COSCINODISCUS DISPAR PERAGALLO AND HÉRIBAUD

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COSCINODISCUS DISPAR PERAGALLO AND

HÉRIBAUD

Karen Serieyssol

To cite this version:

Karen Serieyssol. COSCINODISCUS DISPAR PERAGALLO AND HÉRIBAUD. Vie et Milieu / Life

& Environment, Observatoire Océanologique - Laboratoire Arago, 1995, pp.279-281. �hal-03052731�

VIE MILIEU, 1995, 45 (3/4) : 279-281

COSCINODISCUS DISPAR PERAGALLO AND HÉRIBAUD

KAREN K. SERIEYSSOL

ABSTRACT - Coscinodiscus dispar Héribaud described by Héribaud in 1893 has been reported from Miocène, Pliocène sites of France. A detailed light and scanning électron microscope study was undertaken to détermine a description of the species.

Coscinodiscus dispar Héribaud does not fit in any of the known fossil gênera and

this is discussed.

RÉSUMÉ - Coscinodiscus dispar Héribaud, décrit par F. Héribaud en 1893, a été trouvé dans le Miocène et le Pliocène de France. Une étude détaillée en micro-scopie optique et en micromicro-scopie électronique à balayage a mis en évidence les caractères ultrastructuraux de l'espèce. Une discussion basée sur les caractères ul-trastructuraux de Coscinodiscus dispar Héribaud montre pourquoi cette espèce n'appartient pas aux genres fossiles connus.

DIATOMS MORPHOLOGY CENTRICS MIOCENE PLIOCENE DIATOMÉES MORPHOLOGIE CENTRIQUES MIOCÈNE PLIOCÈNE

INTRODUCTION

In order to compile a biostratigraphy of France, it is important to have a detailed knowledge of the différent species contained within the différent deposits. Therefore, a detailed examination of the différent species using the light and scanning élec-tron microscopes was undertaken. One species re-ported from Miocène, Pliocène deposits is

Coscinodiscus dispar Héribaud.

Coscinodiscus dispar was first described by

Héribaud (1893) from fossil material near Puy-de-Dôme (Massif Central, France). After describing the species, he stated the species was found in material coming from the Dépôt de Varennes no. 2 where it was very common and a characteristic species of this deposit. In 1903, Héribaud wrote that Coscinodiscus dispar and its varieties are one of the most common species found in the Dépôt de Faufouilhoux. The Varennes sédiments were not found in the Héribaud collection at the British Muséum (Natural History), but examination of the Faufouilhoux material found that C. dispar was présent in a sample labeled Faufouilhoux 3092. Therefore, this material was used to détermine the characteristics of Coscinodiscus dispar.

MATERIAL AND METHODS

The French material collected by Héribaud and labeled Faufouilhoux 3092 cornes from an explosion crater located near the city of Murât. Several names have been used by différent authors (Héribaud 1893, 1903 ; Fournier 1965 ; Gasse and Serieyssol 1991 ; Gôer de Hervé et Tempier, 1991; Serieyssol 1993) for the same site : Faufouilhoux, Sainte Reine, Murât, and Aux-illac. The most commonly used name is that of Auxil-lac. The Auxillac diatomite is eut by a basait intrusion that has been dated at 5.34 ± 0.30 Ma. (Rey 1975). This places the formation of the diatomite at the end of the upper Miocène - beginning of the Pliocène.

The sample was cleaned using the method described in Serieyssol (1993). The cover slips for the light mi-croscope (LM) were mounted with Naphrax. The scan-ning électron micrographs (SEM) were taken on a Jeol JSM 841A after standard coating and mounting pro-cédures.

RESULTS

Coscinodiscus dispar Peragallo and Héribaud in

Héribaud (1893).

The valves are circular, almost fiât varying in diameter between 3 and 14 um. Valve face

are-280 K.K. SERIEYSSOL

Plate 1. - Figs. 1 to 7. Coscinodiscus dispar Peragallo and Héribaud. Figs. 1 to 4. SEM. Scale bars = 1 um. 1 = labiate process, c = central strutted process, m = marginal strutted process. Figs. 5 to 7. LM. Scale bar = 10|0.m.

olae, clearly visible in the LM, are arranged in radial rows with the larger areolae in the center and decreasing in size towards the margin (Pl. 1, Figs. 1 & 2). The valve face areolae are covered on the internai face by domed cribra, visible in the SEM (Pl. I, Figs. 3 & 4). There is a distinct break between the valve face areolae and those of the mantle (Pl. I, 1, 20). The mantle pores are smaller in size and arranged in linear rows (Pl. I,

2). The marginal strutted processes, présent on every third to fourth striae, are large in size and have a relatively large extension of the process towards the center (Pl. I, 3 & 4). The number of satellite pores could not be determined but at least two could be observed. The external expression of the marginal strutted process is marked by an interruption of the mantle alveoli and by a slight doming of the silica around the pore (Pl. I, 2). One

COSCINODISCUS DISPAR PERAGALLO & HERIBAUD 281

slightly excentric central strutted process was noted with either two or three satellite pores (Pl. I, 1, 2, 3, 4). A sessile labiate process is situated at the valve face mantle border and is located on the opposite half of the valve from the central strutted process. The labiate process appears on the exter-nal face as an elongated slit situated at the valve face - mantle junction near a marginal strutted process. The external extension of the labiate process (typical of the genus Stephanodiscus) is lacking.

DISCUSSION

If we accept the expanded définition of Thalas-siosiraceae Lebour emend. Hasle, used by Theriot and Bradbury (1987), then this species best belongs to the family Thalassiosiraceae, based on the présence of strutted processes. However, there is a problem fitting the species into the main freshwater gênera of this family. The species can-not be added to the genus, Mesodictyon, by the présence of areolae with raised or domed internai cribra, because Mesodictyon has been defined as having loculate areolae, each having a cribrum in-side the loculus and a foramen on both the internai and external valve surfaces (Theriot and Bradbury, 1987). Coscinodiscus dispar also has a central strutted process ; a character that has not been ob-served on the known species of Mesodictyon.

Coscinodiscus dispar could belong to any of

the following gênera, Cyclotella, Cyclostephanos and Stephanodiscus, by the présence of raised or domed cribria on the internai valve face. But, both

Cyclotella and Cyclostephanos have alveolar

chambers which this species does not have. It differs from Stephanodiscus by the lacking the fasciculate radial areolae, an external tubulus on the labiate process and the external tubulus on the marginal strutted processes.

ACKNOWLEDGEMENTS - I would like to thank Dr. F. Gasse for constructive discussions and for allowing me to work in the Laboratoire d'Hydrologie et de Géo-chimie isotopique, Université Paris-Sud and the Labora-toire de Micropaléolontologie, Université Paris VI for allowing me to use their SEM. I would also like to thank Mr. P. Blanc for his assistance with the SEM.

REFERENCES

FOURNIER F. (GASSE), 1965. Etude de quelques gise-ments diatomifères d'Auvergne, Faculté des Sciences d'Orsay, D.E.S.

GASSE F and K. SERIEYSSOL, 1991. Diatomées néo-gènes du Massif Central français : quelques faits bi-ostratigraphiques. C. R. Acad. Sci. Paris

312,

GOER H.A. DE et P. TEMPER, 1991. Carte géologique à 1/50000, St-Flour, B.R.G.M., Orléans.

HÉRIBAUD J., 1893. Les Diatomées d'Auvergne. Ed. Librairie des Sciences Naturelles, Clermont-Ferrand, 255 p., 8 pl.

HÉRIBAUD J., 1903. Les Diatomées fossiles d'Au-vergne (Second Mémoire). Ed. Librairie des Sciences Naturelles, Clermont-Ferrand, 166 p., 4 pl.

REY R., 1975. Premières données radiométriques rela-tives à l'âge du niveau pollinique de Reuver. C.R.

Acad. Se. Paris,

281

SERIEYSSOL K.K., 1993. Les Diatomées des sédi-ments lacustres d'âge Miocène supérieur d'Andance et Rochessauve (Ardèche). Paléoécologie et Bios-tratigraphie. Thèse Univ. Paris 6, 311 p.

THERIOT E., J.P. BRADBURY, 1987. Mesodictyon, a new fossil genus of the centric diatom family Thalas-siosiraceae from the Miocène Chalk Hills Formation, western Snake River Plain, Idaho.

Micropaleon-tology,

33

Reçu le 19 décembre 1994 ; received December 19, 1994 Accepté le 2 juin 1995 ; accepted June 2, 1995