Hard animal tissues rich in minerals, such as bones and shells, turn into fossils easily, but it's much rarer to find fossils of softer tissues. Now, new analyses of fossils of a scorpion and one of its distant arthropod relatives suggest that some of the original organic material in those creatures' exoskeletons can survive for millions of years. Better yet, researchers say these remains help solve the long-standing mystery about how such materials can become fossilized in the first place.
The exoskeletons of arthropods—a group that includes living creatures such as spiders, scorpions, and crabs, as well as long-extinct trilobites—are made of a composite of protein and polymerized sugars. This so-called chitin-protein complex has a particular ratio of carbon, nitrogen, and oxygen atoms bound together in chemically distinct ways, says George Cody, an organic geochemist at the Carnegie Institution for Science in Washington, D.C. Although microorganisms generally gobble up these flexible-yet-crunchy tissues within days, exoskeletons are sometimes partially preserved in fossils.
Using a high-resolution, scanning transmission x-ray microscope that can detect certain chemical bonds, Cody and his colleagues looked at the 310-million-year-old fossils of a scorpion and the 417-million-year-old fossils of a eurypterid, a marine arthropod that may be related to today's horseshoe crabs. "These fossils are the remains of something that was once more substantial," says Cody. Assuming the animals' exoskeletons were comparable to those of modern scorpions, more than 90% of the original thicknesses of the exoskeletons were gone, he says. But the researchers found chemical-bond fingerprints of chitin-protein complex in thin, nitrogen-rich layers in both fossils. Analyses show that 59% of the chitin-protein complex from the ancient scorpion's exoskeleton, and 53% of that from the eurypterid's, was preserved in those thin veneers, the researchers reported online 3 February in Geology.
Those telltale chemical bonds provide clues about how the material was preserved, says Cody. Like many arthropods, the scorpion and eurypterid had a waxy coating on their exoskeletons to keep them from drying out. Soon after they died, the waxy coating began to decompose, releasing carbon-rich substances called fatty acids. Then those compounds chemically bonded to the chitin-protein complex, forming a material resistant to further decay. "Without the chitin complex, you lose the fatty acids, and without the fatty acids, you lose everything," says Cody.
The new findings are a nice confirmation that under the right conditions, at least some nonmineralized parts of arthropods can be preserved, says Andrew Knoll, a paleontologist at Harvard University. Nicholas Butterfield, a paleobiologist at the University of Cambridge in the United Kingdom, isn't so sure that the bonding between fatty acids and chitin-protein complex is what made these specimens turn into fossils. "Other sorts of chemical reactions may be at work, because this type of preservation isn't limited to arthropods," he says.
Nevertheless, Butterfield notes, it's pretty clear that in some cases, the products of decomposition play a critical part in the preservation of what's left for posterity.