No. 85: Jan-Feb 1993
"Zoologists and geologists agree that specimens of Mya arenaria (the American soft-shell clam) from the Holocene found in Europe first appeared in the sixteenth century, after the voyage of Columbus. But we have dated a sample from the Kattegat region on the east coast of the Skaw in northern Jutland, Denmark, that predates Columbus's voyage. This result implies that contact between America and Europe existed before the sixteenth century."
In fact, one sample of the American soft-shell clam was carbon-dated by the authors of the above paragraph at 1245-1295±1 standard deviation. This particular shell was discovered in the sand barrier farthest from the Danish coast. Since it seems highly unlikely that the larvae of the American clam could have swam across the Atlantic, the authors wonder if the Vikings might have carried them home. (Petersen, K.S., et al; "Clams before Columbus," Nature, 359:679, 1992.)
Comment. Certainly the Vikings might have consumed these clams on their voyages to the New World, but why would they or any other explorer carry live clams or their larvae back to the Old World? Or was the transfer accidental in ships' ballast?