This news article is based on the open-access journal article (Lakeram et al., 2026) published in the Journal of Paleontology. The study is published under a Creative Commons Attribution 4.0 license.

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Introduction: Scientists Found Poop Inside Poop

Sometimes fossil poop tells scientists what an animal ate. This time, it revealed something stranger: another animal was apparently eating the poop.

Researchers studying a Carboniferous coal ball from Illinois discovered two ancient tunnels through the underground roots of Psaronius, a giant tree fern. The tunnels were filled with fossil fecal pellets from terrestrial arthropods.

Then, under the microscope, they found a second kind of coprolite. These tiny pellets were probably produced by oribatid mites. Some occur around the larger coprolites. Others occur inside disturbed larger coprolites.

The best explanation is wonderfully simple: nearly 300 million years ago, tiny mites were eating the partially digested plant material in another arthropod's feces. The fossil record had preserved poop inside poop.

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A 300-Million-Year-Old Coal Swamp Frozen in Stone

The fossil comes from the Mt. Rorah Coal Member of the Tradewater Formation in southern Illinois. These rocks formed during the Middle Pennsylvanian, when tropical peat swamps covered large parts of Euramerica.

The specimen is a coal ball, a calcium-carbonate-rich nodule that formed within ancient peat before the plant material was compressed into coal. Coal balls can preserve plant cells, roots, spores, fungi, and tiny traces of animal activity in remarkable three-dimensional detail.

The Mt. Rorah coal flora included giant lycopsids, seed ferns, cordaitaleans, and marattialean tree ferns. It also preserves an unusual amount of arthropod fecal material and feeding damage, making it a valuable record of what small animals were doing on the forest floor.

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The Tree Fern Roots Became a Tiny Ecosystem

The two borings cut through the root mantle of Psaronius, one of the iconic tree ferns of Pennsylvanian coal forests. One tunnel measures about 3 centimeters long; the other about 6 centimeters.

Both follow the margins of underground roots and are packed with fossil fecal pellets. The surrounding roots do not show the kind of wound response expected if an animal had burrowed through healthy living tissue.

That suggests the arthropods were probably feeding in dead or decaying root material. In other words, they were not attacking a thriving tree fern. They were helping dismantle plant tissues that had already entered the decomposition process.

The Larger Coprolites: Who Made Them?

The larger type 1 coprolites measure roughly 0.5 to 2 millimeters in diameter. They contain ground-up pieces of Psaronius root tissue.

The researchers cannot identify the producer with certainty. Based on the width of the tunnels, the size and shape of the fecal pellets, and comparisons with living and fossil arthropods, they suggest an early roach-like insect, a millipede, or another terrestrial arthropod.

That uncertainty is important. The fossil records a behavior very clearly - tunneling, feeding, and defecating - even though the animal itself was not preserved.

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Then Came the Tiny Mite Poop

The type 2 coprolites are dramatically smaller: less than about 60 micrometers across. Their size, shape, contents, and distribution closely match coprolites produced by oribatid mites.

Oribatid mites are tiny chelicerate arthropods. Living species are usually only fractions of a millimeter long, and fossil evidence shows that their relatives were already important decomposers in Paleozoic forests.

The high concentration of type 2 pellets suggests that multiple mites were feeding in the borings. Different pellet sizes may even reflect different growth stages of the mites.

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The Evidence for Coprophagy: Yes, They Ate the Poop

The strongest evidence comes from the spatial relationship between the two kinds of coprolites.

Tiny mite coprolites occur around the larger fecal pellets, but some occur inside type 1 coprolites that appear to have been disturbed. Both coprolite types contain digested Psaronius tissue.

That pattern led the researchers to conclude that the mites were practicing coprophagy, the consumption of feces. They were likely mining the larger arthropod's droppings for partially digested tree-fern tissue.

So the sequence was probably: Psaronius root tissue was eaten by a larger arthropod, the arthropod produced fecal pellets, mites ate those fecal pellets, and the mites produced still smaller fecal pellets. A 300-million-year-old recycling system was preserved in place.

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Why Poop-Eating Mites Mattered to a Carboniferous Forest

The funny part of the story is obvious. The ecological part is more important.

Pennsylvanian peat swamps produced enormous amounts of plant debris. Dead roots, leaves, wood, spores, and other tissues had to be broken down before their nutrients could be recycled through the ecosystem.

The larger arthropod began that process by consuming dead Psaronius root tissue. Mites then processed the same material again after it had passed through the first animal's digestive system.

Modern decomposer communities work in much the same way. One organism's waste becomes another organism's resource. Repeated feeding breaks organic material into smaller pieces, changes its chemistry, and helps release nutrients back into the environment.

The fossils show that complex detritivore food webs were already operating in Carboniferous peat swamps nearly 300 million years ago. Even tiny mites were part of the machinery that kept ancient forests functioning.

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Why a Coal Ball Can Preserve Behavior

Body fossils tell us which animals existed. Trace fossils can tell us what those animals were doing.

Here, no roachoid, millipede, or mite body had to survive. The tunnels, the two sizes of coprolites, the chewed plant tissue, and the placement of one kind of feces inside another were enough to reconstruct a sequence of feeding behaviors.

It is an unusually intimate view of a Pennsylvanian forest floor: a dead tree-fern root became food for one arthropod, its waste became food for mites, and both animals helped return plant nutrients to the peat-swamp ecosystem.

Sometimes fossil poop reveals an ancient diet. Sometimes it reconstructs a food web. And occasionally, if the fossil record is feeling especially generous, it preserves poop inside poop.

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Journal Article:

Lakeram, S. R., Donovan, M., Labandeira, C. C., Elrick, S. D., & Punyasena, S. W. (2026). Coprolite-filled borings: insight into the life history of detritivorous Pennsylvanian terrestrial arthropods. Journal of Paleontology, 99(6), 1369-1383. Published online February 5, 2026. https://doi.org/10.1017/jpa.2025.10201.

Image and reuse license: The article is published under the Creative Commons Attribution 4.0 International License, which permits reuse, distribution, and reproduction with proper attribution.