310-million-year-old brain exceptionally preserved in unique fossil
Researchers have discovered one of the oldest and best-preserved brains in the fossil record. A 310-million-year-old horseshoe crab was found with its complete brain intact, thanks to a previously unknown preservation method.
The majority of our knowledge of ancient creatures comes from bones – soft tissues don’t fossilize very well. Some mechanisms are better than others at preserving these fragile tissues though, most famously amber. Scientists can then scan amber-encased creatures to image their brains and other organs.
But that record only goes back so far. The oldest amber inclusions date back about 230 million years ago, to the Triassic period. Burgess Shale-type deposits, however, extend as far back as 520 million years ago, to the early Cambrian. These mudstone deposits can also preserve soft tissues as carbon films – most commonly the gut, but on rare occasions imprints of parts of the nervous system can be found.
Normally, Burgess Shale-type fossils are thought to be produced when animals are buried in mud that slowly turns into a solid iron carbonate mineral called siderite. That preserves the whole body, creating many of the detailed, famous fossils of trilobites and horseshoe crabs.
The newly discovered specimen, a horseshoe crab called Euproops danae, adds a unique wrinkle to the story. It was encased in siderite as usual, but a second mineral is in play that causes its brain to show up as a stark white shape, contrasting clearly against the brown rock.
“In our fossil, the brain of Euproops is replicated by a white-colored clay mineral called kaolinite,” says Professor John Paterson, co-author of the study. “This mineral cast would have formed later within the void left by the brain, long after it had decayed. Without this conspicuous white mineral, we may have never spotted the brain.”
When the researchers compared this ancient brain to those of its living relatives, they found that the basic structure seems to have changed very little in the hundreds of millions of years since.
“The fossil’s central nervous system is closely comparable to that of living horseshoe crabs and match up in their arrangement of nerves to the eyes and appendages,” says Paterson. “It also shows the same central opening for the oesophagus to pass through. This is quite remarkable, given the substantial morphological and ecological diversification that has taken place in the group over the intervening 310 million years.”
Discoveries like this can help scientists better understand the biology of ancient animals, as well as the process of evolution over long time periods.
The research was published in the journal Geology.