Frozen in Time: 365 Million-Year-Old Fossil Shows Trilobite in the Act of Molting

Frozen in Time: 365 Million-Year-Old Fossil Shows Trilobite in the Act of Molting
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Fossilization preserves bones -- everybody knows that. But in rare cases, fossilization does something even cooler: it preserves moments. Recently, Błażej Błażejowski, based out of the Institute of Paleobiology at the Polish Academy of Sciences, was lucky enough to stumble upon a fossil containing a very special moment frozen in time. He shared his discovery last Friday in the journal Scientific Reports.

The piece of shale, excavated from the Kowala Quarry in Poland's Holy Cross Mountains, held what appeared to be a trilobite -- an ancient ocean-dwelling animal resembling a large armored bug -- in the act of shedding its exoskeleton. Subsequent X-ray modeling both confirmed the discovery and revealed the fossilized animal's identity. It was Trimerocephalus, an eyeless trilobite roughly 6.3 inches long and 3 inches wide.

In shedding it's armored exoskeleton, the fossilized creature was molting. Molting is without a doubt one of nature's most incredible acts. Every so often, certain creatures cast off parts of their bodies in a process of biological renewal. Cats and dogs do it with their fur (often shedding hair all over your sofa). Chickens do it with their feathers. Snakes do it with their skin. Most remarkably, many arthropods, including spiders, crabs, and insects, discard their entire exoskeleton! Ripping themselves out of their bodies in an unnerving and alien manner, they emerge revitalized and renewed, leaving behind a near-complete husk of their former selves.

But this particular ancient bug did not emerge renewed. Quite the opposite. It appears to have died in the process.

"The animal died within the sediment shortly after extricating itself from the old exoskeleton," Błażejowski and his co-authors write. "The cause of mortality is, of course, not known but might be due to stress of molting or possibly, given the context, death by hypoxia or hydrogen sulfide toxicity within the sediment."

Scientists always assumed that ancient arthropods like trilobites molted in similar manners to modern arthropods, but Błażejowski says that this discovery is the oldest to directly confirm that educated hunch.

Source: Błażejowski, B. et al. A moment from before 365 Ma frozen in time and space. Sci. Rep. 5, 14191; doi: 10.1038/srep14191 (2015).

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