig. 1 ). In valve view, frustules circular, 7–22 µm in diameter ( Figs 2–24 ). Marginal area (outer ¼ of valve) concentrically undulated and consisting of an external ring of small bright ʻchambersʼ transforming into strong radiating costae – 10 to 12 striae in 10 µm. The central ¾ of valve surface strongly tangentially undulated, forming an S shape in large specimens ( Figs 7–8 ). The central area with radial anastomosing striae of unequal length, some extending deep in the central zone to a central bright punctum. Marginal and central areas not distinctively structured. In small specimens, strong transversal undulation is attenuated and radiating striae are converging to a central ring.

Scanning electron microscopy ( Figs 25 –40)

Valves strongly undulated externally ( Figs 25–26 ). Cingulum present, consisting of an open valvocopula and several copulae ( Figs 27–28 ). Valve surface is irregular with sprinkled granules and prominent embossed ribs. The marginal striated area is circumferentially undulated with numerous granules; the mantle is gently sloping ( Fig. 27 ). Striae consisting of 3–4 rows of finely aligned areolae alternating with reduced hyaline interstriae bearing near the valve margin big rounded openings corresponding to the external openings of marginal fultoportulae ( Figs 27–28 ). The central area displays a steep tangential undulation ( Fig. 29 ). If the junction between the valve face and the mantle is steeply marked vertically both on the elevated and depressed sides ( Fig. 30 ), horizontally, from the elevated to depressed parts, the incline is smooth and gradual. On the internal side of the marginal area, striae become single rows of large rounded to oblong areolae that progressively become smaller and arranged to some extent into a stellate pattern ( Fig. 31 ). Where ribs are fusing, external areolae are bigger and occluded by volae. Broken specimen displays a simple valve structure: a basal siliceous layer composed of anastomosing ribs starting from a central hollow and continuing to the valve rim where intercostal spaces are covered by a finely perforated silica layer ( Fig. 32 ).

Internally, there is no central lamina inside the valve (Fig. 33). Costae are strongly silicified and elevated, extending from the valve rim to the valve center and fusing into a thick silicified hollow (Figs 34–35). The alveolar structure could be classified as complex as the alveolus bears in its middle a recessed / sunken costa that carries the marginal fultoportula (mfp) (Fig. 36). As such, mfp are located on every second striae and are composed of one tube and three satellite pores (Figs 37–38). The ring of mfp and rm stands just beneath marginal lamina. One single rimoportula (rm) positioned on a recessed costa consisting of a short tube with a tangential slit that is always diametrally opposed to the raised side (Fig. 39). One single valve face fultoportula (vffp) – composed of one tube and three satellite pores – is always diametrically opposed to the rimoportula (Fig. 39). It appears eccentric since positioned on the raised part (Fig. 40). The external opening of the vffp is difficult to observe as it is positioned on the external slope of the raised central part (see in Fig. 26 ).

Time range

23–28 ka, abruptly absent after, considered extinct.

Remarks

Cyclocostis rolfii gen et sp. nov. belongs unequivocally to the family Stephanodiscaceae ( Glezer & Makarova 1986) . In LM, it resembles Discostella woltereckii (Hust.) Houk & Klee in Klee & Houk (1996) . However, SEM observations reveal a completely different structure in C. rolfii gen et sp. nov., particularly in the position of mfp and rm on costae, ruling out its belonging to the genus Discostella . With alternating and unequal striation pattern on the valve face, C. rolfii gen et sp. nov. also resembles Cyclotella stoermeri Khursevich & Kociolek in Kociolek & Khursevich (2013) in LM. Even though they share the absence of a central area, unequal striation, reduced alveoli and position of mfp and rm on recessed costae, C. rolfii gen et sp. nov. differs by the absence of centripetal roofing (central lamina) and thus the alveoli have no distinct border at the valve center side. The unequal striation visible in LM in C. rolfii gen et sp. nov. only corresponds to the way ribs are arranged internally: elevated, strongly silicified, anastomosing and joining in a central hollow. These thick radial ribs are the external characteristic features of the genus Stephanocostis Genkal & Kuzmina represented by S. chantaica Genkal & Kuzmina. However , these structures are internal in C. rolfii gen et sp. nov., whereas they are external in S. chantaica whose internal structure is flat with fine areolae forming ± radial rows of starlike criba.

The strongly tangentially undulated valve is a character shared by Cyclotella and Pliocaenicus Round & Håk. , the latter genera being included into Lindavia by Nakov et al. (2015). The position and structure of mfp are also a common character. But unlike in some Cyclotella or Pliocaenicus , no domed criba are observed internally in Cyclocostis gen. nov. As the position of the rm within the ring of mfp in C. rolfii gen et sp. nov. differs from Pliocaenicus , it rules out its belonging to this genus.

The alveoli of C. rolfii gen et sp. nov. could be classified as complex, because there is a median fultoportula born on a recessed costa. However, structurally they are simple since not partially occluded by central lamina and widely open towards the valve center. The structure of alveoli of C. rolfii gen et sp. nov. rather corresponds to some extent to the marginal chambers defined by Lange & Syvertsen (1989) as “a marginal space characterized by an opening on the inside of the valve encompassing two or more alveolus openings that is limited by coarse interstriae”. These marginal chambers are present in Cyclostephanos novaezeelandiae (Cleve) Round in Theriot et al. (1987) or C. dubius (Hust.) Round in Theriot et al. (1987) . However, the genus Cyclostephanos Round is largely heterogenous in terms of frustule morphology as it includes species with and without alveolar chambers. Except the marginal chambers, C. rolfii gen et sp. nov. shares no other characters with the genus Cyclostephanos .

The characteristics of the striae, composed of fine pori on the mantle and becoming uniseriate with large radially arranged areolae towards the center of the valve, as observed in C. rolfii gen et sp. nov., are shared with Paleotertiarius. Even though C. rolfii gen et sp. nov. and Paleotertiarius share characters such as strong ribs and the structure and position of mfp, in Paleotertiarius, flat or concentrically undulated valves, internal domed criba and rimoportula located on the side of a costa inside the alveolus are morphological differences that exclude C. rolfii gen et sp. nov. from belonging to Paleotertiarius.

Figures

Figs 1–24. Type material of fossil lacustrine diatom Cyclocostis rolfii Paillès gen. et sp. nov., Lake Petén- Itzá (Guatemala); LM girdle view (1) and valve views (2–24). 4. Holotype (MNHN, slide PC0608731). 7–8. Valve surface strongly tangentially undulated, forming an S shape. Scale bar = 10 µm.

Figs 25–32. Type material of fossil lacustrine diatom Cyclocostis rolfii Paillès gen. et sp. nov.; SEM external valve views. 25. Valve view of concentrically undulated marginal area and tangentially undulated central area; punctuated striae become in the central area rows of larger areolae arranged in a stellate pattern. 26. Valve surface with scattered papillae; the external opening of the single valve face fultoportula is located on the raised part (white arrowhead). 27. Marginal area showing the external openings of marginal fultoportulae, collared but with no projections (white arrowheads). 28. Side view of marginal area showing striation, papillae, external openings of marginal fultoportulae (mfp – two white arrows), and the cingulum consisting of an open valvocopula and several copulae (white arrow). 29. Detail of the central area with large areolae; external areolae are bigger and occluded by volae in places where ribs are fusing. 30. Broken valve view showing the different striation between the margins and the center, the steep transversal undulation and the valve thickness. 31. Marginal area with the external openings of marginal fultoportulae (white arrowheads), papillae, and the cingulum. 32. Broken valve view showing the simple structure of anastomosing ribs covered by a finely perforated silica layer. Scale bars: 25 = 5 µm; 26, 28–32 =2 µm; 27 = 1 µm (27).