Conventional wisdom says that brains don't fossilise, but these seven fossilised brains beg to differ.
An arthropod called Fuxianhuia protensa, which lived on the ocean floor about 520 million years ago, would have looked much like today's shrimp, say paleontologists. Thanks to fossilized remains, we now know that its brain was also similar to those of today's crustaceans. A team led by paleontologist Xiaoya Ma examined seven specimens of the now-extinct creature with a scanning electron microscope, and they found traces of a brain - in the form of a flattened carbon film - in each one.
It's something of a vindication for the scientists. Back in 2012, four of the current paper's co-authors made what they claimed as the first report of a fossilized brain, in the journal Nature. Other paleontologists greeted the claim with scepticism. The Nature paper was based on a single F. protensa specimen, and many paleontologists said that at best, it had been a one-off fluke, not likely to be repeated.
Their new paper, in the journal Current Biology, is based on seven specimens of F. protensa, all newly unearthed from the rich fossil beds of southwestern China's Chengjiang Shales. And this time, Ma and her colleagues say they have a better idea of what conditions led to the unlikely preservation of brains in the fossil record.
First, the animals would have to be buried almost immediately to keep scavengers from picking apart the remains to eat the soft tissue. And the sediment and surrounding sea water would have to contain almost no oxygen; otherwise, bacteria would cause the soft tissue to decay. How could that happen? An underwater mudslide could do the trick, burying the shrimp alive and then preserving them - and their brains - for posterity.
Of course, brains are squishy, and fossilised remains are usually buried under many layers of very heavy sediment. Why weren't these tiny brains simply squashed into oblivion? Arthropods, it turns out, have very dense brains; their cells are closely packed, which makes it easier for them to withstand pressure without just bursting.
As sediments piled up atop the dead F. protensa, the pressure would have slowly pressed the water out of their brain cells, causing the tissue to gradually flatten without losing its structure. Then the heat and pressure from millions of years of burial would have caused that tissue to give up its oxygen and hydrogen, leaving behind a flat film of carbon in the shape of an ancient arthropod brain.
Top image: Xiaoya Ma