According to a news release from the University of Arizona, researchers have found the earliest known complete nervous system superbly preserved in the fossilized remains of a never-before characterized creature that crawled or swam in the ocean 520 million years ago.

The discovery suggests that the ancestors of chelicerates split off from the family tree of other arthropods more than half a billion years ago. According to the researchers, the specimen belongs to a departed group of marine arthropods called megacheirans.

“We now know that the megacheirans had central nervous systems very similar to today’s horseshoe crabs and scorpions,” said senior author Nick Strausfeld, a Regents’ Professor in the University of Arizona’s Department of Neuroscience. “This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian.”

The researchers recognized the 3-centimeter-long creature discovered from the Chengjiang formation near Kunming in southwest China, as a member of the archaic genus Alalcomenaeus. Animals in this group had an elongated, segmented body armed with approximately a dozen pair of body appendages giving the animal the ability to swim or crawl or both.

The researchers examined the fossil by bringing into play several different imaging and processing methods, using the iron deposits that had selectively gathered in the nervous system during fossilization to their advantage.

The researchers applied complex imaging methods to several scans of the specimen, first overlaying the magenta color of the iron deposit scan with the green color of the computed tomography (CT) scan, then subtracting the two. This culminated in what looked like a negative X-ray photograph of the fossil.

According to Strausfeld, the negative X-ray photograph revealed a nervous system in surprising detail.

Examining in contrast the outline of the fossil nervous system to nervous systems of horseshoe crabs convinced the researchers that the Alalcomenaeus was a member of the chelicerates. Specifically, the fossil reveals the usual signs of the brains discovered in scorpions and spiders: three groups of nerve cells known as ganglia fused together as a brain also fused with some of the animal’s body ganglia.

A short time ago, the same researchers described the finding of a fossilized brain in the 520-million-year-old fossil Fuxianhuia protensa, revealing startling similarity to the advanced brain of a modern crustacean.

“Our new find is exciting because it shows that mandibulates (to which crustaceans belong) and chelicerates were already present as two distinct evolutionary trajectories 520 million years ago, which means their common ancestor must have existed much deeper in time,” Strausfeld noted. “We expect to find fossils of animals that have persisted from more ancient times, and I’m hopeful we will one day find the ancestral type of both the mandibulate and chelicerate nervous system ground patterns. They had to come from somewhere. Now the search is on.”

The study’s findings are described in detail in the journal Nature.