Lamp shells, old and new

Press, 17 January 1986, Page 16

Lamp shells, old and new

Fossils are a constant source of fascination to nearly everyone, young or old, professional or amateur, there is something intriguing about a once living thing beautifully preserved by being “turned into stone.” Because there are so many fossils around, often easily collectable, they are among the more common items featured in many of the museum displays that I see throughout the Canterbury region. It is no coincidence that most fossils are of marine rather than terrestrial origin. This is because the chances of something which dies on land being preserved long enough to become fossilised are very small indeed. The odds in the sea are much better, where dead organisms fall into the encompassing muds of the sea floor. So whale fossils commonly represent the mammals, as do penguins the birds, while sea shells are the most common fossils of all.

Some of the most interesting shell fossils occur in great abundance throughout the rocks — mostly limestones — of the South Island. These are the brachiopods or lamp shells. The brachiopod has a bivalved shell and is often thought to be just another one of the bivalved (twoshelled) molluscs commonly found today, such as oysters, scallops, pipis. In fact, not only do the brachiopod shells differ in many respects from those of molluscs, but the respective animals which live within are no relation to each other at all, each belonging to a completely different group or phylum.

What this means is that despite the outward similarity of their

bivalved shell, a brachiopod is related no more closely to an oyster than it is to a polar bear or an earthworm. Although there are many different species of brachiopod fossils to be found, most have some common features by which they can be distinguished from fossilised molluscs. The two valves or shells are usually unequal in size and, where they are hinged together, one of the valves may protrude or curve around the hinge like a beak. In most brachiopods one of the valves can be seen to be pierced near the hinge (it is actually the beak part which is pierced) by a small hole called a foramen. In life, a fleshy stalk is passed out through this hole by the animal to anchor itself somewhere firm on the sea-floor. It is from this hole that the brachiopod derives its popular name of “lamp shell” because of the shell’s resemblance to a primitive oil lamp with a hole from which the wick protruded. Another important difference between the bivalved molluscs and brachiopods is that in the former, the shells fit around the left and right sides of the animal, while the brachiopod wears its shells above and below its body. Brachiopods are a fascinating group with a long history; they have actually existed at least since the Cambrian period, more than 500 million years ago. Their heyday was probably between 100 and 200 million years ago. Since then they have declined in numbers, largely replaced by the enormous mollusc group. In New Zealand, besides the fossils, we are fortunate to have

some of the world’s last colonies of living brachiopods At Stewart Island is a small, pinkish, smooth shelled species often referred to as a “toenail

shell.” Around Otago coasts is a small dark, ribbed form which resembles a small fan shell. And there is a colony of brachiopods living in Lyttelton Harbour.

By

BEVERLEY McCULLOCH