Earliest Known Complete Nervous System Discovered in Ancient 520-Million-Year Fossil

An international team of scientists has unearthed one of the best preserved and earliest known central nervous system dating back to 520 million years from South China.

The fossilized remains of the extinct ancient animal- a creature new to science that crawled or swam in the ocean, was unearthed at the Chengjiang fossil site near Kunming in Southwest China. The finding of the new ocean creature helps in decoding the long standing mystery of the ancestors of the modern day spiders and scorpions.

The remains indicate that ancestors of chelicerates (spiders, scorpions and their kin) split off from other arthropods more than half a billion years ago. 

The findings of the University of Arizona and London Natural History Museum suggest that the 3 centimeter long specimen belongs to an extinct group of marine arthropods called megacheirans (Greek-large claws) and is a representative of the extinct genus Alalcomenaeus. The specimens belonging to this group are known for their elongated and segmented bodies with a dozen pairs of body appendages that help them to swim to crawl or both.  Similar to the scorpions, these specimens had scissor- like claws attached to the head, used to grasp and sense their surroundings.

"We have now managed to add direct evidence from which segment the brain sends nerves into the great appendage. It's the second one, the same as in the fangs, or chelicerae. For the first time we can analyze how the segments of these fossil arthropods line up with each other the same way as we do with living species - using their nervous systems," co-author Greg Edgecombe, said in a press statement.

Since the fossilized nervous system is not clearly visible to the naked eye, the researchers used several image processing techniques to separate the iron deposits that had settled in the nervous system during the process of fossilization.  Further, with the help of computed tomography (CT) the researchers made the neural structure more visible. CT reconstructs the 3D image of the specimen.

"However the CT scan didn't show the outline of the nervous systems unambiguously enough," Strausfeld said, "while a scanning laser technique mapping the distribution of chemical elements showed iron deposits outlining the nervous system almost as convincingly but with minor differences." 

Hence, they availed another scanning technique to trace the distribution of the iron deposits outlining the nervous system.  They overlaid magenta color of the iron deposits scan with color green of the CT scan. The image data that was not there in both scans was discarded by the researchers. On overlapping of the magenta and green color, they noticed white structure of the preserved nervous system.

The researchers then compared the outline of the fossil nervous system to the nervous system of the horseshoe crabs and scorpions and confirmed that the 520 million- year Alalcomenaeus was a member of the chelicerates.

Strausfeld concluded saying, "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. 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 detail of the new specimen was described in the journal Nature.