Neritilia neritinoides, INHABIT FRESH OR

Lozouet, Pierre, 2004, The European Tertiary Neritiliidae (Mollusca, Gastropoda, Neritopsina): indicators of tropical submarine cave environments and freshwater faunas, Zoological Journal of the Linnean Society 140 (4), pp. 447-467 : 456-457

DID NERITILIA NERITINOIDES INHABIT FRESH OR

BRACKISH WATER?

In the Lower Oligocene (Orist), four juvenile specimens of Neritilia have been collected on a beach sand deposit and are very abraded. In the Upper Oligocene, N. neritinoides has been found in a very fine sand layer (5–10 mm thick) included in a marly sequence. The predominantly marine molluscs are mixed with species which inhabit fresh and brackish water, suggesting that they originate from a variety of environments ( Lozouet, 2003). The temporarily accessible outcrops of Pessac (Lower Miocene) have also yielded a rich mixed fauna ( Lozouet & Gourgues, 1995) in a sequence 4 m thick of very fossiliferous calcareous sand, including cross-bedded and shell lenses, indicating the influence of tidal currents. The studied sample of Cap de Bos contains 1345 gastropod specimens representing 125 species. Of these 114 (1123 specimens) are marine, six (129 specimens) are freshwater and five (93 specimens) are possibly brackish-water species. Brackish and freshwater specimens account for 16.5% of the total fauna. This layer also contains many pieces of lignitized wood and a few fossils of mammals and crocodilians have been reported ( Duranthon & Cahuzac, 1997), which confirms the influence of a river.

In the Lower Miocene (Lucbardez ‘Mondiet’), N. neritinoides occurs at the top of a 5-m thick section with alternating sandy marls, limestone banks and sandy limestones. It is one of the commonest species, and the specimens are perfectly preserved. Analysis of the variation of the faunistic components of this outcrop should help provide a more precise description of the environment of N. neritinoides . The sequence from base to top is as follows:

(1) 0-35 cm, calcareous sandy marl with numerous potamid gastropods ( Granulolabium plicatum (Bruguière, 1792)) .

(2) 35-135 cm, sandy marls including two limestone banks (10 and 20 cm thick); molluscs are dominated by one carditid ( Bivalvia) species accumulated in the shelly limestones.

(6) 320-370 cm, brown-red humic sand, with very fine lignitic beds including Melanopsis .

(7) 370-395 cm, weathered clay with preserved fauna ( Melanopsis , Congeria ) only at the bottom.

(8) 395-495 cm, Quaternary sand, ‘Sable des Landes’.

Layers 1 and 2 were probably deposited under shallow marine/lagoon conditions as indicated by the genus Granulolabium (see Lozouet et al., 2001a). This Lower Miocene biocenosis, of low diversity and dominated by Venericardia and Granulolabium , strikingly resembles the lagoonal community described by Gitton, Lozouet & Maestrati (1986) at the top of the marine sequence of the Lower Oligocene of the Paris Basin (Ormoy fauna).

In layer 3, the shells are very scattered. A more careful study of a sample rich in mollusc fragments revealed that the dominant species are the small Ervilia and Lentidium bivalves. A species of Lucinidae was also recorded. The most commonly found gastropods are young specimens of Hydrobia cf. aturensis and Granulolabium plicatum . In the finest fraction, the species Pelecydion sp. (Pelecydionidae) is unusually abundant. Species of this tropical family are generally considered as very rare. However, during the ‘Expédition Montrouzier’ to New Caledonia ( Bouchet et al., 2002), an association of small Mollusca including Pelecydion, Iravadiidae , Cornirostridae was collected in coarse beach sediments (C. Erseus, pers. comm.). The bivalve Lentidium is a characteristic genus of the ‘Upper clean sand’ assemblages of the Mediterranean Sea ( Pérès, 1982) having been reported in the same environment since the Oligocene of the Paris Basin ( Gitton et al., 1986). As this corresponds to the surf zone of the lower beach, it concurs with the presence of Pelecydion sp.

Layer 3 may be interpreted as a spillover deposited on the subtidal sediment of the estuarine side of a littoral spit. Intrusion of material from a dynamic marine environment into relatively static and brackish areas is characteristic of storm deposits. An additional corroboration of this interpretation is the ecology of the more abundant species of layers 5–7, in decreasing order of frequency: Melanopsis dufourii Férussac, 1822 , Gyraulus balizacencis (Peyrot, 1931) , Neritilia neritinoides , Hydrobia spp. (three species). The predominance of Congeria basteroti (Deshayes, 1836) is significant. All these species are very well preserved. The numerous N. neritinoides have a glossy surface and details of the protoconch are clearly preserved. In contrast, the rare brackish elements of the malacofauna (estuarine or lagoonal) have been collected only in layer 5 ( Gastropoda: Granulolabium plicatum , Bivalvia: Anadara cardiiformis (Basterot, 1825) , Venericardia pinnula (Basterot, 1825)) , and are highly abraded.

Melanopsis dufourii Férussac, 1822 is very close to the extant M. praemorsa (Linné, 1758) , a circum-Mediterranean species living in running water (brooks, irrigation canals, rapids and springs) rather than stagnant water, and tolerating desiccation and moderate salinity ( Brown, 1994). Dreissenidae also live in freshwater and tolerate moderate salinity ( Archambault-Guezou, 1976). The Planorbidae , such as Gyraulus , are a typically freshwater element but the extant species G. laevis (Alder, 1838) (very similar to G. balizacencis ) may occur in inland waters close to the sea, tolerating slightly brackish conditions ( Kerney, 1999). Gyraulus species generally live in quiet water, among weeds. The two most common hydrobiid species do not belong to the brackish-water fossil group of Hydrobia andreaei Boettger, 1892 ( Lozouet et al., 2001a) but resemble hydrobiids that inhabit waters of low to moderate salinity. Another species is suggestive of the extant Mercuria confusa (Frauenfeld, 1863) , a hydrobiid mostly restricted to freshwater in estuaries and pools. I also add one species of Unionidae and the gastropods Acroloxus cestasensis (Peyrot, 1932) (Acroloxidae) and Ferrissia sp. (Planorbidae) ; these species belong to freshwater groups. The extant Ferrissia wautieri (Mirolli, 1960) lives in stagnant or slowly moving water with ample vegetation ( Kerney, 1999). A displaced terrestrial found in this assemblage species belongs to the genus Carychium (Ellobiidae) .

Analysis of the fauna of this sequence shows the evolution of the shallow water and lagoonal community towards a freshwater environment. As indicated by the good state of preservation of the molluscs and the habitats of their modern counterparts, layers 5–7 are freshwater deposits. This provides evidence that N. neritinoides belongs to a freshwater community probably able to tolerate slight salinity. Like the other species of Neritilia , it has a planktotrophic stage (as indicated by the protoconch); it has been postulated that this type of larval veliger, after hatching in freshwater, needs to reach the sea and then return to an inland environment (Kano et al., 2001). Due to the lack of palaeotopographic information one cannot be precise about previous environmental conditions where N. neritinoides has been collected, but it is presumed that it was an estuary or freshwater lake close to the sea.