It has long been believed that brains do not fossilize because brain tissue, being soft and wet, starts to decompose minutes after death. The discovery of fossilized brains of Fuxianhuia protensa, an ancient marine arthropod that thrived 520 million years ago, debunks this notion.
In a study published in Current Biology on Oct. 29, Nicholas Strausfeld of the University of Arizona neuroscience department and his colleagues used electron microscopy to analyze the fossils of the prehistoric arthropod that were unearthed in Chengjiang Shales of Southwest China.
They found that the remains of the now-extinct arthropod have preserved brains. The imaged brains of the shrimp-like creature also revealed a neural structure that looked similar to those of present-day crustaceans, suggesting that some arthropods eventually evolved to have a simpler nervous system.
Strausfeld and colleagues already provided proof of brain fossilization in 2012 when they described the remains of Fuxianhuia with remarkably well-preserved brain and optic lobes but paleontologists dismissed the findings, saying that the specimens were experimental artifacts or the tissue had fossilized under rare circumstances that would make the discovery a one-off event.
In the new study, however, Strausfeld's team reported seven other newly-found specimens of Fuxianhuia that also have remnants of nervous tissue. The brains were found to have been preserved as flattened carbon films with some naturally laminated with tiny iron pyrite crystals. The findings have shed light on how and why the neural tissue fossilizes.
In a separate study, Strausfeld's team reconstructed the fossilization process, showing how the prehistoric arthropod's brain was successfully preserved. Replicating the process in the laboratory could strengthen the researchers' claim that brains can indeed be fossilized under ideal conditions.
Strausfeld and colleagues covered cockroaches and sandworms in mud. The research team believed that the neural tissue would not be broken down by bacteria or get eaten by predators if buried deep enough and the water is sufficiently anoxic, or low in oxygen.
The brain tissue should also be capable of withstanding the pressure of being buried. Experiments in the laboratory revealed that arthropods have dense neural tissues that can survive these conditions.
"During this process, the brain maintains its overall integrity leading to its gradual flattening and preservation," said Strausfeld. "F. protensa's tissue density appears to have made all the difference."