Clypeina tibanai , sp . nov . ( Polyphysacea , Dasycladales , Chlorophyta ) , mid-Cretaceous green alga from the Potiguar Basin , Brazilian margin of the young South Atlantic Ocean

Dasycladales. Polyphysaceae. Clypeina. Cladosporate. Cretaceous. Albian. Cenomanian.


INTRODUCTION
Many Dasycladalean genera are monospecific or comprise few species.By contrast, Clypeina (Michelin, 1845) looks like it had a relatively stable and successful history with more than forty species (see Appendix I) found over almost 200My, from the Late Triassic up to the Palaeogene times.However moving from simple counting of the morphospecies and going to the morphogroup reveal it was probably more heterogeneous than previously thought.This new taxon is part of the earliest benthic assemblages, poorly diversified, colonizing the shallow-water marine carbonate ramps of the young South Atlantic Ocean.
A summary review of genus Clypeina (Michelin, 1845) The type species of Clypeina, the fossil Clypeina marginiporella (Michelin, 1845), was originally described as a coral-like animal by Michelin (1840Michelin ( -1847;;p. 177-178, pl. 46, fig. 26 a-b).The generic name was given after the Latin word "clypeus", meaning shield, because the first specimens collected by Michelin looked like "perfect rings" and "half rings".These individual calcareous structures were easily picked from the loose Cenozoic calcareous sands of the Paris Basin.In contrast, our specimens are embedded in a hard limestone and studying them requires petrographic thin sections, roughly 30µm thick.
There were many revisions of the generic diagnosis of Clypeina (see Rezak, 1957;Elliott, 1968;Bassoullet et al., 1978).The last one by the "French Group Studying Fossil Algae" (Bassoullet et al., 1978) takes into account the observation of J. and L. Morellet (1918) who found pores on the cap (top) of the algal thallus corresponding to the mark of a terminal tuft of sterile "hairs".It also incorporates an opinion since J. and L. Morellet (1918;p. 104, § "4°") report the occurrence of tubes with rows of pores, i.e. verticils of sterile laterals.Such features are observed in some modern Acetabularia (see for instance, Granier, 1994;pl. 4, figs. 6-9).According to them, these tubes should represent the bottom parts of some Clypeina thalli.However neither were they part of the material they studied, which is now deposited at the Muséum National d'Histoire Naturelle in Paris, nor were they observed in newly collected material (P.Génot, personal communication to B.G., June 6, 2012).Finally, Bassoullet et al. (1978) introduced a new feature, i.e. the possible occurrence of sterile verticils between fertile ones.This feature was observed in Clypeina sulcata (Alth, 1881), but it does not exist in the type-species, i.e. C. marginiporella (Michelin, 1845).In our opinion, other key features, such as the presence or absence of large interverticilar spacings, should also be taken into consideration.However the revision of the whole group is beyond the aim of this study that focuses on the sole description of a new species found in ?upper Albian-Cenomanian strata of Brazil.

Stratigraphic level
Uppermost part of the Ponta do Mel Formation, ?late Albian-Cenomanian in age (Tibana and Terra, 1981;Granier et al., 2008).Ponta do Mel is a facies-driven lithostratigraphic unit represented by a dominantly calcareous interval sandwiched in the dominantly siliciclastic Açu Formation (Neto et al., 2007); locally, the upper siliciclastics are missing and replaced by the pelagic facies of the Jandaíra Formation (see Granier et al., 2008;Fig. 1).

Facies and assemblage
The microfacies (Fig. 2A) displays a grain-dominated fabric, commonly bioturbated: it is a floatstone of large aggregates and solenoporacean nodules with a poorly sorted grainstone matrix.The smaller grains are pelletoids and micritic ooids.There are few bioclasts, partly micritized and commonly microbially coated: echinoderms, small gastropods, mollusc shells, few benthic foraminifers, including Trocholina silvai Petri, 1962, and calcareous green algal remains (Neomeris srivastavai Granier et al., 2012, the new species of Clypeina, and Linoporella ?sp.).The intergranular space is filled with drusy calcitic cements and locally on echinodermal remains by syntaxial rim cements.Locally there are patches consisting of micrite and small grains, geopetally arranged that possibly percolated through the grainy column above.Type material.The holotype consists of an oblique section (Fig. 2F) from the thin-section LPB 27381 cut from a core sample at 1638.00m, well 1-RNS-11 (offshore Rio Grande do Norte, Brazil).
Description.The thallus is roughly cylindrical, actually slightly club-shaped, bearing closely set verticils of laterals.The narrow part, i.e. with a stem diameter not exceeding 0.4mm (Fig. 2B, F), corresponds probably to the lower part of the thallus; it is also the part where the laterals are the shortest and less numerous.Complementary to this hypothesis we assume that the wider part, i.e. with a stem diameter larger than 0.7mm (Fig. 3A-C), represents the upper part of the thallus where the laterals reach their maximum length and are more numerous.The laterals are tubular in shape, with a diameter of 0.08mm on average.They communicate with the axial cavity through a proximal, narrow pore.The laterals forming a verticil are arranged at an upward acute angle on the axial stem, they rapidly bend to be almost orthogonal to the axis for the longest ones.They are closely set in the proximal part before the curve and the resulting general shape of a verticil is that of a funnel as for many representatives of the genus Clypeina.Beyond the bending the laterals diverge radially and form discrete rays (Fig. 3A-C , 1 2 ( 3 ) , 2 2 6 -2 3 7 ( 2 0 1 4    example in Clypeina stelliformis L. & J. Morellet 1939 (see Génot, 2009;pl. 10) and in Clypeina digitata (Parker & Jones, 1860) (see Génot and Granier, 2011;pl. 4).Cysts were not detected: they were probably sited in the primary laterals (there are no secondaries); the corresponding reproductive structure is cladosporate.
Diagnosis.Clypeina with a slightly club-shaped thallus.Closely set, funnel-like verticils consisting of 14 to 16 laterals, tubular in shape, with a diameter of 0.08mm on average.Laterals inserted at an upward acute angle on the axial stem, then bent to be almost orthogonal to the main axis distally.Joined together in their proximal part, neighboring laterals diverge radially and form discrete rays after the bending.Laterals are/grew longer in the upper part of the thallus, giving the calcareous coating a spiny morphology.Its measurements are given in the first column of Table 1.
Differences.The new species shows some affinity to Clypeina caliciformis Nikler & Sokač in Granier & Deloffre, 1992, non 1970, a junior synonym of Clypeina hanabataensis Yabe & Toyama, 1949, according to Granier (2002).However, it is easy to distinguish them when comparing the general measurements: there are less laterals per "whorl" (w) in the new species and these laterals are broader (p).The external, spiny shape of the thin calcareous (originally aragonitic) coating of the new Clypeina reminds that of Pseudoactinoporella fragilis (Conrad, 1970), however the lack of a proximal "short secondary branchlet" on the tubular laterals allow us not to increase the comparison.

DISCUSSION
With the exception of the Solenoporaceae, relatively few Cretaceous algal species from the Tethyan tropical realm are found in the young South Atlantic Ocean.With respect to the Dasycladalean algae, many Tethyan genera for the Albian -Cenomanian interval, such as the classical genera Cylindroporella, Cymopolia, Salpingoporella, and Triploporella, or the Harlanjohnsonella and Pseudocymopolia, are not found in these southern areas.Exceptions include Genotella pfenderae (Konishi & Epis, 1962), Heteroporella lepina (Praturlon, 1967), Neomeris cretacea Steinmann, 1899, and Trinocladus tripolitanus Raineri, 1922 (Fig. 4): i) within Family Dasycladaceae, Neomeris cretacea is often mentioned in the Cretaceous strata (Bassoullet et al., 1978).However, after the taxonomic revision carried out by Barattolo (1990), only two occurrences (Mexico and Brazil) can be taken into account.Similarly, Genotella pfenderae, quoted as "Neomeris pfenderae", was also commonly reported from the Atlantic (Bassoullet et al., 1978;Granier et al., 1991) but the valid records are restricted to Arizona (USA), Nigeria and Portugal (Granier and Berthou, 2002).So far the two remaining Brazilian Dasycladacean species, Brasiliporella nkossaensis (P.Masse, 1995) and Neomeris srivastavai Granier et al., 2012, are only known from the South Atlantic Ocean.Note that the first one was originally described from the Republic of the Congo as "Holosporella nkossaensis"; ii) Heteroporella lepina is the only Thyrsoporellacean species widely distributed in both the Tethys realm and the South Atlantic Ocean (stars on Fig. 4).The species names "Neomeris budaense Johnson, 1968" (from Texas) or "Heteroporella potiguraensis Srivastava, 1982" (from Brazil) are both junior synonyms of H. lepina (Granier et al., 1994); iii) finally, the newly described Clypeina tibanai, which represents the first record of a fossil Polyphysacean alga in South America, is completing the short list of calcareous green algae occurring in the young South Atlantic Ocean (Granier et al., 1991(Granier et al., , 2008(Granier et al., , 2012)).This assemblage consisting of few cosmopolitan species and fewer endemic species allow us to define  (Conrad, 1970) for comparisons.(L: maximum length; D: external diameter of a verticil; d: diameter of the stem; w: number of laterals per "whorl"; h: spacing of two successive verticils, from center to center; α: angle of the laterals to the main axis; l: length of a lateral; p: width of a lateral) a phycological paleobioprovince, i.e. the South-Atlantic province, discrete from that of the Tethyan realm.Although the list of Dasycladalean species is not that short when compared to some other classical groups of Tethyan organisms.Specifically, rudists and some large benthic foraminifers are notably absent within the narrow oceanic corridor between Africa and South America (Granier and Dias-Brito, 2013).This absence was thought to be due to salinity anomalies in the Atlantic basins of the north-eastern margin of Brazil.However, because we found these green calcareous algae associated to red algae we can exclude this hypothesis.For multiple reasons, including paleogeography (Rio Grande Rise-Walvis Ridge barrier, Fig. 4) or inappropriate paleolatitude, these green calcareous algae have probably colonized the South Atlantic seaway from a northern route, not a southern route.In conclusion some taxa or fossil groups known in the Tethyan realm are probably missing along the South Atlantic coasts only because they their ancestors did not find a route to the South.Clypeina tibanai, a fossil green alga from Brazil D O I : 1 0 .Radoičić, 1975, non 1969(transferred to Milanovicella Granier & Berthou, 1994, by Bucur (2000))

FIGURE 1 .
FIGURE 1. Location of the Petrobras well 1-RNS-11, 30km North of Macau, offshore of the State of Rio Grande do Norte (Brazil).