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?

Several taxa of Stephanodiscaceae were found in the upper section of Pleistocene sediments from Lake Petén-Itzá (Guatemala). A new fossil genus Cyclocostis Paillès gen. nov. and new extant species Discostella gabinii Paillès & Sylvestre sp. nov. are described. Cyclocostis gen. nov. is characterized by a strongly tangentially undulated valve surface, coarse unequal striation reaching a central punctum in the valve center, an absence of central lamina and domed criba, widely open alveoli with one median recessed costa bearing marginal fultoportulae and a single rimoportula all within a ring. A single valve face fultoportula is present on the raised part of the valve opposite the rimoportula. Diff erences relative to similar genera and the delimitation of a new genus are discussed. Discostella gabinii sp. nov. is distinguished by circular and fl at valves, a small central area bearing 5 to > 30 scattered large areolae giving a colliculate appearance, medium-sized alveoli, marginal fultoportulae on every 4–5th costa, a single rimoportula and internally smooth valve center. Diff erences to similar taxa in the genus Discostella are discussed. The succession of the species of Cyclotella, Discostella and Cyclocostis gen. nov. in our record could represent eco-phenotypic responses to particular environmental stress / change.


Introduction
Publications describing freshwater diatoms from Central America are scarce. If few paleolimnological studies based on diatoms have been conducted in Costa Rica (e.g., Chá vez & Haberyan 1996;Haberyan & Horn 1999, 2005, Nicaragua (e.g., Slate et al. 2013) and Panama (e.g., Temoltzin-Loranca et al. 2018), even fewer have been conducted in the Yucatan Peninsula, and these are generally limited to the Holocene (Whitmore et al. 1996;Rosenmeier et al. 2004). Longer diatom records originated from the Mexico Basin (Bradbury 2000;Ortega et al. 2010). Only the record of Cohuo et al. (2018) is based on the same sedimentary sequence as the present study. Furthermore, since the majority of fresh water bodies in this region are alkaline, calcium-bicarbonate dominated systems (Pérez et al. 2013), valve preservation is usually poor (Metcalfe et al. 2000). A few taxonomic studies have been recently published on diatom species from, e.g., Panama (Lange-Bertalot & Metzeltin 2009) or El Salvador (Wetzel & Ector 2014;Krahn et al. 2018), with one dealing with centric diatom species of Cyclotella (Kütz.) Bréb. from Guatemala ).
In the last 50 years, the genus Cyclotella (Brébisson 1838: 19) has received signifi cant attention. There have been many attempts to classify species within the genus according to the morphological features by Lowe (1975), McFarland & Collins (1978), Serieyssol (1981), Servant-Vildary (1986), Loginova (1990), Håkansson (1990), Håkansson et al. (1993) and Tanaka (2007). In a revision of the genus Cyclotella, Håkansson (2002) subdivided it on the basis of the type of undulation of the valve, the morphology of the central area, the position of the rimoportula, and the position and number of satellite pores of marginal fultoportulae. This author also suggested that the number of satellite pores of marginal fultoportulae may be the most important character for defi ning phylogenetic groups within Cyclotella. Interestingly, Loginova (1990) observed that only fossil species of Cyclotella have marginal fultoportulae with three satellite pores. Under Håkansson's (2002) assumption, Prasad & Nienow (2006) identifi ed one lineage of Cyclotella with three satellite pores and another with two satellite pores. Similarly, using the presence / absence of a central lamina (Servant-Vildary 1986) and the presence / absence of marginal chambers (Lange & Syversten 1989), the authors identifi ed three groups of Cyclotella. Khursevich & Kociolek (2012) distinguished 11 groups of Cyclotella based on the structure of alveolae, the number and location of rimoportula, and the structure of striae. For Houk et al. (2010), the genus Cyclotella is subdivided into three groups. A revision of the classifi cation of Cyclotella by Nakov et al. (2015) identifi ed the position of the rimoportula as a synapomorphy. Cyclotella s. str. has a rimoportula located on a costa within the ring of marginal fultoportulae, as opposed to those of Discostella Houk & Klee, Lindavia (F.Schütt) De Toni & Forti and Paleotertiarius (Håk. & Khursevich) S.Blanco (Nakov et al. 2015). Àcs et al. (2016) separated the genera Lindavia, Pantocsekiella K.T. Kiss & Àcs and Edtheriotia Kociolek, You, Stepanek, R.L.Lowe & Wang from the genus Cyclotella on the basis of morphological and genetic characters. At this point, these new concepts make it diffi cult to disentangle the phylogeny and taxonomy of this group.
In 2006, the Petén-Itzá Scientifi c Drilling Project (PISDP) recovered a total of 1327 m of sediment from seven sites in Lake Petén-Itzá, Petén, northern Guatemala, for paleoclimate and paleoenvironmental studies (Hodell et al. 2008). The fossil diatom fl ora of Lake Petén-Itzá was analyzed in core PI-6 dated by tephro-chronology and covering the last 84 ka (Kutterolf et al. 2016). From this record, the new species Cyclotella petenensis Sylvestre, Paillès & Escobar and C. cassandrae Paillès & Sylvestre were described (Paillès et al. 2018). Based only on morphological observations, we herein describe a new genus belonging to the family Stephanodiscaceae Glezer & Makarova, Cyclocostis Paillès gen. nov., and a new species of Discostella from the same core, bringing the total to three new species and one new genus described from the Pleistocene sediments of Lake Petén-Itzá. So far, except for C. cassandrae and the new genus Cyclocostis gen. nov. (described herein) that are absent from the modern dataset (Pérez et al. 2013), the majority of species in the fossil record are still extant and will therefore provide a solid basis for paleolimnological reconstructions.

Material and methods
During the International Continental Scientifi c Drilling Program (ICDP) expedition PISDP in 2006, sediment cores were collected at site PI-6 in Lake Petén-Itzá (Hodell et al. 2008), providing a continuous record of sediment accumulation for the last 84 cal ka. Cores were stored on board at 5°C, then transferred to the Minneapolis Core-Repository in Minnesota (USA) where samples were taken from depths ranging between 0 and 70 mcd (meter composite depth) at 10-cm intervals. The core was dated by 14 C (44 ages) on terrestrial remains and three tephra layers dated by Ar / Ar (Escobar et al. 2012). An age model was derived using a weighted fi t through 36 age-depth points. The sediments consisted of laminated light brown to greenish clays, gypsum and tephra layers (for more details see Hodell et al. 2008).
A 5 mm thick slice of sediment (0.5 g) was fi rst decarbonated with hot 37% HCl for a few hours, the organic matter was then oxidized with hot 33% H 2 O 2 for a few hours. Diatom suspensions were successively rinsed and decanted with distilled water. Aliquots of cleaned diatoms were diluted and evaporated at room temperature onto coverslips then mounted on glass slides using Naphrax ® mounting medium. Diatom slides were examined under oil immersion using light microscopy (LM) at 630 × or 1000 × magnifi cation using a Nikon Eclipse 80i microscope equipped with diff erential interference contrast optics and a Nikon D300 camera. Counting was generally performed on three slides. The total number of valves counted per sample varied from 200 in nearly sterile samples to > 1200 in rich samples. Diatom identifi cation and taxonomy followed Krammer & Lange-Bertalot (1986, 1988, 1991a, 1991b with the revised nomenclature in AlgaeBase (Guiry & Guiry 2020). Number of striae in 10 μm were determined following Genkal (1977), i.e., as the number of striae in 10 μm of the valve circumference, not in 10 μm of a transect (chord).
For scanning electron microscopy (SEM) observations, a few drops of cleaned diatom material were air-dried on circular coverslips which were then attached to aluminum stubs and gold-coated with Cressington 108 auto (Watford, UK). Diatoms were examined with a XL 30 ESEM Philips SEM at an accelerating voltage of 1-30 kV.

Results
Sediments from core PI-6 revealed a rich and diverse diatom fl ora (153 species belonging to 42 genera). Ninety-six species are extant and were found in diverse waterbodies from the Yucatan Peninsula (Pérez et al. 2013). The upper part of the sedimentary sequence between 60-16 ka is characterized by an alternation of the species of Cyclotella and Discostella ( Fig. 1

Etymology
The genus name refers to the circular morphology of the valve and marked radiating costae.

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

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

Description
Light microscopy  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 ⅓ 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 ⅔ 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 . 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 fi nely 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 fi nely perforated silica layer (Fig. 32).
Internally, there is no central lamina inside the valve (Fig. 33). Costae are strongly silicifi ed and elevated, extending from the valve rim to the valve center and fusing into a thick silicifi ed hollow . The alveolar structure could be classifi ed 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 (vff p) -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 vff p is diffi cult to observe as it is positioned on the external slope of the raised central part (see in Fig. 26).

Remarks
Cyclocostis rolfi i 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 diff erent structure in C. rolfi i 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. rolfi i gen et sp. nov. also resembles Cyclotella stoermeri Khursevich & Kociolek in Kociolek & Khursevich (2013)  However, these structures are internal in C. rolfi i gen et sp. nov., whereas they are external in S. chantaica whose internal structure is fl at with fi ne 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. rolfi i gen et sp. nov. diff ers from Pliocaenicus, it rules out its belonging to this genus.
The alveoli of C. rolfi i gen et sp. nov. could be classifi ed 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. rolfi i gen et sp. nov. rather corresponds to some extent to the marginal chambers defi ned 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. rolfi i gen et sp. nov. shares no other characters with the genus Cyclostephanos.
The characteristics of the striae, composed of fi ne pori on the mantle and becoming uniseriate with large radially arranged areolae towards the center of the valve, as observed in C. rolfi i gen et sp. nov., are shared with Paleotertiarius. Even though C. rolfi i gen et sp. nov. and Paleotertiarius share characters such as strong ribs and the structure and position of mfp, in Paleotertiarius, fl at or concentrically undulated valves, internal domed criba and rimoportula located on the side of a costa inside the alveolus are morphological diff erences that exclude C. rolfi i gen et sp. nov. from belonging to Paleotertiarius.

Etymology
This taxon is named in honor of deceased Gabin Sylvestre, the courageous 7 years old nephew of F. Sylvestre.

Isotype
Slide ZU 11/31 and sediment R1285 deposited at the Friedrich Hustedt Diatom Center in Bremerhaven, Germany.

Other material examined
Modern specimens collected from Cenote Juarez and Lake Amatitlan (see Table 1).   (Figs 41-50). When numerous, the scattered areolae give the impression of a colliculate / granular fl at center. The marginal area of the valve face has radial striae numbering from 10 to 14 in 10 μm. The striae are long (⅔ of the valve radius) and of equal length. On large specimens, marginal striation is crossed circumferentially by a ring (ʻSchattenlinieʼ = ʻshadow lineʼ) close to the valve center (Figs 48-50).

Type locality
Scanning electron microscopy  Valves fl at to barely concave externally with gently sloping mantle. Central area covered with several scattered punctae separated by knots (colliculate) bearing papillae (Figs 51-52). Radiating striae starting on the mantle as crescents of three to fi ve rows of fi ne areolae (60-70 areolae / 10 μm), merging into two rows near the central area and ending with a single large pore (Figs 53-54). The central area is thus bordered by a ring of large areolae. On the valve face, striae are depressed, whereas they are smooth on the mantle. Near the valve margin, every third to fi fth striae, pores just below the crescent of fi ne areolae mark the external openings of marginal fultoportulae (Fig. 54). Interstriae are narrow, domed and granular on the valve face, whereas smooth on the mantle. The mantle is unornamented except for the large round openings of marginal fultoportulae and few papillae. The external opening of the rimoportula was not observed, although it should be positioned at the same level since it is within the ring of marginal fultoportulae.
Interior views of the valve show a fl at to slightly concave but smooth central area with none or single areola (Figs 55-56). The internal lamina spread from the valve center to ⅔ of the valve radius. The alveoli are thus medium sized, oblong and of unequal length, those bearing marginal fultoportulae being longer (Fig. 56). Marginal fultoportulae with two laterally positioned satellite pores surrounding a short tubulus are located at the distal extremity of every third to fourth alveoli (Fig. 57). One nearly sessile rimoportula with vertically orientated lips located between two costae at the edge of an alveolus and within the ring of marginal fultoportulae (Fig. 58). Girdle bands present, an open valvocopula with two copulae (Fig. 56); a row of fi ne pores is noted on the interior of the girdle band (Fig. 58).

Remarks
With marginal fultoportulae and rimoportula being located between costae on the marginal side of the alveolus, Discostella gabinii sp. nov. belongs undoubtedly to the genus Discostella. Amongst the 15 species of Discostella described so far (Kociolek et al. 2018), D. gabinii sp. nov. showed some resemblance to D. areolata (Hust.) Houk & Klee. However, in LM they look somewhat diff erent, the unique holotype of D. areolata having coarser striation (6-9 striae in 10 μm) and a large colliculate central area (Houk et al. 2010: table 330, fi gs 1-7). A reexamination of the original material of D. areolata from Hustedt by Tagliaventi & Cavinaci (2002) provided unambiguous SEM images of external views but only ambiguous internal views since D. areolata was rare and mixed with D. stelligera (Cleve & Grunow) Houk & Klee var. robusta (Hust.) Houk & Klee in the original material. The central area of D. areolata is concave or convex, smooth or consisting of alternating impressions and protrusions of various size with small punctae being mainly located in the depressions. Sometimes domed radiating striae resembling a poorly defi ned rosette are present in the central area. In D. gabinii sp. nov., the central area is always fl at, indeed colliculate but with large punctae inserted in the depressions. Moreover, in D. areolata, striae are depressed and costae elevated on their entire length, whereas in D. gabinii sp. nov., this feature is restricted to the valve face, the mantle being smooth. Internally, two types of central area could be attributed to D. areolata: smooth with no central fultoportula or smooth with a punctum. These variations are also visible in D. gabinii sp. nov. Despite uncertainties related to the species described as D. areolata, D. stelligera var. robusta and D. stelligera var. hyalina (Hust.) Houk & Klee, the structure of marginal costae and the position of marginal fultoportulae and rimoportula are quite diff erent compared to that of D. gabinii sp. nov. Marginal costae can be forked or not. Furthermore, marginal fultoportulae (composed of one tube and two satellite pores placed horizontally) and rimoportula (vertically orientated slit) are inserted within the alveolar chamber.
Another somewhat similar species is D. elentarii (Alfi nito & Tagliaventi) Houk & Klee with fl at valves, although it has a large central area with radiate rows of granules and scattered punctae, coarsely striated (9-10 striae in 10μm) and reduced marginal area, and a marginal row of small spinae. Internally, it has similar smooth central area (sometimes with a faint stellate pattern) and similar structure and position of mfp and rm. The only diff erence is that, internally, in D. elantarii costae are broadening toward the valve margin with a punctum in the middle giving the impression of forked costae. After reexamination of D. elantarii by Knapp et al. (2006), it appears that the correct striae density is 8-14 and that each collared marginal fultoportula and the single rimoportula are surrounded by satellite pores covered by a cribum. Although we did not use a fi eld emission variable pressure SEM, such structures are absent in D. gabinii sp. nov. Interestingly, it is the only morphological feature used to diff erentiate D. elantarii from D. stelligera in SEM (Knapp et al. 2006). The presence of pores in the girdle band is also a subtle character shared by D. elentarii and D. gabinii sp. nov. that requires further investigation. Despite morphological similarities with D. areolata and D. elentarii, D. gabinii sp. nov. possesses distinctive characteristics that are suffi cient to defi ne a new species. Stelligeroid species of Cyclotella have been transferred to the genus Discostella on the basis of the unique position of strutted and labiate processes (Houk et al. 2010). However, diffi culties arise because these species are often heterovalvate and size and morphological variations exist. As reported by Tagliaventi & Cavinaci (2002), Alfi nito & Tagliaventi (2002) and Knapp et al. (2006), only minute distinctive features allow one to diff erentiate D. areolata, D. stelligera, D. stelligera var. robusta, D. stelligera var hyalina and D. elantarii. This latter species is endemic to New Zealand and coexists with D. stelligera in two lakes. Knapp et al. (2006) suggest that considering the diffi culty in diff erentiating them, they could be sibling species and D. elantarii may descend from D. stelligera.

Ecology and associated diatom fl ora
In the modern dataset (Pérez et al. 2013), Discostella gabinii sp. nov. was initially identifi ed as "Cyclotella sp22" (code CP22) and another species was identifi ed as "Discostella aff . pseudostelligera" (CYAP). When analyzing the fossil fl ora and diagnosing D. gabinii sp. nov., we re-examined modern samples and observed that the two species were similar. "Discostella aff . pseudostelligera" and "Cyclotella sp22" were therefore combined together under the name Discostella gabinii sp. nov.
Cyclotella petenensis, although considered to be fossil at the time of description (Paillès et al. 2018), was identifi ed as C. meneghiniana (CYMG) in the modern dataset mainly due to the fact that specimens were small in size, tangentially undulated with < 5 valve face fultoportulae on the raised part. Once diagnosed as a new species in the sedimentary record, a re-examination of modern samples revealed that C. petenensis was present in low percentages (< 4%) in fi ve water bodies. Only in Lake Yalahau (Yucatan lowlands), C. petenensis reached 17.8% (Table 1). Of all water bodies investigated, Lake Yalahau had the highest diatom species richness. In this lake, water is shallow, warm (28.8°C) and alkaline (pH 8.9) with a high dissolved oxygen content (8.7 mg / L). Electrical conductivity is high 2350 μS / cm. Water was magnesium (136.8 mg / L) and bicarbonate (707.4 mg / L) rich. Its diatom population was composed of 33% of C. meneghiniana accompanied by Brachysira australofollis Lange-Bert. & Gerd Moser, B. neoexilis Lange-Bert., Encyonema densistriata Novelo, Tavera & Ibarra and Fragilaria famelica (Kütz.) Lange-Bert. In coastal Lake Progreso where C. petenensis represents 4% of the fl ora, water conductivity was 2040 μS / cm. In the modern samples, it appears that C. petenensis seems to favor waters with conductivities close to 2000 μS / cm.

Discussion
In terms of morphology, Stephanodiscaceae encompass a wide range of valve structures and thus genera. Interestingly, Cyclocostis gen. nov. possess distinctive morphological characters that can be found in species of genera Cyclostephanos, Cyclotella (Lindavia, Discostella, Pantocsekiella), Stephanocostis, Pliocaenicus, and Paleotertiarius. Conversely, the combination of these peculiar morphological features, which diff erentiate it from these genera, can be used to justify the defi nition of a new genus. In terms of valve structure, when looking at internal valve views of species of Stephanodiscus Ehrenb., Stephanocostis and most species of Cyclostephanos, the striking features are 1) the absence of alveoli and thus ribs, these being rather hyaline interfascicles and 2) the presence of domed criba covering internal areolae. Nevertheless, internal domed criba are always present in Paleotertiarius and Pliocaenicus and in some species of Lindavia. But Paleotertiarius and Pliocaenicus as well as some species of Cyclostephanos (C. novaezeelandiae and C. dubius) are also characterized by ghost or reduced alveoli. In contrast, internal alveolate structure -simple or complex -is a widespread structural component of Cyclotella s. str., Lindavia, Pantocsekiella and Discostella. As in Paleotertiarius and Pliocaenicus, the alveolus (Anonymous 1975;Houk et al. 2010) is a chamber opening to the inside of the cell by small or large opening with a perforate outer layer. Thus, the degree of occlusion of the alveolus is determined by centrifugal roofi ng. Consequently, contact between the cell and the exterior is highly reduced by the presence of internal central lamina (Servant-Vildary 1986). Similarly, in more simply structured valves of Stephanodiscus, Cyclostephanos and Stephanocostis, the presence of domed criba is assumed to reduce contact between the cell and the exterior.
In morphogenesis of centric diatoms, valves are systems of silica ribs, which grow out from a circular center during valve formation (Round et al. 1990). They are formed within silica deposition vesicle (SDV) enclosed in the silicalemma, starting with a small, thin disc composed of radial bunched siliceous strands from the center (Kaluzhnaya 2006). According to Bedoshvili & Likhoshway (2019), valve morphogenesis in centric diatoms begins with a formation of a ring (annulus) from which the ribs radiate centrifugally as the SDV grows and until the mantle is formed (= horizontal growth). Vertical growth occurs by silica deposition to thicken the valve and to diff erentiate fi ne structures such as areolae and various processes. Structurally, Cyclocostis gen. nov. appears simple: a web of radiating ribs starting from a central silicifi ed ring covered by a coarsely perforated silica layer. The central ring represents a remainder of the annulus that was never fi lled in. Additionally, striae are reduced to two ribs covered with a perforated layer, and alveoli are widely open to the valve center. The frustule is strongly deformed and, consequently, has a larger surface area compared to a fl at disc. Futhermore, the cell  Pérez et al. (2013).
cavity is reduced (see the thickness of the frustule in Fig. 33). It could be anticipated that such simple construction combined with a large surface area would greatly expose the cell to surrounding waters as well as facilitate communication with the outside environment.
The species of diatoms are discerned by phenotypic characters. According to Benton & Pearson (2001), the process of speciation is too slow to be observed directly. However, along an upper Miocene lacustrine sequence, populations of triangular Cyclotella sp. exhibit complexifi cation of their alveoli suggesting a trend toward a reduced contact between the cell and its surrounding environment (Servant-Vildary 1986). Similarly, Theriot et al. (2006) observed a morphological shift in the Stephanodiscus niagarae C.Ehrenb. / S. yellowstonensis E.C.Ther. & Stoermer complex between 13.7 and 10.0 ka. They suggest that directional morphological evolution strongly associated with continuous environmental change would account for the evolution of S. yellowstonensis. Additionally, in Pleistocene sedimentary sequences from lakes Ohrid and Prespa, morphological variations (valve diameter, striae morphology, number of ribs and valve face fultoportulae) in Cyclotella populations suggested the expression of environmental factors (Cvetkoska et al. 2014). Interestingly, these studies only concern centric diatoms, the only study on pennate diatoms in lacustrine sequences concluded that levels of morphological diff erentiation were likely a consequence of limited dispersal (Evans et al. 2009).
In the PI-6 sedimentary record, from around 60 to 16.1 ka, we observe a succession of Cyclotella meneghiniana, Discostella stelligera, D. gabinii sp. nov., C. petenensis and Cyclocostis rolfi i gen et sp. nov. that could evoke fl uctuations of the lacustrine environment. A similar interpretation was made by Bradbury (1971) in a > 46 ka diatom record from Lake Texcoco in Mexico where Cyclotella cf. stylorum (probably C. petenensis), C. quillensis L.W.Bailey and C. striata (Kütz.) Grunow alternated and refl ected changes in salinity. He noticed that their distribution throughout the core was an alternation of ecotypes. From a morphological point of view, C. petenensis and C. cassandrae are probably variations of C. meneghiniana, initially present in our record (Paillès et al. 2018). Similarly, Discostella gabinii sp. nov. could also be a variation of D. stelligera fi rstly present in the record. For the newly described Cyclocostis gen. nov., its basic structure and the strong deformation of the valve suggest enhanced communication between the cell and the exterior. During 5 ka, Cyclocostis gen. nov. alternates with benthic species such as Nitzschia amphibioides, Mastogloia smithii, M. elliptica and Navicula seminuloides. As such, its punctual presence could also be the expression of local and rapid changes in the environment.
Overall, the succession of the species of Cyclotella, Discostella and Cyclocostis gen. nov. in our record could represent eco-phenotypic responses to environmental change / stress. Changes in water conductivity and / or water level fl uctuations could be evoked as suggested by the presence of benthic mesosaline to hypersaline species developing between Cyclotella, Discostella and Cyclocostis gen. nov. episodes. Except for Cyclotella cassandrae and Cyclocostis rolfi i gen et sp. nov. that were not identifi ed in the modern dataset, all other centric species are still extant. Thus, a calibration of the modern data set in order to perform pH and conductivity transfer functions in the fossil record will be considered in future studies.
this publication and L. Ector for providing extensive literature. SEM investigations were funded by the Institute for Research and Development (France). The Laboratoire Préparation Micropaléontologique (CEREGE, France) provided all facilities for sample processing. Finally, the constructive comments of two anonymous reviewers is gratefully acknowledged.