Rhagor - Opening our national collections

Golden Wonder! Rare fossil trilobite preserved in stunning detail

[image: The Museum's golden fossil from Martin Quarry, New York State. Larger trilobite approx. 3 cm long]

The Museum's golden fossil from Martin Quarry, New York State. Larger trilobite approx. 3 cm long

[image: A tiny Triarthrus eatoni specimen lies next to the bigger one. ]

A tiny Triarthrus eatoni specimen lies next to the bigger one. Trilobites of various ages were fossilized together and must have lived in the same place. Only larvae are missing.

[image: A pair of delicate antennae and lots of legs are preserved in fine detail.]

A pair of delicate antennae and lots of legs are preserved in fine detail.

[image: C.E. Beecher's 1839 reconstruction of Triarthrus eatoni based on fossils from his Trilobite Bed. ]

C.E. Beecher's 1839 reconstruction of Triarthrus eatoni based on fossils from his Trilobite Bed. The legs have two branches, an inner walking leg and an outer gill with fine filaments.

Trilobites are common in the rocks in Wales, but this rare specimen differs from others in our collection. Preserved beneath the carapace are the legs and on the head a pair of delicate antennae ('feelers'). These features stand out vividly in gold against a black shale background. Such exceptional fossils give us great insights into how trilobites moved, fed and sensed the world around them.

All trilobites had legs and antennae when they were alive, but these were quite soft and usually rotted away before they could be fossilized. Most trilobite fossils are just parts of the hard exoskeleton or carapace and tell us little about the softer parts of the body.

Why is the trilobite golden?

The golden colour is because the animal has been fossilized in pyrite, also known as iron pyrites or Fool's Gold. Fossilization of soft body parts in pyrite is very rare, and is only known from a couple of places in the world. This particular fossil comes from rocks of Ordovician age (approx. 455 million years ago) from New York State in the USA. Soft-bodied fossils preserved in pyrite are also found in the much younger Hunsrück Slate in Germany, of early Devonian age (approx. 390 million years ago).

Pyrite is an iron sulphide mineral (FeS2), and it can form where there are low oxygen levels and lots of iron. The trilobites were probably swept up by an underwater avalanche and buried in deep sea mud.

The mud would have been rich in sulphates and dissolved iron, but low in oxygen. Sulphate-reducing bacteria would have helped decay the trilobites, releasing sulphides. The sulphides combined with the dissolved iron to form pyrite, which replaced or coated the trilobite tissues as they decayed.

Beecher's Trilobite Bed

Pyritized trilobites have been known from the famous Beecher's Trilobite Bed in New York State for over a century. The bed was discovered by amateur fossil collector William S. Valiant in 1892, but is named after Charles Emerson Beecher, an academic from Yale University to whom Valiant showed his amazing trilobite finds. Beecher leased the land between 1893 and 1895, and quarried out as many fossils as he could, until he thought there was nothing left to be found. He wrote many scientific papers about the trilobites until his untimely death in 1904. The trilobites were found in just one thin (4 cm) layer of rock, laid down around 455 million years ago, during the Ordovician period.

The Trilobite Bed was rediscovered in 1984 and since then, more beds containing golden trilobites have been found in New York State. In 2004, an amateur collector started searching about 50 miles away, and eventually found a rock layer of the same age containing trilobites. Our specimen comes from this new quarry, now known as Martin Quarry after its finder. Many important fossils have been found in Martin Quarry and studied by Professor Derek Briggs of the Yale Peabody Museum, and his colleagues.

Growing Up

Our specimen (Triarthrus eatoni) has a second, tiny trilobite next to the larger one. Trilobites grew from larva to adult by going through a series of moults. As they got older, they regularly moulted off their old exoskeleton to grow bigger. Many different sizes of Triarthrus have been found in the Trilobite Bed, but none of its earliest larval stage. Trilobites of various ages clearly lived together, but the larvae must have lived somewhere else. They may have floated around as plankton in the water column, while larger juveniles and adults lived on the sea bed.

Article by: Lucy McCobb: Palaeontologist

Article Date: 2 August 2013

3 comments

John Mason on 31 January 2014, 11:18

Hi Sue -

I've also found that pyrite decay is temperature-sensitive, so storage in a fridge will help slow the problem down a lot.

Cindy Howells (Geology Collections Manager, National Museum of Wales) on 23 September 2013, 09:10

Hi Sue,
The pyrite that you have had problems with is probably from much younger fossils - Jurassic ammonites etc. These trilobite fossils are preserved in a more stable form of pyrite and they don't seem to be at all sensitive to damp environments. We do keep it in a prtective box, just to be sure though.
Some pyritised fossils will decay no matter what, but others are more stable. As a general rule though, keep them very dry, in a dry room (not on a windowsill or bathroom). They react strongly to changes in humidity, so ideally they might be in a sealed plastic box with a sachet of silica gel beside them. This will absorb moisture and can be refreshed once a month or so by drying it out on a radiator.

Sue Harris on 18 September 2013, 20:32

This sounds great but how are you going to keep the pyrite from disintegrating? I have had many fossils and tried ways to stop the process happening with not much success.

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Glossary

Invertebrates
Animals without a backbone.

Trilobite
An extinct group of arthropods that lived in the seas from the Cambrian Period (520 million years ago) to the end of the Permian Period (250 million years ago). Their name comes from division of their body into three lobes.

Ordovician
Period of geological time from 488-443 million years ago

Arthropod
An invertebrate animal with a hard outer carapace and lots of jointed legs, e.g. insects, crabs.

Sulphate-reducing bacteria
Bacteria that break down organic matter when there is no oxygen present, producing sulphides as a by-product.

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