Loculicytheretta morkhoveni sp. nov. (Ostracoda) from West Africa and its relevance to the history of the Mediterranean Seaway

Loculicytheretta morkhoveni sp. nov., of which the females have six loculi per valve, was found in Recent beachsand samples from Senegal and The Gambia (West Africa). A displaced specimen of the same species was present in a sample from deeper water off Guinea, where it occurred with L. aff. L. pavonia (having four loculi per valve). The presence of the Mediterranean species L. pavonia (with three loculi) in deep samples West of Gibralter is interpreted as a result of transportation by outflowing currents. The discontinuous distribution pattern of the genus Loculicytheretta is tentatively related to a reversal of the flow regime in the Mediterranean basin during the Pliocene.


INTRODUCTION
The genus Loculicytheretta Ruggieri, 1954 has received relatively much attention from ostracod workers, not only because of its stratigraphical importance, but also because of its morphological peculiarities. The females are in the possession of posteroventral loculi; this feature is common in Palaeozoic Ostracoda but among the post-Palaeozoic Ostracoda only known to occur in the genera Loculicytheretta and Heptaloculites Ruggieri, 1963. The true function of these structures is as yet poorly understood, and various hypotheses have been proposed (McKenzie, 1971;Athersuch & Bonaduce, 1976).
Until van Morkhoven's (1962/63) extensive review of post-Palaeozoic ostracods was published, only the type species, L. pavonia (Brady, 1866) was known to science. In his treatment of the genus, van Morkhoven reported and illustrated a species from West Africa (The Gambia), quite similar to the type species, but differing in having six instead of three loculi in the female valve. In the present paper, this species is reported from two West African localities, and a formal description is presented.
At this moment a total of 10 species is attributed to the genus Loculicytheretta s . s . , ranging in age from Miocene to Recent. The morphologically similar genus Heptaloculites Ruggieri, 1963 includes 12 species, all of which are of Eocene age. Excellent illustrations of numerous species of both genera were given in Bismuth et al. (1978), and the evolution and palaeogeographic distribution was discussed. By these authors, Heptaloculites was treated as a junior synonym of Loculicytheretta and included in the latter. Carbonel & Colin (1982) demonstrated that the Palaeogene and the Neogene to Recent groups of loculate ostracods, although morphologically similar, belong to separate phylogenetic lineages, and reintroduced Heptaloculites for the Palaeogene species.
This paper forms part of a larger study on West African ostracods. The Loculicytheretta material is deposited in the Geological Museum of the University of Amsterdam, collection nos. K 7530 -K 7540 and K 7550 -K 7556.

RESULTS
In three samples from the coast of Senegal and The Gambia, a species of Loculicytheretta was found that appeared to be identical to the species van Morkhoven (1962/63) reported from the Subrecent of The Gambia. The samples were taken from the sediment surface at approx. 50cm below the low-tide line, from open beach, high energy environments. A volume of about lOOcc, consisting of siliceous fine to medium grained sand, was examined. In all samples, very rich ostracod faunas were found; other constituents were shell debris and benthic foraminifera.
The same Loculicytheretta species was found in a sand sample collected at a depth of 133m off the coast of Guinea. This sample was the shallowest out of a series of boxcore grabs collected during Geotropex '83 ("Meteor" expedition no. 65). The single carapace found in this material has a smooth, polished surface, probably resulting from downslope transport.
This sample also yielded anotner representative of the genus: L. aff. L. pavonia (sensu Bismuth et al., 1978), which also had a smooth surface and may also be displaced. The latter form closely resembles Loculicytheretta pavonia (Brady), but is distinguished by having four loculi, one of which is often poorly developed. Bismuth et al. (1978) reported this form from a number of West African localities and a description and Loculicytheretta pavonia (Brady, 1866) 1. Spanish coast, Recent: van Morkhoven, 1962/63;McKenzie, 1973;Doruk, 1974;Athersuch & Bonaduce, 1976;Bismuth et al., 1978;Carbonel & Colin, 1982. 2. Ligurian Sea, Pleistocene: Ruggieri, 1954. 3. Aegean Sea, Recent: Brady, 1866Muller, 1912;Barbeito-Gonzalez, 1971. 4. Eastern Levant, Recent, Plio-Pleistocene: McKenzie, 1973Doruk, 1974;Athersuch & Bonaduce, 1976;Athersuch, 1979. 5  discussion of its affinities was given. The suggestion was made by these authors that this form, as well as the form with six loculi, are Pleistocene offshoots of ancestral L. pavonia. L. pavonia ranges from Pliocene to Recent and is known from a considerable number of localities in t h e Mediterranean area (Fig. 1). Benson & Sylvester-Bradley (1971) reported the species from samples collected in the Alboran Sea at depths of 620m and 700m as its westernmost occurrence. A new locality that can be added to the list is the Gulf of Cadiz (approx. 12km W. of the Strait of Gibralter), where it was found in three samples collected during a cruise in 1982 with B/O Cornide de Saavedra (Istituto Jaime Almeria, Barcelona).

DISCUSSION
The presence of L. pavonia in material from the Gulf of Cadiz is the first report of this species from outside the Mediterranean. It is, however, obvious that the faunas found in these samples do not represent a biocoenosis. The species was found in three surface samples (upper 2cm), collected at depths of 510m, 836m and 1166m. The mixed origin of the material can be inferred from the presence of ostracod species characteristic of neritic Mediterranean environments (such as Semicytherura sulcata (G. W. Miiller) and Cytheretta adriatica Ruggieri) , in combination with other species that are found today only in colder, more northern environments (Finmarchinella angulata (Sars), Cythere lutea 0. F. Miiller). Bottom currents through the Strait of Gibralter will have picked up material moving downslope, and subsequently transported this over the sill into the Atlantic. It cannot be decided whether such a westward transportation, or a reworking of older sediments, or even a combination of both processes can explain the reported occurrences. But one thing that can be safely assumed is that Loculicytheretta pavonia, being a typical inhabitant of very shallow water (Bismuth et al., 1978), is not able to survive in deeper waters. The present-day Mediterranean watermass circulation is of an anti-estuarine type; surface currents bring in water from the Atlantic, bottom waters leave along the floor of the Strait of Gibralter (Fig. 2). Thus, migration of a strictly shallow water genus out of the Mediterranean as a result of a current pattern as it exists today is unlikely. There is, however, evidence that in the Early Pliocene this circulation pattern was reversed, with a wide and deep connection situated approximately where Gibralter is Mediterranean watermass circulation (after van Harten, 1984). A. Modern type anti-estuarine system: surface currents enter the basin, deep waters leave along the floor of the Strait of Gibralter. B. Estuarine system, model probably applicable until the Late Pliocene: surface currents are directed towards the Atlantic, deep waters flow over the sill into the basin. Broad arrows symbolise the balance between evaporation (open arrows) and freshwater input (black arrows).
now, but extending southward into Morocco and northward into southern Spain. During this period, deep water ostracods could invade the Mediterranean (Benson & Sylvester-Bradley, 1971;Benson, 1973Benson, , 1976van Harten, 1984). It is considered that at the same time, facilitated by the outflowing surface currents directed towards the Atlantic, shallow water genera, such as Loculicytheretta, could have enlarged their realm to include not only the southern beaches of the Mediterranean, but the adjacent areas in Northwest Africa as well. Subsequent speciation as a result of adaptation to slightly different environments may have given rise to a number of closely related forms, distinguished in particular by the number of loculi.
But where, at the beginning of the Pliocene, did the genus come from? The distribution of the post-Palaeozoic loculate ostracods shows considerable gaps, both in a horizontal and in a vertical sense. A total of 22 species is known (Bismuth et al., 1978); six of these are Pliocene to Recent and found along the coasts of West and Northwest Africa and in the Mediterranean. This is the "pavonia" group considered in this paper. A second group, with four species, is restricted to the Miocene of Southwest France, Portugal and North Africa. These two groups together were named group I1 by Carbonel & Colin (1982) and these authors suggested for this group an origin different from that of yet another, strictly Eocene group, which is found predominantly in North Africa and includes at least 12 species.
Oligocene species of loculate Ostracoda are not known and this is one reason why Carbonel & Colin (1982) make a clear distinction between the Palaeogene and Neogene species. For the Palaeogene species group, they proposed to retain the genus Heptaloculites Ruggieri, 1963, whereas the Neogene taxa were allocated to Loculicytheretta Ruggieri, 1954. Another break in the record of Loculicytheretta can be found between the Late Miocene and Early Pliocene. Although the record of the genus is continuous, no species is known to be present in both periods. The end of the Miocene is marked in the Mediterranean by the Messinian Salinity Crisis, and it seems probable that the break in the Loculicytheretta lineage correlates with this catastrophic event. As suggested by Benson & Sylvester-Bradley (1971) for deep-water ostracods, the development of new faunas, after refilling of the basin, may have started with forms that invaded the basin or survived in isolated areas. Possibly the shallow water species Loculicytheretta pavonia evolved from such a Miocene survivor. After the end of the Salinity Crisis, immense areas with a suitable habitat were generated, and a rapid occupation of these areas could be effected, followed by the migration of the genus into the Atlantic. Here, from the ancestral Loculicytheretta with three loculi, other Since the Late Pliocene reversal of the current system, the Strait of Gibralter acts as a barrier to ocean-directed migration of the shallow water inhabitant L. pavonia. taxa evolved, of which most developed more than three loculi (Fig. 3). Pliocene Loculicytheretta are known from the Mediterranean (Doruk, 1974;Yassini, 1979b) as well as Morocco (Carbonel & Colin, 1982).
We imagine that when in the Late Pliocene the circulation pattern reversed and the wide and deep connection with the Atlantic was reduced to no more than a narrow strait, a wedge was driven in the until then continuous distribution of the genus around northern Africa. This effect was intensified by the drastic climatic changes that followed and Loculicytheretta pavonia became isolated from its Atlantic descendants permanently.  lateral view (x90). Specimen K 7.5.56, offshore Guinea; specimens K 7530 and K 7.539, type locality.

Remarks.
The triloculate species Locuficytherettu pavoniu is reported from quite a number of Recent localities in the Mediterranean (see Fig. 1 in this paper; Athersuch & Bonaduce, 1976;Bismuth et al., 1978). Doruk (1974) mentioned its presence in the Plio -Pleistocene of Italy and Turkey, and Yassini (1979b) in the Pliocene of Algeria. The stations in the Gulf of Cadiz where it was found are the following: Station G 127, depth 510m, lat. 36"10,2'N, long. 06"41,9'W (two female valves); Station G 128,depth 836m,lat. 35"57,0'N,long. 07"00,1'W (one male carapace); Station G 129, depth 1166m, lat. 35"50,2'N, long. 07"15,7 Three longitudinal ridges are present, dorsal one shortest, medium and ventral ones connected near the posterior margin. This feature is especially distinct in males and juveniles, less so in females. Surface covered with numerous pits, arranged in a characteristic pattern of rows. Intensity of the pitting, particularly of the females, variable, as nearly smooth to reticulate surfaces were found.
Distribution. The species was found in three samples from West African beaches; Cap Skirring (Senegal), Bakau and Fajara (both Cape St. Mary, The Gambia). One carapace was found in a sample from deeper water off Guinea; Van Morkhoven's report (1962/63) is also from The Gambia. Possibly also Morocco (Pliocene, see Remarks). Remarks. The females with their characteristic loculi cannot be mistaken for any other Recent ostracod species. Males of L. aff. L. pavonia seem to be quite similar, according to the illustrations in Bismuth et al. (1978). Unfortunately, we have no males of this form in our collection. Males of L. pavoniu have a different ornamentation: the pitting is much finer (especially along the posterodorsal margin), and the ventral ridge is clearly threefold.

Explanation of Plate 2
Figs. 1-11. Locuficytherettu morkhoveni sp. nov., fig. 1, 9  fig. 11, 9 LV, K 7535, hinge structure (x175). All specimens from the type locality. Oertli, in Bismuth et al. (1978, Addendum 3) mentions material that was sent t o him by P. Carbonel (University of Bordeaux I) from Upper Miocene and Lower Pliocene sections of Morocco. Three species appeared t o be present; two of these ( L . sp. 1 and L . sp. 2) were later described by Carbonel & Colin (1982) who assigned them an Early Pliocene and Pliocene age. T h e third species, represented by one badly preserved specimen only, probably also from the Pliocene, was, according to Oertli (op. cit.), most likely L. n. sp. van Morkhoven, 1963. If that identification is correct, it implies a considerable extension of the range and distribution of the new species. Dimensions. Length males 0.63-0.67mm, height males 0.30-O.33mm; length females 0.65-0.7 lmm, height females 0.33-0.36mm.