Dating the Cassis rufa shell from the Mousterian levels of the Grotte du Prince, Monaco.

Hayden, Brian ; Nelson, D.E. ; Cataliotti-Valdina, Jean 等

A fragment of Cassis rufa shell, in modern times a species of the Indian Ocean, was reliably reported from the deep Mousterian deposits excavated at the beginning of the century from the Grotte du Prince, Monaco. Because its known habitat is so distant and exotic, there has always been question about the specimen's authenticity. A radiocarbon determination shows it to be recent, and no evidence for long-distance movement of shell in the European Middle Palaeolithic.

In 1906, Boule (1906: 123) and Villeneuve (1906: 244) reported an unusual shell fragment among the faunal remains from one of the lowest hearths in the deep Mousterian deposits excavated from the Grotte du Prince, one of the Grimaldi Caves along the Mediterranean coast of Monaco. The excavations were conducted from 1895 to 1902 under the direction of Le Chanoine Villeneuve. The Mousterian deposits were contained in a talus cone that progressively filled the cavern almost to its 20-m height. Although the deposits at the entrance to the cave had been disturbed by railway and quarrying activities, the inner deposits were 'intact' and very voluminous. Five distinct hearth zones were identified in the interior; hearth D was the second from the bottom. It was from this hearth that the fragment of Cassis (Cypraecassis) rufa was reportedly excavated. In referring to this shell, Villeneuve (1906: 244) stated in his report that: 'Il n'y a pas de doute sur l'origine de cette piece, elle gisait dans le foyer D, par consequent tres bas'. However, in his report, Boule (1906: 123) wrote a long footnote expressing surprise at the recovery of this specimen from Mousterian deposits in Monaco, as it was not a Mediterranean species; the closest known habitat was the Aden coast and the Indian Ocean, implying long-distance transport at a very early time.

The implications of this find remain of considerable interest today, and some prehistorians have expressed scepticism about the Mousterian age of the specimen in present-day reflections similar to those of Boule (Renfrew 1986; Reese 1989; 1991; pers. comm.; D. Bar-Yosef pers. comm.). While the transport of shells and other materials (e.g. pyrites) up to 90 km from their source has been documented from Mousterian sites in France and the Levant, such transport was still relatively rare (O. Bar-Yosef 1989: 178; pers. comm.; D. Bar-Yosef 1989: 170; J.-M. Geneste pers. comm.; see also Hayden 1993). The distance of transport implied by the Cassis shell would have been most unusual for a Mousterian assemblage, and indeed far greater than anything else documented for the later Upper Palaeolithic and even Neolithic periods. It is difficult to reconcile such movement with our current understanding of Palaeolithic hunter/gatherer mobility and socio-economic organization.

Subsequent research has not added new information to resolve this difficulty. Malacologists agree on the identification, as the species has a very distinctive morphology, size and colouring. No closer present-day habitat has been identified (Abbott 1969), and it seems unlikely that a species from warm present oceans would have been found further north in the Pleistocene. It seems indisputable that the only possible means that the shell could have been deposited at the Grotte du Prince was via human transport.

The remaining question is the age of the sample, and fortunately that can be tested. The specimen itself is in the collections of the Musee d'Anthropologie Prehistorique de Monaco, and we received permission to photograph and remove a small portion for radiocarbon dating.

Radiocarbon dates on marine shells may be affected by two potential problems:

1 Exchange of the shell carbonate with carbonates in the ground water. In the calcareous terrain of southern Europe, ground-water carbonates are likely derived from the old calcareous rocks and modern biogenic and atmospheric carbon dioxide. Any exchange processes would thus make very old shell samples appear considerably younger, and very young shell samples appear somewhat older.

2 Since marine organisms obtain their carbon from the ocean, the radiocarbon ages of the organisms are apparently too old. In this case, that is not a problem, as the magnitude of this affect is seldom more than a few hundred years, and the problem here is to determine whether the shell age is, or is not, many tens of thousands of years.

The exchange problem is the primary difficulty. Our approach was to obtain a sufficiently large piece of the shell so that we could examine its microscopic structure and remove a portion from the interior. This portion would then be radiocarbon dated using the accelerator mass spectrometry (AMS) method, with one of two anticipated outcomes:

i a very young age, close to the time of excavation if the sample in some way reflects excavation problems, or

ii an intermediate or apparently Mousterian age which would indicate that the sample was either truly of some antiquity or that it had been contaminated by exchange.

If the latter result were obtained, then we would have to do further work to establish the true age. To do this, we planned to use a new method for radiocarbon dating protein (Nelson 1991) which would yield an age not affected by carbonate contamination.

The fragment removed for analysis (FIGURE 1) had approximate dimensions of 2 x 1 x 1 cm and weighed 2.5 g. Examination of the newly-cut surface clearly showed that the shell interior remains in pristine condition. The surface is lustrous and retains the brownish-pink colouring for which the shell is named. Detailed, individual growth layering is clearly visible under the microscope. Our immediate impression was that the shell is remarkably well preserved if it is indeed 50,000 years old.

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A small portion of the sample was cleaned in distilled water and placed in an evacuated reaction vessel. Phosphoric acid was added, and the first carbon dioxide released was pumped away, so that we would be sampling only interior material. The subsequent [CO.sub.2] was collected and converted into two portions of graphite for separate AMS determinations. These measurements were made in the normal fashion (Davis et al. 1990) at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, USA. While the normal procedure is to determine the stable isotope ([delta][.sup.13C]) value for each individual sample, we have here assumed a value typical of marine shells. The uncertainty in so doing is at most a few decades, which is of no consequence to this study. The two ages obtained are given in TABLE 1.

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The two measures obtained are self-consistent, giving a weighted average of 670 [+ or -] 50 Before Present. Can this be a contaminated Mousterian shell? It is highly unlikely. For a shell of true age 40,000--50,000 years to have an apparent age of 670 years, > 90% of its carbon must have been exchanged for carbon of Modern [.sup.14C] concentration. This is highly unlikely even if the shell could have been contaminated by groundwater containing [.sup.14C] from nuclear weapons testing, and in this case the specimen was collected a half-century before the advent of nuclear fission. Clearly, the shell is recent, not of Mousterian age.

How recent? As discussed above, to obtain the true age we must adjust the age obtained for the ocean-water effect. Using data on the apparent ages of recent (pre-bomb) shells from the Indian Ocean area (J. Southon pers. comm.) and the calibration provided by Stuiver et al. (1988), we obtain a possible calendrical age range for this sample extending from about 1700 AD to the time of the excavation.

Thus, the shell is not evidence for Mousterian long-range trade, but rather it appears to be an example of the problems inherent in archaeological excavation. How could it have got into hearth D?

The measured age provides only fuel for speculation. Cassis rufa shells were traded into the Mediterranean during this time period; as Villeneuve (1906: 244) himself notes, 'C'est un morceau d'une grande coquille bien connue dans nos ports ou les marins l'apportent de l'ocean Indien ou du Pacifique; Cassis rufa, le casque rouge utilise en Italie pour la confection des camees'.

How such an item would then find its way into the cave is open to conjecture. It may have been a prank. Even today, various objects that are considered to be 'obvious' fakes are sometimes slipped into the matrices of unsuspecting co-workers. Unfortunately, these jokes are not always evident to everyone involved, and once taken seriously by prominent investigators, it becomes difficult for perpetrators to admit responsibility. Fortunately, with modern dating techniques, it has now become possible to correct some of these distortions of our archaeological data.

Acknowledgements. We are extremely grateful for the generous cooperation of the Musee d'Anthropologie Prehistorique de Monaco and for the kind assistance of Daniella Bar-Yosef, Henk Mienis and David Reese. We thank N. Shackleton for critical comments and suggestions. The measurements and interpretations were undertaken at LLNL with the kind permission of Jay Davis and the expert assistance of John Southon and other staff members.

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