An early record of the genus Cytheridella Daday, 1905 (Ostracoda, Limnocytheridae, Timiriaseviinae) from the Upper Cretaceous of Mali, West Africa: palaeobiogeographical and palaeoecological considerations

The limnic ostracod genus Cytheridella Daday, 1905 (Limnocytheridae, Timiriaseviinae), previously only known from Plio–Pleistocene sediments and Recent lacustrine environments of South and Central America, the Caribbean Islands, Florida and Equatorial Africa, and from earliest Eocene to Early Oligocene Eurasian localities, is found for the first time in the Upper Cretaceous (undifferentiated Campanian–Early Maastrichtian) of northern Mali, West Africa, suggesting an African origin for the genus. The association with the brackish-water ostracod genus Sarlatina suggests a mixo- or oligohaline environment for the Cretaceous species of Cytheridella.


INTRODUCTION 0
The limnic ostracod genus Cytheridella Daday, 1905 (Cytheracea, Limnocytheridae, Timiriaseviinae, Cytheridellini) is represented by several Plio-Pleistocene and Recent species geographically restricted to South and Central America, the Other fossil species assigned to the genus Cytheridella by various authors have been removed from it by Purper (1974: pp. 655656). The oldest species previously included in this genus, Cytheridella? barnstorfensis Martin, 1957(in Martin & Weiler, 1957, from the Late Jurassic of Germany, has been recently reattributed to the genus Marslatourella (Marslatourella?) Malz, by Schudack (1994). It is also possible that ?Gomphocythere sp.
L 234 Grtkoff, 1958 from the Tertiary (Eocene, 'Grks Polymorphes') of Central Zaire, lacking the lateral ventral ridge on both valves typical of most species of the genus Gomphocythere Sars, 1924, belongs to the genus Cytheridellu, as already suggested by Pinto & Sanguinetti (1962).

Stratigraphic framework
Recent study of material from northern Mali, West Africa (Fig.  2), led to the discovery of rare specimens of a new species of Cytheridella in sediments of Upper Cretaceous age (Colin et al., 1996). The level which has yielded this species, in the borehole. In Talack, contains also the ostracod genera Virgatocypris, Zonocypris, Cypria, Darwinula and Sarlatina, as well as charophyte oogonia of Platychara caudata Grambast and Porochara globosa Grambast. It is located below the first welldated Late Maastrichtian marine beds (previously dated as Danian by Berggren, 1974) containing the foraminifera LuBteina bibensis Marie and the ammonite Lybicoceras ismaeli Zittel (Krasheninnikov & Trofimov, 1969). According to Bellion et ul. (1990), the age of this horizon is not very clear but is probably Campanian to Early Maastrichtian on the basis of molluscs faunas. Charophyte oogonia suggest a Campanian age, although a Maastrichtian age cannot be discounted (M. Feist, pers. comm.)

Taxonomic comments
Subclass In Australia, the closely related genus Gomphodella is considered to be a 'freshwater species which can withstand a slight amount of dissolved solid in water, with its highest salinity recorded at 2.3%' (De Deckker, 1981). The other related African genus Gomphocythere can also occur in slightly saline conditions (K. Martens, pers. comm.) and highly alkaline waters, up to > 30 meq-' (Cohen et al., 1983).
Fossil species are also often found in strictly freshwater environments, as in the Middle Eocene of Grabels near Montpellier (southern France), where Cytheridella is associated with Frambocythere and mammals. However, in several cases, Cytheridella is known to occur in mixohaline environments: Pleistocene and Holocene of Argentina (Zabert & Herbst, 1986;Bertels & Martinez, 1990), in which Cytheridella ilosvayi and Cytheridella aff. ilosvayi occur with truly limnic ostracods and the brackish-water species Cyprideis salebrosa van den Bold. Sub-recent and Pleistocene of Venezuela and Plio-Pleistocene of the Dominican Republic and Trinidad (van den Bold, 1971Bold, , 1986, in which Cytheridella boldii Purper, 1974  Eocene of Hungary in which Cytheridella gantensis is associated with lagoonal molluscs and dasycladacean algae (Monostori, 1977(Monostori, , 1993. Early Eocene of India in which Cytheridella gujratensis is found with the brackish-water genus Neocyprideis and rare arenaceous foraminifera (Bhandari, 1993). Earliest Eocene (Ilerdian) of southern France in which Cytheridella audensis occuring with the limnic ostracod Frambocythere, Cypris, Candona and Virgatocypris can also be associated with the brackish-water genus Neocyprideis (Tambareau et al., 1991).
In Mali, although typical freshwater genera occur in the assemblage ( Virgatocypris, Cypria, Darwinula and Zonocypris), a lagoonal oligohaline environment is suggested by the presence in the sample of a great number of the cytherid genus Sarlatina wich represents more than 50% of the fauna (Fig. 4), and whose ecological requirement was probably very close to that of Cyprideis (Babinot & Colin, 1976).
Whether Cytheridella actually lived in the lagoonal environ- ment or was transported into it, as suggested by Monostori (1993) for the Eocene of Hungary and by Keyser (1977) for recent environments of southern Florida, cannot be proved. However, the fact that in Mali the specimens found are closed carapaces does not favour a transportation hypothesis. The high percentage of closed carapaces observed in Gomphodella maia De Deckker, 1981 in sub-Recent sediments of Tasmania, southern Australia, has been explained (De Deckker, 1981, 1982 by the supposition that this species could burrow in the sediment to withstand short periods of desiccation. Concerning Cytheridella postornata from the Plio-Pleistocene of Colombia, Sheppard & Bate (1980) argue that 'Although a large species, the shell is thin and delicate and no extensive transport of the material is envisaged'.
Similar problems concerning the salinity tolerance and transportation into marine environments of the Mesozoic related genera Timiriasevia and Theriosynoecum have been discussed by various authours (see synthesis in Carbonel et al., 1988, pp. 443447).
In conclusion it can be said that Recent species of Cytheridella are present in various types of permanent limnic environments, from truely lacustrine (Cytheridella chariessa Rome, 1977(in Rome & De Deckker, 1977 in Lake Kivu in Zaire), to marginal environments (for example Cytheridella alosa in Florida (Keyser, 1977)). It is assumed that fossil species had the same ecological requirements.
Palaeobiogeographical considerations (Fig. 5) This species of Cytheridella of Campanian-Early Maastrichtian age is the oldest known representative of the genus, the previously known oldest species being of earliest Eocene age (Cytheridella audensis Tambareau et a/., 1991). Its presence in Mali during this period suggests that the genus, like other members of the Timiriaseviinae such as Frambocythere, recently reported from the Albian of Zaire (Colin, 1993), probably originated in Africa during the Cretaceous.
During the Early Tertiary (earliest Eocene to Early Oligocene), it spread over Europe and India from which it subsequently disappeared, probably as the result of climatic deterioration (cooling). In fact, the major change in lacustrine ostracod fauna and the appearance of Cytheridella in southern France occur during the Middle Ilerdian (earliest Eocene) and is associated with a marked climatic warming event well documented by subtropical to tropical floras (Tambareau et a/., 1991).
Until now, despite a great number of studies, no Miocene representative of the genus has been reported. The only Miocene species formerly assigned to Cytheridella, Cytheridella mediterranea Zalanyi, 1913 from the Miocene of Hungary, being coarsely reticulate and lacking the distinctive vertical sulcus, certainly does not belong to the genus, as already suggested by Purper (1 974).
In the PliePleistocene, the genus is only known from few South American localities (Brazil, Colombia and Venezuela) and its present day distribution is limited to Equatorial Africa (Cameroon and Zaire), South America (Argentina, Paraguay, Brazil, Venezuela and Chile), Central America (Nicaragua), the Caribbean Islands (Dominican Republic, Cuba and Trinidad) and the southern United States (Florida). The absence of Cytheridella from pre-PliePleistocene sediments in South and Central America is most likely due to the lack of studies. Other recent members of the Cytheridellini are geographically restricted to South and East Africa (Ethiopian Realm) and Israel for Gomphocythere (Martens, 1993), and Australia for Gomphodella (De Deckker, 1981)

CONCLUSION
The early occurrence of the limnic ostracod genus Cytheridella in the Campanian-Early Maastrichtian of Mali, suggests that this genus, which had a strictly Eurasian distribution during the Eocene-Oligocene and since the Plio-Pleistocene has been geographically restricted to South and Central America, the Caribbean Islands, Florida and Equatorial Africa, probably originated in West Africa during the Late Cretaceous. The association with other freshwater ostracods and the brackishwater genus Sarlatina supports a mixo-or oligohaline environment for the Cretaceous species.