Fossilized poop, formally called coprolites, looks like smooth, dark-colored stones. Most specimens are brown, gray, tan, or black, though some are white or pale yellow depending on what the animal ate and what minerals replaced the original material. They range from marble-sized pellets to logs the size of your forearm, and at first glance, many people walk right past them thinking they’re ordinary rocks.
Shape, Color, and Texture
The shape of a coprolite depends entirely on the animal that produced it. Many are cylindrical or tube-shaped, resembling stubby sausages with rounded or pinched ends. Others are oval, spherical, or lumpy and irregular. Some carnivore coprolites have a distinctive triangular flat face on one side. The surface can be smooth and polished from mineral replacement, or rough and pitted with small holes. A few are so porous they almost look like sponges, yet feel surprisingly hard and heavy when you pick them up.
Color varies widely. Coprolites from meat-eating animals tend to be whitish, pale yellow, or light gray because of the high calcium and phosphorus content from digested bones. Plant-eater coprolites lean toward darker browns and blacks. Over millions of years, the surrounding sediment also influences color, so you can find coprolites in shades of red, orange, or even greenish hues depending on the iron and mineral content of the soil they were buried in.
Size is another giveaway. Insect-eating animals produced coprolites as small as a few centimeters, while large dinosaur coprolites can exceed 30 centimeters (about a foot) in length. One of the most famous human coprolites, found beneath a Lloyds Bank site in York, England, during a 1972 excavation of a Viking-age settlement, measures 20 centimeters long and 5 centimeters wide. It dates to the 9th century and is likely the largest fossilized human feces ever recovered.
What’s Inside Them
Breaking open or scanning a coprolite reveals a cross-section of what the animal was eating. Carnivore coprolites often contain visible bone fragments, fish scales, and hair or fur embedded in the matrix. Herbivore specimens hold plant fibers, seed husks, and bits of wood. One study of hyena coprolites found numerous hairs throughout the interior, while coprolites from an insect-eating species called the aardwolf preserved recognizable termite remains on the surface and inside broken pieces.
These inclusions are one of the main reasons scientists study coprolites in the first place. They’re direct evidence of what an animal actually ate, not just what it could have eaten. Modern labs use CT scanning to peer inside coprolites without cutting them open, generating three-dimensional images of bone fragments, seeds, and other contents still locked in the mineral matrix. When researchers do need chemical data, they can extract proteins and fats, or use scanning electron microscopy to identify microscopic structures like pollen grains or parasite eggs.
Shark Coprolites Have a Unique Spiral
One of the most visually distinctive coprolites comes from sharks and rays. These specimens have a clear spiral or corkscrew pattern running through them, sometimes visible on the outer surface and always obvious when sliced open. The pattern comes from the spiral intestine, a corkscrew-shaped section of gut unique to sharks that contains anywhere from 2 to 50 folds of intestinal tissue. This structure works like a one-way valve, slowing food down and increasing nutrient absorption. As waste passes through, it gets molded into that distinctive spiral shape before being expelled. Fossil collectors prize spiral coprolites because they’re immediately recognizable and nearly impossible to confuse with ordinary rocks.
How Poop Turns to Stone
Fossilization happens when feces are buried quickly in sediment before they can decompose. Bacteria in the surrounding mud break down organic material under low-oxygen conditions, and this microbial activity triggers a chemical chain reaction. Minerals dissolved in groundwater, primarily calcium phosphate, begin to crystallize within the poop, gradually replacing the original organic matter molecule by molecule. This process can begin within weeks to months of burial.
Carnivore droppings fossilize especially well because bones in the diet supply extra calcium and phosphorus ions, which accelerate mineral precipitation. The result is a phosphate-rich coprolite that is dense, hard, and preserves fine internal details. Coprolites can also mineralize with calcium carbonate, iron carbonate (siderite), or other minerals depending on the chemistry of the surrounding sediment. The specific mineral that forms depends on local pH levels and what ions are available in the pore water around the buried specimen.
How to Tell a Coprolite From a Rock
This is trickier than it sounds. Siderite and iron-oxide nodules that form entirely through geological processes can mimic the shape and size of coprolites so convincingly that even researchers have been fooled. These “pseudo-coprolites” have caused decades of debate in paleontology. Masses of siderite weighing several kilograms have been found eroding out of clay beds, shaped like large droppings but containing no biological inclusions whatsoever. The key differences: real coprolites typically contain visible fragments of bone, plant material, scales, or other dietary evidence. Pseudo-coprolites are chemically uniform inside with no trace of biological content.
If you find something in the field that looks promising, there’s a simple test that fossil hunters have used for years. Touch the specimen to the tip of your tongue. If it sticks slightly, it likely has a high calcium phosphate content, which is a strong indicator of a genuine coprolite. The phosphate mineral is porous and pulls moisture from your tongue, creating that sticky sensation. If you’d rather skip the tongue test, pressing the surface with a wet fingertip can produce a similar sticky feeling, though it’s less reliable.
Other clues to look for: coprolites are often heavier than similarly sized sedimentary rocks. They may have a slightly waxy or resinous luster on fresh breaks. And their shape, while sometimes irregular, often retains a recognizably biological form with tapered ends, surface grooves, or folded textures that pure mineral concretions rarely replicate convincingly.
Where Coprolites Are Found
Coprolites turn up in sedimentary rock formations worldwide, from Permian deposits over 250 million years old to relatively recent archaeological sites just a few centuries old. They’re particularly common in areas that were once lakebeds, river floodplains, or shallow marine environments, where rapid burial in fine-grained sediment was likely. Large concentrations have been found in Upper Cretaceous formations in Saskatchewan, Canada, Miocene deposits in Madagascar, and various sites across the American West. Beach and cliff erosion regularly exposes new specimens, making coastal fossil-hunting sites some of the most productive places to look.
In archaeological contexts, human coprolites from sites like Skara Brae in Scotland’s Orkney Islands (dating to the third millennium BCE) have been recovered intact at 3 to 5 centimeters in length. These specimens, analyzed with CT scanning and protein analysis, revealed details about Neolithic diets that bones and pottery alone could never provide. Whether from a Tyrannosaurus or a Viking settler, coprolites remain one of the most direct windows into what ancient creatures actually consumed on a daily basis.