It's rare that a press release email grabs my attention, but this one did. “New AMNH Study Shows that Early Trilobites Had Stomachs”, the message read. All I could think was “What?! How did we not know this?”

The fossil record is incomplete. Paleontologists are constantly having to think in the space between life and death – what ancient species were really like and how remnants of those lives were preserved. So some amount of hypothetical gap filling is necessary. If a dinosaur skeleton is found without a skull, for example, you wouldn’t automatically assume it was the Jurassic equivalent of Headless Horseman. The head just wasn’t preserved or found. So, following from that, when I thought of the pillbug-like arthropods called trilobites, I just assumed they had stomachs. I didn’t even know there was a mystery.

Paleontologist Melanie Hopkins and colleagues set things straight in a new paper on early trilobites found among a roughly 525 million year old site in Yunnan, China. Dozens of beautifully-preserved trilobites representing eleven species have been found in the area – part of what’s called the Guanshan Biota – and representatives of at least two species show parts of their digestive tracts. Fossils of the trilobites Palaeolenus lantenoisi and Redlichia mansuyi display an expanded part of the stomach called a crop, and one particular Palaeolenus fossil also shows “paired digestive glands along the alimentary tract.”

Now, while other trilobites have been found with preserved parts of their digestive tracts, none have been this old. The next oldest representative is a single juvenile trilobite from Sweden. And the arrangement of the parts matters. Previously, Hopkins and colleagues explain, paleontologists drew from trilobites with remnants of internal organs to propose that there were two kinds of digestive systems in these arthropods – a more primitive system with digestive glands and a more advanced arrangement with a crop expanding from the alimentary canal. The discovery of exceptionally old trilobites with both systems changes what paleontologists expected.

Early trilobites showing remnants of the digestive tract. Credit: Hopkins et al 2017

The most obvious change is that the crop – basically the trilobite stomach – evolved much earlier than previously thought. It wasn’t an advancement over the other system but was there from the beginning. But the presence of both a crop and digestive glands, Hopkins and colleagues write, means that the evolution of trilobite innards was more complex than previously supposed. 

At least some early trilobites had both sets of digestive structures, which raises new questions about why other trilobites seem to have one or the other. Is it a matter of taphonomy, some specimens preserving one kind of digestive route and not the other? Or are they alternatives that dovetailed early and have broader implications for how these animals fed? With any luck the fossils that will inform and invigorate these questions will soon be found, leading paleontologists to wonder if they have the guts. 

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Hopkins, M., Chen, F., Hu, S., Zhang, Z. 2017. The oldest known digestive system consisting of both paired digestive glands and a crop from exceptionally preserved trilobites of the Guanshan Biota (Early Cambrian, China). PLOS ONE. doi: 10.1371/journal.pone.0184982