Ancient trilobite stuffed itself with food almost to bursting point

For the first time, a trilobite fossil has been found with preserved gut contents, and an analysis of the specimen suggests the iconic ancient animal had a voracious and unfussy appetite.

Trilobites, arthropods that lived from the Early Cambrian to the end of the Permian Period (around 521–252 million years ago), were some of the earliest animals to scuttle through the sea. Scientists have uncovered tens of thousands of trilobite fossils over the centuries, but this example of the species Bohemolichas incola is distinct from the others because its gut contents are preserved inside it.

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“Most fossil trilobites are not dead whole animals, but rather shed carapaces from the moulting process, like modern crabs,” says Per Ahlberg at Uppsala University in Sweden. As a result, none of their internal organs are present in the fossil. But this trilobite, unearthed in the Czech Republic, was buried alive with a full gut in an undersea mudslide.

To find out what was inside the 465-million-year-old trilobite, Ahlberg and colleagues examined the fossil using high-resolution X-rays, which they used to create three-dimensional digital reconstructions. “After that, it was just a matter of comparing the shell fragments in the gut with known fossil organisms to work out what our trilobite had been eating,” says Ahlberg.

The scans revealed crushed shells from ancient crustaceans, echinoderms and bivalves – the early relatives of crabs, sea stars and mussels. They concluded that the trilobite ate a wide variety of marine organisms, both living and dead. Shell fragments in the animal’s gut suggest it was able to crunch through fragile shells or swallow animals whole, and probably made meal choices primarily based on its prey’s size and shell strength.

Judging by signs of digestion, “surprisingly, it seems to have been eating very quickly, stuffing itself almost to bursting”, says Ahlberg. He suspects the trilobite was over-eating to trigger moulting, a strategy shared by some modern arthropods.