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Shifts in Earth’s orbit increase the chances of spectacular fossils


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Changes in the seasons mean changes in preservation conditions


For palaeontologists, fossils are buried treasure, and, like treasure of the more conventional sort, such finds are not all of equal value. Fossilised bones, while useful, are reasonably common. Preserved impressions in fine sediment of soft parts like skin and organs are rarer and concomitantly more helpful when it comes to understanding what ancient life was like. But the palaeontological equivalent of finding royal jewels is the discovery of soft tissues that have themselves become preserved. Until now it has been assumed that soft-tissue preservation is a chance, and therefore unpredictable, event. But work published in Geologyby Farid Saleh of Claude Bernard University in Lyon, France, suggests that regular variations in Earth’s orbit can affect the preservation of soft tissue in predictable ways.

For such tissue to be preserved, minerals that impede the activities of tissue-consuming bacteria need to surround the body of a dead organism quickly, before it can rot away. Iron-rich minerals are particularly good at keeping flesh-eating bacteria at bay and are thus commonly found in the sediments around soft-tissue fossils. These sorts of minerals appear in the geological record seemingly at random but, while studying the Fezouata shale, a 500m-year-old formation in Morocco, Mr Saleh noted that exquisitely preserved soft-tissue fossils of annelid worms, sponges, arthropods (pictured) and echinoderms seemed to turn up at regular intervals.



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