Melanie J. Hopkins, Feiyang Chen, Shixue Hu, Zhifei Zhang

A rare glimpse inside a 510-million-year-old digestive system suggests feeding was a complicated business for the first arthropods. Even this early in animal evolution, some animals had a variety of structures in their gut for storing and processing food.

Arthropods fared especially well following the Cambrian Explosion about 541 million years ago, particularly the now-extinct trilobites, which occupied the oceans for about 270 million years.

About 20,000 species of trilobite are known, but only about 40 species preserve any traces of their digestive system.


Melanie Hopkins at the American Museum of Natural History, New York, and Zhifei Zhang at Northwest University in Xi’an, China and colleagues have now added two trilobites to this short list.

Gutsy trilobites

Palaeolenus lantenoisi and Redlichia mansuyi lived between 509 and 514 million years ago, and are preserved in the rocks of eastern Yunnan, China. Both are fairly early trilobites, so they should have had simple guts, but no.

Both show evidence of an expanded stomach or “crop” beneath the head shield. This was probably used to store food early in digestion, says Hopkins. It has never been seen in such ancient trilobites.

P. lantenoisi also had a series of paired digestive glands – perhaps for further breakdown of food – running along its alimentary tract.

It’s a combination of features that is virtually unheard-of in trilobites. Many researchers had assumed trilobites either had a crop or digestive glands, but never both.

“I think it is possible that many trilobites had both,” says Hopkins. If a specimen seems to lack one, that might simply be due to poor preservation.

Mystery diet

“There was more variation in digestive system anatomy than previously thought,” says Allison Daley at the University of Lausanne, Switzerland. “These new findings suggest a great variation in how trilobites processed their food, even as geologically early as in the Cambrian.”

This mirrors the complex feeding behaviours seen in other early animals like the enigmatic anomalocaridids, which looked like a cross between squid and huge woodlice. Seemingly, some targeted specific prey while others were filter feeders.

It would help to know what trilobites ate, but this is “controversial”, says Daley. “It’s been difficult to pinpoint exactly what they ate and how.”

A January 2017 study pointed out that paired digestive glands like those of P. lantenoisi are often found in predatory arthropods today, suggesting trilobites were active predators. But other researchers argue that the trilobite glands seem far simpler than those seen today, so this argument may not hold.

It would be better to infer what they ate from their mouthparts and limbs, says Thomas Hegna at Western Illinois University in Macomb.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0184982

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