What Is a Concretion? Fossils Sealed Inside Their Own Stone Capsules

A concretion is a hard mass of mineral cement that grows within sediment around a nucleus, and organic matter is an excellent nucleus. When a carcass is buried, bacterial decay of the soft tissue alters the chemistry of the surrounding pore water, typically raising alkalinity, which causes carbonate minerals to precipitate out of solution in the immediate vicinity. The cement fills the pore spaces between sediment grains and hardens them into a mass distinctly harder than the mud around it. The important point is timing, since this can happen within weeks to a few years of burial, which is far faster than the compaction and lithification of the surrounding sediment. The result is a rigid capsule formed before the mud was squashed, so the animal inside is preserved in three dimensions while equivalent fossils in the enclosing rock are flattened to films.

Early cementation solves several preservation problems at once:

• •It resists compaction, so bodies keep their original shape instead of being crushed flat
• •It seals the fossil from circulating water, which limits later dissolution and mineral replacement
• •It happens fast enough that soft tissues can be captured before they fully decay, which is why concretions yield muscle, gut contents, gills and eyes
• •It separates the fossil from the host rock mechanically, so the nodule splits cleanly along the specimen rather than through it
• •It makes the fossil findable, since a hard lump weathers out of soft mud and lies on the surface intact
• •It protects the specimen during transport and storage, since the capsule is its own container

Several of the most important fossil sites in the world are concretion deposits, and the pattern of what they preserve is consistent. The Mazon Creek deposit in Illinois yields ironstone nodules containing an extraordinary range of soft-bodied animals and plants from a coastal environment, including jellyfish, worms and the problematic animal that took decades to place in the tree of life. Concretions from Jurassic and Cretaceous marine rocks yield ammonites in full three-dimensional relief and occasionally with soft parts. Christmas tree formations, phosphatic nodules and various ironstone deposits across several continents do similar work. The collecting method is distinctive, since nodules are gathered whole and split, traditionally by repeated freezing and thawing which exploits water in the natural parting plane, and most contain nothing, which makes the process a long exercise in patience for an occasional remarkable result.

Concretions occur in enormous numbers without any fossil inside, since the cement can nucleate on almost any irregularity in the sediment, and the resulting shapes are frequently mistaken for organic objects. Rounded, lobed and elongated nodules have been reported as eggs, bones, footprints and artefacts, and the resemblance can be striking, particularly where a nodule has weathered into a smooth ovoid. Some genuinely famous claims have turned out to be concretions, including supposed eggs and supposed remains from several localities, and the distinction requires looking at internal structure rather than external shape. A fossil has anatomical organisation, with layers, cavities and surfaces that relate to each other in a way a mineral mass does not, so sectioning or scanning settles the question quickly. The reverse error also happens, with genuine specimens discarded as ordinary stones, which is why productive sites are worked by people who know what the local nodules look like when they contain something.

Working with concretions requires its own techniques and its own caution. Splitting is the standard approach and it is destructive if done badly, since a nodule broken along the wrong plane cuts through the specimen, and the freeze and thaw method is preferred precisely because the rock chooses its own weakness. Computed tomography has changed the field considerably, since scanning a nodule reveals whether anything is inside and where it sits before any decision to open it, and in some cases the specimen can be studied digitally without opening at all. Acid preparation dissolves carbonate where the fossil itself is resistant. The chemistry of the nodule is also informative in its own right, since the mineral composition records the conditions of decay and burial, and studying the concretion rather than only its contents has become a recognised approach. That inversion, treating the container as data, has produced useful results about how fast burial and decay proceeded.

Decaying soft tissue changes the chemistry of the water around a buried carcass, which precipitates mineral cement into a hard capsule within weeks to years. That capsule forms before the surrounding mud compacts, so the animal inside keeps three dimensions and sometimes soft parts, while equivalent fossils elsewhere are flattened to films. Most nodules contain nothing, and scanning now reveals which ones do before anyone splits them.