How Coprolites Form — and How to Tell One Is Real

PALEODEX · 8 August 2026 · 5 min read

Turning waste into rock is harder than it sounds — and telling the real thing from a rock that just happens to look like it is its own small science.

We've covered what coprolites reveal — a T. rex's crushed-bone dinner, ancient human DNA, a hadrosaur's taste for rotten wood. What we haven't covered is the harder question underneath all of it: how does waste survive 66 million years in the first place, and how does anyone prove a lump of rock actually used to be dung?

The chemistry that makes it possible

Most soft, organic material rots away within weeks or is destroyed by scavengers and bacteria long before fossilization could ever start. Coprolites survive through a specific process called phosphatization: phosphate minerals — often already present in the digested remains themselves, especially in carnivore waste rich in bone and tissue — replace the original organic material before it fully decays, locking the form in mineral rock. Groundwater chemistry matters too; a phosphate-rich, low-oxygen environment dramatically improves the odds. It's a race against decomposition, which is exactly why coprolites are far rarer finds than bones or teeth: the window for it to work is short, and most waste loses that race.

The oldest documented case: Mary Anning's "bezoar stones"

This isn't a new area of study. In the early 1800s, English fossil collector Mary Anning noticed that certain stony lumps she called "bezoar stones," commonly found near ichthyosaur skeletons on the Jurassic Coast, often contained fish scales and bones inside. Geologist William Buckland formally described them in 1829 and coined the term coprolite — from the Greek for "dung stone" — establishing exactly the identification principle still used today: the outside can look like an ordinary rock, but the internal content tells the real story.

How you actually prove it's real

A rock that merely looks like dung isn't enough — geologists have a name for that trap, too.

The pseudocoprolite problem

A pseudocoprolite is a mineral nodule or concretion that resembles fossilized waste purely by coincidence of ordinary geological processes, with no animal involved at all. Because shape alone can mislead, paleontologists rely on a short checklist before calling something a genuine coprolite:

• Internal content: CT scanning or thin-sectioning to look for undigested fragments — crushed bone, fish scales, plant material, phytoliths — that a plain rock would never contain.
• Mineral composition: lab testing for the phosphate-rich chemistry (or silica/calcite in some herbivore cases) that phosphatization actually produces, rather than the mineral profile of the surrounding rock.
• Morphology: a twisted, spiral, or lobed external shape consistent with having been extruded through an intestine — a detail geology alone doesn't produce.

A definitive 2010 study by Hollocher and colleagues in the journal PALAIOS used exactly this combination of chemical and structural analysis to confirm the phosphatic, biologically-formed nature of a set of dinosaur coprolites — the modern version of the same test Buckland was reaching for two centuries earlier, just with better instruments.

Why the detective work matters

Every coprolite in the record — the T. rex's crushed bone, the Paisley Caves DNA, the hadrosaur's rotten wood — only counts as evidence because someone first proved it wasn't just a rock. That verification step is unglamorous, but it's the difference between a real data point and a good guess, which is the same standard the rest of the fossil record has to meet before it earns a place in the Fossil-DEX.

Sources

The chemistry and identification criteria described here, and the specimen in the image — sources below.

• Coprolite — overview, history, and formation (Wikipedia, citing Buckland 1829 and subsequent research)
• Hollocher, K.T. et al., "Phosphatic Coprolites..." — PALAIOS 25:132-140 (2010)
• Ichthyosaur coprolites specimen photo — James St. John, Wikimedia Commons