Coprolites and fossil guts from more than 35 deposits show digestive systems growing more complex through the Cambrian, redistributing carbon as they went.
Around 540 million years ago the fossil record changes character. Animals acquire shells, eyes, limbs and the habit of eating each other, and most of the major animal body plans appear within a geologically short interval. Explanations for the Cambrian Explosion usually reach for rising oxygen, the evolution of predation, or changes in ocean chemistry.
A review in Trends in Ecology & Evolution proposes adding something less dignified to the list. Guts appeared at the start of the Cambrian, and with them the first coprolites — fossilised faeces — and the authors argue that the resulting redistribution of organic matter helped fuel the radiation.
“Next to rising oxygen levels and other contributing factors, the importance of faeces in ancient ecosystems is often overlooked,” said Dr Russell Bicknell of Flinders University’s College of Science and Engineering, an Australian Research Council DECRA fellow and a palaeobiologist specialising in arthropods. The lead author is Dr Julien Kimmig of the Karlsruhe Institute of Technology.
The timing
The chronology is the review’s starting point. Animals first appeared around 600 million years ago, in the Ediacaran, and those early forms largely lacked through-guts — Ediacaran organisms are generally interpreted as absorbing nutrients across their surfaces or filtering passively, without a mouth-to-anus digestive tract.
Complex digestive systems and the first coprolites both appear at the start of the Cambrian. Evidence from more than 35 deposits worldwide shows coprolites and fossilised guts becoming more complex and more varied in shape and size through the period.
“With this shift, we see increasingly complex digestive systems,” Bicknell said.
“The faecal matter includes microscopic pellets through to centimetre-scale coprolites containing shells and other animal fragments,” he said — which is also direct evidence of what was being eaten, since a coprolite with shell fragments in it is a record of a predator that crushed shells.
Why a gut changes an ecosystem
The mechanism the review proposes turns on where organic carbon ends up.
An organism that absorbs nutrients across its surface processes material where it sits. An organism with a gut takes food in at one place and deposits concentrated, partly digested waste somewhere else — and faecal pellets sink. The effect is to move carbon and nutrients downward and to package them into discrete parcels dense enough to fall out of the water column, rather than leaving them dispersed.
That creates a food supply on and in the sediment that did not previously exist, which is an opportunity for deposit feeders, which in turn supports predators on them. The authors argue that feeding strategies through the Cambrian track the production and distribution of organic carbon and nutrients, and that this helped establish conditions resembling those of modern oceans.
Modern oceans run on something similar. The biological pump, in which sinking faecal pellets and marine snow carry carbon from the surface to depth, is one of the planet’s largest carbon fluxes, and it requires animals with guts. On this reading the Cambrian is when that machinery switched on.
The specialised hardware came with it. Early arthropods evolved distinct foreguts and digestive glands, which allowed them to process a wider range of food — and an animal that can digest more things can occupy more niches.
“We have considered these ancient diets, increasingly sophisticated digestive systems and trophic interactions as part of a much bigger picture,” Bicknell said.
What a review can establish
This is a systematic review, not new fossil evidence, and the distinction matters for how much weight the central claim can carry.
What the compiled record supports well is a correlation and a sequence: guts and coprolites appear at the start of the Cambrian, and both diversify through it alongside the animals. That is a solid empirical pattern drawn from more than 35 deposits.
What it cannot do is separate cause from consequence. Complex guts appearing alongside complex animals is exactly what you would expect if both are products of the same radiation, and a gut is as plausibly a result of new feeding opportunities as a driver of them. The review’s framing — faeces fuelling the radiation — is a hypothesis about direction that the co-occurrence does not establish.
The usual preservation caveats apply too. Coprolites fossilise under particular conditions, guts are soft tissue preserved only in exceptional deposits, and the apparent increase in complexity through the Cambrian partly tracks which deposits of which age happen to preserve such things. Burgess Shale-type preservation is not evenly distributed in time.
“It becomes clear in the fossil record that the evolution of feeding strategies aligns with” the production and distribution of organic matter, Bicknell said — alignment being the honest word for what the record shows.
An unfashionable kind of evidence
The review’s more durable contribution may be methodological. Coprolites are among the least studied categories of fossil, partly for obvious reasons, and they carry information nothing else does: direct evidence of diet, of digestive capability, and of trophic links between organisms that are otherwise inferred from body plans and guesswork.
A predator’s jaws tell you what it could have eaten. Its faeces tell you what it did eat. For reconstructing a 540-million-year-old food web, that is a meaningful difference, and the argument for taking coprolites seriously as a dataset holds whether or not they turn out to have driven the radiation.
The research was funded by the Australian Research Council. The release does not state any conflicts of interest.
Notes
- Kimmig, J., Bicknell, R.D.C. “The Cambrian fecal revolution: Fueling the Cambrian Radiation.” Trends in Ecology & Evolution, 2026. DOI: 10.1016/j.tree.2026.06.013.
- Flinders University. “How an ‘explosion of excrement’ changed Earth’s ecosystems forever.” EurekAlert!, 4 August 2026. https://www.eurekalert.org/news-releases/1138389.
Prof. A. El Albani, Univ. Poitiers