There have been some extraordinary examples of soft tissue preservation in fossils over the past few years, from apparent red blood cells in dinosaur bones to feathers and pigments in early birds. A new set of fossils that have come out of south China, dating to around 515 million years ago, show something else astonishing: the preservation of the creatures' nervous systems, including the brain, nerve cord, and individual nerves. Thought to be the oldest and best-preserved nervous systems ever found, they once belonged to an animal known as Chengjiangocaris kunmingensis, and are described in a paper published in PNAS. 

“The animal itself is an early arthropod,” explained Dr. Javier Ortega-Hernández, from the University of Cambridge’s Department of Zoology, to IFLScience. “The fossils are approximately 515 million years old, so really close to the origin of animals. And we have a nerve cord, so that is a really good insight into early animal evolution.” Today, arthropods are represented by insects, crustaceans, and spiders. What made the fossils remarkable was the exquisite level of preservation of the nervous system.   

“You can see this really thin strand which goes all throughout the body, and you can that it has these spots which repeat themselves a lot, and those would correspond with ganglia,” Dr. Ortega-Hernández continues. “Ganglia are masses of nervous cells, which occur in each of the segments of the arthropods.” All arthropods have segmented bodies, and each segment has its own ganglion that controls the pair of legs, not dissimilar to a "mini brain" that keeps everything in motion. Finding this in the fossil arthropods was nice to see, but expected.  

What the researchers didn’t expect, however, was the detail of the nerves preserved branching off from the ganglion. In all living arthropods, each ganglion gives rise to only a couple of long, incredibly fine nerves that serve the individual segments. The fossils, however, showed dozens of such nerves, something that's only seen today in a creature closely related to arthropods known as velvet worms. This heavy branching, suggest the researchers, shows that it is a very ancestral feature that was inherited from a velvet worm-like ancestor. Since then, there has been a loss of these neurological characteristics in modern arthropods.

But how can such fine soft tissue manage to get fossilized and preserved for over 500 million years? The answer might lie in what exactly the nervous system is made up of. “Things like the nervous system or the gut get fossilized counter-intuitively,” says Dr. Ortega-Hernández. Why this happens is believed to be “due to the chemically reactive composition of adjacent lipid-rich ganglia,” he wrote in the paper.

^{Image in text: Complete specimen of Chengjiangocaris kunmingensis, showing the individual ganglia in incredible detail. Jie Yang/Yunnan University, China}