Museum Notebook: Whanganui beaches rich source of fossils

Strolling along a beach on a fine day is a popular activity in any season. Every beach has its own special qualities, whether it is the colour of the sand, the range of shells washed up or the seaside cliffs.

One of the unusual and interesting things about a typical Whanganui beach is the numerous fossils, washed out of the soft sedimentary cliffs that characterise the Whanganui region.

Fossils are easy to spot if they are still fully encased in rock or have fragments of rock clinging to them. Sometimes fossils look very much like any other shell, so it can be hard to tell if they were molluscs that lived recently, or if they were alive millions of years ago.

One of the most common fossils along the Whanganui coastline are extinct oysters (Tiostrea or Patro species). The shells are usually a silvery-grey colour. They are quite smooth and almost circular in shape, very unlike the corrugated uneven shells of a modern oyster.

Sometimes both sides of the oyster can be found, cemented together by rock. Occasionally fragments of a much larger species of oyster can be found. These have eroded from further upriver, where they can be seen exposed in the cliffs above the Whanganui River Rd between Parikino and Ātene.

The huge oysters (Crassostrea ingens) inhabited an offshore reef around 2.5 million years ago. For around 20,000 years, a thick band of these shells accumulated in an area 50km across and many metres deep. Just one of these massive oysters would be more than enough for a meal.

Other commonly found shells are the scallop. Sometimes they are the shells of modern scallops, but a closer inspection might reveal a slightly different profile. The now extinct Pecten novaezelandiae tainui has a more boxy, square profile within the corrugations of the shell. This species is recent in fossil terms, having gone extinct only 0.4 million years ago.

A slightly older scallop, the Pecten novaezelandiae marwicki from 0.6 million years ago, has corrugations with a more softly rounded profile. Usually, by the time these recent fossils have eroded out of the surrounding rock, they have also broken, so it may be only a fragment that washes up on the beach.

Fossilised sand dollar. Whanganui Regional Museum Collection ref: 1802.8692

One of the most interesting finds on Whanganui beaches are fossilised sand dollars (Fellaster species). Often, they are still covered in rock, but their circular shape makes them noticeable. Occasionally the fossil has been washed clean and looks similar to a modern echidnoderm, the snapper biscuit, except that it will be very hard and heavy, compared to the lightweight and fragile modern example.

All these locally found fossils are part of what is known as the "Whanganui series", with the most recent subgroup being named "Castlecliffian", and a slightly older sub-group being "Nukumaruan".

Whanganui Regional Museum's exhibition Young Land - Hāhā te Whenua includes examples of commonly-found Whanganui series fossils. If you find some and you want to know more, comparing your fossils with the examples in the museum's geology cases would be a good place to start.

• Margie Beautrais is the educator at Whanganui Regional Museum.