Coprolites are fossilized feces. They’re the preserved droppings of animals (and sometimes humans) that lived thousands or even millions of years ago, turned to stone through the same mineralization processes that create other fossils. Far from being mere curiosities, coprolites are one of the most information-rich fossils scientists can find, offering direct evidence of what ancient creatures ate, what parasites they carried, and what their ecosystems looked like.
How Feces Become Fossils
For droppings to survive millions of years, they need to mineralize before bacteria break them down completely. In most cases, the original organic material is gradually replaced by calcium phosphate, the same mineral family found in bones and teeth. This happens more readily with carnivore droppings because ingested animal tissues are naturally rich in phosphorus from both soft tissue and bone. That high phosphorus content is actually one of the key ways scientists confirm that a suspicious-looking rock is genuinely a coprolite rather than an ordinary stone: the calcium-to-phosphorus ratio in a confirmed coprolite runs around 1.56, which sets it apart from typical sedimentary rock.
Not every coprolite is millions of years old. Some are only centuries old and haven’t fully mineralized. These are sometimes called paleofeces or desiccated feces rather than true coprolites, though the line between the two is blurry. Scientists also distinguish coprolites from a related fossil type called cololites, which are fecal material still preserved inside the gut of a fossilized animal. Both can look similar and contain the same kinds of information, but coprolites were expelled during life while cololites never left the body.
What’s Inside a Coprolite
Crack open a coprolite and you’ll find a remarkably detailed snapshot of an ancient meal. The contents vary depending on the animal and its diet, but the range of preserved material is striking.
- Plant remains: partially digested tissues, seeds, fibrous material, and cellulose fragments. Microscopic plant structures called phytoliths (tiny silica casts that form inside plant cells) survive especially well. Starch granules and pollen grains also preserve in coprolite material, with researchers recognizing pollen preservation in coprolites as far back as the 1930s.
- Animal remains: bone fragments, hairs, feathers, fish scales, and insect parts. Some of these are visible to the naked eye, while tiny bone fragments only appear when sliced thin and viewed under a microscope.
- Parasite eggs and larvae: intestinal worms deposited their eggs in the host’s gut, and those eggs fossilized right along with everything else. This gives scientists a direct record of ancient infections.
- Fungal spores: some from edible mushrooms, some from medicinal fungi, and some that were simply plant pathogens accidentally swallowed with food.
- Biomolecules: lipids, proteins, and even fragments of DNA can survive in coprolites. Fecal sterols (fat-like compounds produced by gut bacteria) vary depending on whether the animal had a plant-based, meat-based, or mixed diet, and bile acid profiles are unique enough to help identify which species produced the coprolite in the first place.
What Coprolites Reveal About Ancient Diets
Some of the most dramatic coprolite findings come from predators. A set of Triassic coprolites (roughly 230 million years old) from an ancient reptile related to dinosaurs contained up to 50 percent bone by volume, an extraordinarily high proportion that revealed aggressive bone-crushing feeding behavior similar to what Tyrannosaurus rex would later employ. Inside those coprolites, researchers identified fragments of large serrated teeth shed during feeding, bones from a temnospondyl amphibian (a now-extinct group), a possible limb bone from a juvenile dicynodont, rib fragments from an archosaur, and fish remains. A single set of droppings essentially mapped out an entire predator-prey network.
Isotope analysis pushes dietary reconstruction even further. By measuring the ratios of different forms of carbon and nitrogen locked in coprolite material, scientists can determine where an animal sat in the food chain and what type of food fueled the ecosystem. Nitrogen ratios shift predictably at each step up the food chain because animals preferentially excrete the lighter form of nitrogen during digestion, becoming enriched in the heavier form. Carbon ratios, meanwhile, change very little between predator and prey but differ sharply between animals eating land plants versus aquatic food sources. One study of coprolites from a 125-million-year-old Spanish lake ecosystem used these isotopes to show that aquatic food resources were the dominant energy source for the animals living there, and that nitrogen values shifted between wet and dry seasons.
Ancient Parasites and Disease
Coprolites are one of the only ways to study infectious disease in the deep past. Parasite eggs preserve remarkably well in fossilized feces, giving scientists a timeline for how long certain infections have plagued humans and animals.
Roundworm eggs have been found in human coprolites from Peru dating to 2277 BC and from Brazil around 1660 to 1420 BC, confirming that this parasite has been a human companion for at least four thousand years. Hookworm eggs appear in a human coprolite dated somewhere between 3350 BC and 480 AD, and possible hookworm larvae turn up in fecal samples from the Colorado Plateau around 200 BC. These findings matter because they help trace human migration patterns. Hookworms, for example, need warm soil to complete their life cycle, so finding them in cooler regions tells researchers something about the routes early humans traveled.
One of the most famous coprolites in the world illustrates this perfectly. The Lloyds Bank coprolite, discovered in 1972 beneath a former bank site in York, England, is a Viking-age human stool measuring 20 centimeters (about 8 inches) long and 5 centimeters (2 inches) wide. It contained several hundred parasitic eggs, revealing that its producer was heavily infested with maw-worms and whipworms.
How Scientists Study Coprolites
Traditional coprolite analysis involved slicing specimens into paper-thin sections and examining them under light microscopes or scanning electron microscopes. This works, but it has real drawbacks: it destroys part of the sample, only provides flat two-dimensional images, misses inclusions that happen not to fall in the plane of the cut, and reveals little about how the contents are spatially arranged inside.
Newer techniques use high-powered X-ray imaging, specifically synchrotron microtomography, to scan coprolites without cutting them at all. This produces full three-dimensional reconstructions of the interior, allowing researchers to digitally isolate individual bone fragments, teeth, or other inclusions and study them from every angle. The scans generate enormous datasets that can be processed at different resolution levels, with scientists first screening a lower-resolution version to identify points of interest, then zooming into full resolution for detailed analysis. Software tools segment the scan data by identifying connected clusters of similar density, effectively separating bone from mineral matrix from empty space.
For coprolites that are young enough to retain organic molecules, researchers can extract DNA, though this is technically challenging. The goal is to pull out enough genetic material while removing chemical compounds that would interfere with the amplification process needed to read the DNA. Success depends heavily on preservation conditions: dry caves and arid environments tend to preserve DNA far better than waterlogged or tropical sites.
Reconstructing Whole Ecosystems
What makes coprolites uniquely valuable, compared to bones or footprints, is that they capture biological relationships rather than just individual organisms. A single coprolite can document a predator, its prey, the parasites infecting it, the plants growing nearby (via pollen), and even seasonal climate conditions. Taken together, a collection of coprolites from one site can sketch out an entire food web: who ate whom, which resources were most important, and how conditions shifted over time. Few other fossils pack that much ecological information into such a small, unassuming package.