Sunday, 4 June 2006

Ring them bells

I visited Kangaroo Island, South Australia, a few weeks ago with a bunch of friends. We had a picnic at Point Ellen on the island's south coast. Point Ellen lies at the western edge of Vivonne Bay, recently voted the world's best beach apparently. It's not bad, I guess, but there's a lot of nice beaches around Australia. Broad expanses of sand don't do much for me.

Vivonne Bay is interesting for more than its pulchritudinous shoreline. It is the type locality for the Point Ellen formation, which was laid down in the Early Pleistocene during a period of higher sea level. The rocks are stuffed full of shells. Most of them are bivalves, almost as big as your outstretched hand, but there are also many snails. Among them is Nerita milnesi Ludbrook, an extinct relative of Nerita atramentosa Reeve. Nerita atramentosa is almost ubiquitous on rocky shores in southern Australia and New Zealand. Nerita milnesi seemed to have been equally abundant in the Early Pleistocene but has now disappeared.

Campanile symbolicum Iredale is also found as a fossil in the Point Ellen formation. The picture above shows a shell that has been worn away along the midline. Those circles are cross-sections through the whorls. (For scale, those small grey littorinid shells are about 1 cm long.)

Unlike N. milnesi, C. symbolicum is alive and kicking. Well, crawling. Since the Pleistocene, its range has shrunk in a serious way. Although no longer occurring at Kangaroo Island, it is still found in SW Western Australia between Geraldton and the Recherche Archipelago.

Campanile is the only surviving member of the Campanilidae, a family that thrived in the late Mesozoic and early Cenozoic. It's been suggested that the family's dramatic decline in diversity might have been due to competition with other herbivorous snails (conchs in the family Strombidae). But no one really knows. It's nice to think that they're still hanging on in Australia.

[A huge thanks to MM for providing the photograph of C. symbolicum]

Read more
Houbrick, R.S. (1981). Anatomy, biology and systematics of Campanile symbolicum with reference to adaptive radiation of the Cerithiacea (Gastropoda: Prosobranchia). Malacologia 21: 263–289.

Ludbrook, N.H. (1983). Molluscan faunas of the Early Pleistocene Point Ellen formation and Burnham Limestone, South Australia. Transactions of the Royal Society of South Australia 107(1): 37–49.

Ludbrook, N.H. (1984). Quaternary molluscs of South Australia. Department of Mines and Energy, Handbook no. 9.

Milnes, A.R., Ludbrook, N.H., Lindsay, J.M. & Cooper, B.J. (1983). The succession of Cainozoic marine sediments on Kangaroo Island, South Australia. Transactions of the Royal Society of South Australia 107(1): 1–35.