Hundreds of Giant Jellyfish Fossils

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What a storm it must have been! News reports said that hundreds of giant jellyfish once lived about 500 million years ago, but were ‘stranded by a freakish tide or storm’ on an ancient beach. Sand later buried them, forming fossils.1,2 With many specimens measuring over 50 cm (20 in) across, these are the biggest fossil jellyfish known.

Found in a Wisconsin sandstone quarry, it must have been an extraordinary set of circumstances that preserved them, geologists say, for fossilized impressions of jellyfish, which have no skeleton or other hard parts, are extremely uncommon.3

“To preserve a jellyfish, that’s hard, because it has no hard parts. Something is there we don’t understand.”—evolutionary paleontologist James Hagadorn

‘Preservation of a soft-bodied organism is incredibly rare, but a whole deposit of them is like finding your own vein of gold’, said James Hagadorn, one of the paleontologists who reported the find.1,4 Also remarkable is that the rock was sandstone (i.e. the jellyfish were buried in sand which later ‘cemented’ into rock), rather than fine-grained rock like mudstone. In sand, buried jellyfish quickly break down because oxygen readily filters through interconnected air spaces between sand grains, allowing rapid decay. But in fine-grained settings, Dr Hagadorn and his colleagues explain that ‘catastrophic burial and stagnation’ inhibit decay; therefore, jellyfish are more readily preserved. ‘You never get soft bodied preservation in that kind of coarse grain size’, Hagadorn says excitedly.5 ‘When people find a T-rex, that doesn’t excite me that much, because a T-rex has bones and teeth—really easy to fossilize. But to preserve a jellyfish, that’s hard, because it has no hard parts. Something is there we don’t understand.’

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