Oldest Complete Nervous System Ever Discovered Belonged To Large-Clawed Spider-Brained Critter (SLIDESHOW)

Researchers have discovered what is believed to be the oldest-known complete nervous system packed inside an ancient fossil that swam the seas 520 million years ago.

The specimen is most likely from long-extinct group marine arthropods called megacheirans, which is Greek for "large claws," a University of Arizona news release reported.

"We now know that the megacheirans had central nervous systems very similar to today's horseshoe crabs and scorpions," senior author of the study, Nicholas Strausfeld, a Regents' Professor in the University of Arizona's department of neuroscience, said. "This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian."

The ancient critter measures at only a little over an inch and was found in southwest China. It is part of the extinct genus Alalcomenaeus, which is known for characteristics such as a long segmented body, multiple appendages, and "a pair of long, scissor-like appendages attached to the head" believed to have been used for either dexterous or sensory purposes. 

The team analyzed the fossil using different image processing techniques that look at iron deposits in the fossil.

The researchers also used computed tomography (CT) to reconstruct the specimen's likeness in 3-D.

"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."

The researchers then overlaid and then subtracted the images of iron deposits and the green CT scans.

"We discarded any image data that were not present in both scans," Strausfeld said. "Where the two overlapped, the magenta and the green added to each other, revealing the preserved nervous system as a white structure, which we then inverted."

The results show what appears to be an X-ray of the fossil.

"The white structures now showed up as black," Strausfeld said, "and out popped this beautiful nervous system in startling detail."

The creature is believed to have had a similar brain to a scorpion or a spider, which is characterized by three clusters of ganglia that are fused together and include some of the animal's body ganglia.

"Other diagnostic features include the forward position of the gut opening in the brain and the arrangement of optic centers outside and inside the brain supplied by two pairs of eyes, just like in horseshoe crabs," the news release stated.

The team believes the large claw was used for both grasping things and for sensory input. The part of the brain that was wired to this appendage may have developed into the biting mouthpart of a spider.

 "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 said.