Extinct Land Snails from Henderson Island
Dr Richard Preece, Curator of Malacology here at the Museum of Zoology, writes about specimens he collected during field work to Henderson Island:
Henderson Island is an uninhabited island in the middle of the South Pacific. With an area of 37 km2, it is the largest island in the Pitcairn group, and differs markedly from the three other islands. Henderson is a raised coral atoll reaching just over 30 m above sea-level, far lower than Pitcairn (where the Bounty mutineers settled), which is volcanic, but much higher than either Oeno or Ducie, which are both atolls with minimal surface relief. Such uplifted atolls are not common and often support unique wildlife but many have been severely disturbed by humans causing the extinction of many species. Henderson is so remote that it has largely escaped such disturbance, which is why it has been designated a World Heritage Site.
In 1991 I was invited to join the Sir Peter Scott Commemorative Expedition to the Pitcairn islands. This was an international venture master-minded from Cambridge, and included a few other members of the Department of Zoology. Our aim was essentially to undertake a detailed biological survey of this unique ecosystem, conducted by a series of specialists over a 15 month period. I was particularly interested in the molluscs that lived there but also in the fossils from the recent past. Henderson, being largely composed of limestone, proved to be an excellent place to study fossils, both marine shells and minute species of land snail.
The uplifted peripheral rim of Henderson surrounds a shallow central depression that once formed a lagoon. Today the plateau of Henderson is largely covered in dense low forest, supporting several endemic species of plant and animal, but fossil clams and coral in growth position can still be found in the understorey. On the coastal slope, a number of small caves have formed, representing periods of marine erosion at various times during the uplift of the island. Some of these caves had been used by early Polynesians, who settled on the island for a few centuries over 500 years ago. Sediments blackened with charcoal (which could be radiocarbon-dated) containing artefacts and food debris provided clear evidence of this. Beneath the Polynesian levels were pure white cave sediments lacking archaeology but containing abundant bones of birds and snail shells. Most of these could be assigned to species that still occur on the island but some could not be matched up. They included at least half a dozen of land snail, and a species of flightless pigeon, that could not be found living on the island today, despite many months of intense search. These were all species new to science, several of which I have now described formally. It appears that the early Polynesian settlers caused more damage than previously supposed and that Henderson is not as pristine as once thought. It nevertheless remains a wonderful place, still supporting a range of endemic species.