Waxy layers on insects are responsible for fossil chemical make-up.
Richard Van Noorden
A conundrum about the chemical make-up of fossilized insects has been solved this week by scientists who baked up scorpions to find the answer.
Modern arthropods, including cockroaches, scorpions, beetles and shrimps, have shells made up of chitin. Chitin consists of stringy carbohydrate fibres rather like plant cellulose that are embedded in a hard protein matrix, with a waxy layer on top. But their fossilized counterparts, explains Neal Gupta of Yale University in New Haven, Connecticut, have an entirely different chemical make-up. Fossils surviving from 30 million years ago have an outside skeleton containing 'aliphatic' molecules with long chains of carbon atoms, very similar in structure to the chemical compound kerogen, a precursor to components of petrol.
"This is an enigma," says co-author Richard Pancost of Bristol University, UK. "How do you go from chitin, the carbohydrate, to these long-chain hydrocarbon precursors?"
Fossil fuels are made over long time periods when carbon-based material is compressed deep within the Earth. "Kerogens are a stage in the process to fossil fuels," explains Derek Briggs, a palaeontologist also at Yale University, Connecticut. "There are several different types, and there's an ongoing debate about how they form." For marine algae, kerogen is formed by 'selective preservation', as long-chain molecules already in the algae resist decay.
Many assumed the same was true for arthropods and other carbon-based material. Perhaps small quantities of aliphatic molecules in insects are preserved over millions of years while the chitin is lost. But no aliphatic molecules have been found in modern insects. Alternatively, perhaps some outside source provided the hydrocarbons, or maybe waxes on top of the chitin somehow combined to produce them.
To answer the question, the scientists decided to make their own artificial fossils from modern arthropods.
They obtained live emperor scorpions, shrimps and Madagascan hissing cockroaches from Bristol Zoo, and froze them to death. The outside skeleton was removed and baked at 350 °C at around 700 atmospheres pressure for a day in sealed gold containers. This, says Gupta, is an established process that roughly mimics the way fossils form over millions of years, compressed into a single day by upping the temperature.
The team analysed the resulting 'fossilized' arthropods, and found new aliphatic compounds that hadn't been there before the treatment, they report in Proceedings of the Royal Society B1. But chitin extracted from the skeleton, when baked on its own, did not produce these compounds. Nor did bits of skeleton that had their waxy layer removed.
The conclusion: the waxy 'lipid' layer on the outside of the skeleton and in some of its internal tissues is responsible. "Though we still don't know exactly how," admits Briggs.
The researchers are now applying their technique to fossilized plants as well, to work out the intimate details of how fossil fuels can be made from material such as decaying leaves.