Golden Wonder! Rare fossil trilobite preserved in stunning detail

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.

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.

C.E. Beecher's 1893 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.

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.