Scientists have found traces of a tough material that once formed the outer shells of ancient sea creatures called trilobites, preserved inside fossils that are more than 500 million years old.
The discovery shows that parts of living organisms can last far longer in rock than experts once believed, and it changes how scientists think about how carbon is stored deep inside the Earth over long periods of time.
Finding chitin in a fossil
Inside layers of shale rock near Death Valley in eastern California, researchers found a trilobite fossil from the species Olenellus, an extinct marine animal that lived on the ocean floor more than 500 million years ago, that still contained a small piece of its original shell.
Lab work at the University of Texas at San Antonio (UTSA) pulled a clear chemical signal from that shell patch.
Elizabeth Bailey, an Earth and planetary scientist at UTSA, linked that signal to a shell polymer that older searches missed.
Even with just a few fossils, discovering chitin hints that some carbon-rich compounds can stay trapped under ordinary burial conditions.
Modern crab shells and insect skins rely on chitin, a tough sugar-based material in many outer coverings, to stay stiff and light.
Cells string chitin into long chains, then pack them into fibers that resist tearing and don’t dissolve easily.
“However, after cellulose, chitin is considered the second most abundant naturally occurring polymer on Earth,” said Dr. Bailey. Because so much life makes chitin, its decay rate shapes whether organic carbon cycles back quickly or stays buried longer.
Why decay can stall
Under normal seafloor conditions, bacteria and fungi attack chitin soon after death, using enzymes to cut it apart.
In oceans and soils, enzymes made by microbes snap chitin chains into smaller sugars that cells can consume.
Heat and pressure during burial usually rearrange those molecules, leaving little more than a carbon film or mineral copy.
That history makes contamination concerns real, so any positive signal has to clear a high bar.
Checks behind claim
To rule out modern contamination, the UTSA team looked for chemical fingerprints that would only appear if the old polymer remained.
A fluorescent stain made the fossil material glow, and two follow-up chemistry checks confirmed the same signature.
Infrared light and a mass test detected chitin’s building blocks in the study and strengthened the identification.
Burial history likely controlled the outcome, since higher heat or circulating fluids can erase organic traces even when shells persist.
Rocks that protect
Minerals seeped into tiny spaces in the shell, then hardened, blocking water and microbes from reaching the polymer.
Once rock sealed the shell, oxygen dropped and decay slowed, because most microbes need oxygen to keep breaking molecules down.
Similar chemistry turned up in Cambrian sponges, and the paper linked that survival to rapid mineral sealing.
Small changes in temperature, pressure, or groundwater flow can erase the same polymer, so long survival should never be assumed.
Carbon locked in stone
Much of Earth’s carbon ends up in rocks, and the U.S. Geological Survey notes that sinks can store it for long spans.
Buried remains can avoid full decay, then pressure turns sediment into rock, keeping some of that carbon out of air.
Chitin carries both carbon and nitrogen, adding to carbon sequestration, long-term carbon storage away from the atmosphere, in layers that persist.
“The results of the study show that chitin persists much longer in the geological record than originally thought,” said Bailey.
Limestone in the loop
Limestones form when marine remains pile up and cement together, leaving thick limestone layers that people also quarry as stone.
Within many shell-making animals, chitin acts as a flexible framework, so carbonate sediments can bury it along with minerals.
Along with calcium carbonate shells, those sediments may also trap bits of chitin, adding more organic carbon to limestone.
Still, natural burial works on long time scales, so it cannot counter the fast rise of carbon dioxide today.
When rocks heat up
Burial can warm and squeeze rocks into metamorphism, rock changes driven by heat and pressure during time.
Higher temperatures can break long chains apart, and minerals can swap in, leaving no clear trace of the original polymer.
Yet the Carrara shell still carried the signal after mild heating, showing that chitin can survive more than chemistry predicts.
Mapping that breaking point will require fossils from hotter settings and different rock types, not just well-preserved shells.
Lessons from this chitin fossil
Beyond trilobites, many fossils hide thin organic layers, and those layers could now be checked for chitin residues.
Comparing similar shells across different burial histories could help UTSA researchers reveal when chitin survives, and when it turns into simpler carbon.
Finding chitin alongside mineral grains also helps paleontologists separate original biology from later alteration during fossilization.
Careful controls will still matter, because modern organic residues can enter cracks and blur the line between old and new.
Chitin’s survival inside one ancient shell shows that the rock record can keep more biology than expected.
Tracking where that polymer persists and where it vanishes will sharpen carbon-cycle estimates and guide future fossil searches.
The study is published in Palaios.
Photo: АП/Hong Kong’s Antiquities and Monuments Office
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