A fossil Cenozoic monoplacophoran MARC0 TAVIANI, BRUNO SABELLI AND FRANC0 CANDINI

Taviani, M., Sabelli, B. & Candini, F. 1990 04 15: A fossil Cenozoic monoplacophoran. Lethaia, Vol. W H A I A 23, pp. zis216. Oslo. ISSN oo24-iiu.

A few shells of Micropilina (Mollusca, Monoplacophora) have been found in middle Pleistocene (Sicilian) epibathyal clays at Archi (Reggio Calabria, southern Italy). The species appears conchologically indis- tinguishable from the Recent Atlantic taxon Micropilina minuta Wartn, 1989. This finding represents the first record of a Cenozoic fossil monoplacophoran.

Marco Tauiani, Istituto per la Geologia Marina del CNR, Via Zamboni 65, 140127 Bologna, Italy; Bruno Sabelli & Franco Candini, Museo di Zoologia dell' Uniuersita', Via S . Giacomo 9, 140126 Bologna, Italy; 15th October, 1989.

Primitive molluscs belonging to the class Mono- placophora (Odhner in Wenz 1940) were con- sidered to be exclusively Cambro-Devonian in distribution until the discovery, in the 1950s, of a living representative in the abyssal depth of the Peru-Chile Trench (Lemche 1957). Since then, records of Recent monoplacophorans have multi- plied (Filatova et al. 1969, 1976; Rokop 1972; Rosewater 1970; Lowenstam 1979; McLean 1979; Bouchet et al. 1983; Moskalev et al. 1983; WarCn 1988, 1989) and at the present time at least 14 species are known from the world oceans (WarCn 1989). Although monoplacophorans have been recorded from a number of Palaeozoic outcrops (e.g. Knight & Yochelson 1960; Runnegar & Jell 1976; Berg-Madsen & Peel 1978), no post- Devonian records of fossil monoplacophorans have been published so far.

We report here on the finding of Micropilina (a genus recently introduced by WarCn 1989) from the upper Cenozoic of southern Italy: our finding also represents the first palaeontological docu- mentation of the family Neopilinidae (Knight & Yochelson 1958) previously known only for the Recent.

The finding The discovery took place in 1987 during the ARCH1 87 scientific expedition in southern Italy, when a team of palaeontologists collected fossils in a quarry at Archi, near Reggio Calabria, located at 38"09'08"N Lat N - 15"39'09 Long E. The fossiliferous site consists of a c. 30m thick sequence of epybathyal clays of middle Pleis- tocene (Sicilian) age relatively rich in scleractinian

and mollusc remains (Placella 1978; Guadagno et al. 1979; Micali & Villari 1986).

Six adult shells in good state of preservation were found by sieving 1 m3 of a sandy-clay bulk sample. Shell characteristics of the six specimens (Fig. 1) undoubtedly reveal that the molluscs belong to the genus Micropilina WarCn, 1989. Compared with Recent monoplacophorans (Cesari & Guidastri 1976, 1979; Bouchet et al. 1983; WarCn 1988, 1989) the Cenozoic fossils show obvious shell analogies with M. minuta WarCn (1989), one of the four monoplacophorans reported so far for the North Atlantic (Daut- zenberg & Fischer 1896; Di Geronimo & Li Gioi 1980; Bouchet et al. 1983; Cesari et al. 1987; Waren 1988, 1989; Smriglio et al. 1989). Our specimens show evident pores on the external surface, a feature known in M. minuta and in two Atlantic Rokopella, i.e. R. zografi Dautzenberg & Fischer, 1896, and R. goesi WarCn, 1988. The specimens described by WarCn (1989) show the presence of 5-7 'pits' in the central areaof the apex. However, possibly because of partial shell dissolution, SEM magnification of the apical area of the best preserved specimen from Archi failed to show this same feature in the Cenozoic popu- lation.

No live specimens of Micropilina minuta have yet been found. Thus, ultimately, its attribution to Monoplacophora rests on the shell structure, which is typical of the class (Bouchet et al. 1983). M . minuta is so far known only from eight shells trawled off Iceland between 770 and 926 m depth (Waren 1989). The present finding considerably expands the latitudinal range of this taxon south- wards. However, the fact that this species is found in Sicilian deposits of the Mediterranean basin

214 Marco Taviani and others LETHAIA 23 (1990)

Fig. 1. Micropiha from the middle Pleistocene section of Archi. 0 a. Shell (bar = 200 pm). 0 b. Detail of the internal nacreous layer (bar = 20 pm). 0 c. Apical area showing the semi-globular larval shell scar (bar = 100 pm). 0 d. Pores on the external surface of the shell (bar = 50 pm). The specimens are deposited in the Malacological Laboratory of the Zoological Museum, Bologna (cat. no. 007053).

LETHAIA 23 (1990) A Cenozoic monoplacophoran 215

may indicate that its introduction to this basin took place during one of the many incursions of 'boreal' species during the colder phases of the Pleistocene (see Malatesta & Zarlenga 1988). Thus, this species can perhaps be considered another boreal guest to be added to the many others already cited for the Mediterranean. The minute size of M . minuta and the fact that the inventory of the Recent Mediterranean deep sea mollusc fauna is far from completed (Bouchet & Taviani 1989) do not firmly establish whether this monoplacophoran is still living or is now extinct in this basin. The only other Monoplacophora recorded for the Mediterranean is Rokopella zografi (Dautzenberg & Fischer, 1896) recently found as empty shells dredged in the western basin (Smriglio ef af. 1989, with references).

The finding of an extant taxon in the Sicilian deposits of Archi is not surprising because most of the accompanying fauna is known from the Recent of the North Atlantic, where it lives in depths in excess of c. 500 m (Placella 1978; Micali & Villari 1986; Rindone & Vazzana, in prep.). Moreover, it is now well established that the Recent deep-sea mollusc fauna of the Medi- terranean basin was established as early as the beginning of the glacial Pleistocene at least (Raffi & Taviani 1984).

A remark of interest is that the palaeontological data from Archi further indicate that geologically recent monoplacophorans are typically distrib- uted, with rare exceptions (McLean 1979), in deep water. By contrast, Palaeozoic records are all related to comparatively shallow epishelfal seas (Knight & Yochelson 1960). Although we cannot rule out that even in the past mono- placophorans had a wider vertical range, it seems reasonable to hypothesize that, possibly because of ecological pressure (competitors?), Cenozoic to Recent species are confined to outer slope down to abyssal depths. This depth shift seems to parallel the one hypothesized for other primitive groups. such as hydrothermal vent limpets (McLean 1985).

Acknowledgements. -The ARCH1 87 expedition was co-organ- ized by the Museum of Zoology (University of Bologna) and thc Institute of Marine Geology (C.N.R., Bologna). The field activity was carried out by M. Taviani, M. Bigazzi, F. Candini, P. Crovato and N. Taviani. V. Rindone and A. Vazzana kindly helped with sampling. A . Waren and M. Marani critically

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