Complete nervous system discovered preserved in 520-million-year-old fossil in southwest China
Scientists have discovered the earliest known complete nervous system preserved in the fossilised remains of a creature that crawled or swam in the ocean 520 million years ago.
The 3cm-long fossil, found in the Chengjiang formation near Kunming in southwest China, is a distant relative of scorpions and spiders and belongs to an extinct group of marine arthropods known as megacheirans (Greek for “large claws”). Researchers from the University of Arizona’s department of neuroscience and London’s Natural History Museum believe that the ancestors of spiders and scorpions evolved from other arthropods including insects, crustaceans and millipedes, more than half a billion years ago.
“We now know that the megacheirans had central nervous systems very similar to today’s horseshoe crabs and scorpions,” said Professor Nicholas Strausfeld at the University of Arizona, senior author of the study.
“This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian.”
The fossil belongs to the extinct genus Alalcomenaeus and animals in this group had an elongated, segmented body with about a dozen pairs of body appendages enabling the animal to swim or crawl or both.
They all have a pair of long, scissor-like appendages on the head, which scientists believe are used for grasping or sensory purposes.
Greg Edgecombe at the Natural History Museum, co-author of the study, said that some paleontologists had used the external appearance of the appendage to infer that the megacheirans were related to chelicerates - spiders, scorpions and their kin.
This was based on the fact that the appendage and the fangs of a spider or scorpion both have an “elbow joint” between their basal part and their pincer-like tip.
“However, this wasn’t rock solid because others lined up the great appendage either a segment in front of spider fangs or one segment behind them,” Edgecombe said.
“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 analyse 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.”
The scientists analysed the fossil using different imaging and image processing techniques, making use of the iron deposits that had selectively accumulated in the nervous system during fossilisation, and came up with a negative X-ray photograph of the fossil.
“The white structures now showed up as black and out popped this beautiful nervous system in startling detail,” said Professor Strausfeld.
They compared the outline of the fossil nervous system to nervous systems of horseshoe crabs and scorpions, and found that it had the typical hallmarks of the brains found in scorpions.
Three clusters of nerve cells known as ganglia fused together as a brain also fused with some of the animal’s body ganglia. This differs from crustaceans where ganglia are further apart and connected by long nerves, like the rungs of a rope ladder.
The forward position of the gut opening in the brain and the arrangement of optic centres outside and inside the brain supplied by two pairs of eyes were the same features found in horseshoe crabs.
When the scientists plugged these characteristics into a computer analysis programme it allowed them to determine that the 520-million-year–old Alalcomenaeus fossil was a member of the chelicerates.
“The prominent appendages that gave the megacheirans their name were clearly used for grasping and holding and probably for sensory inputs,” said Professor Strausfeld.
“The parts of the brain that provide the wiring for where these large appendages arise are very large in this fossil. Based on their location, we can now say that the biting mouthparts in spiders and their relatives evolved from these appendages.”
Less than a year ago, the same research team published the discovery of a fossilised brain in the 520-million-year-old fossil Fuxianhuia protensa, showing unexpected similarity to the complex 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 said.
“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 results have been published in the latest issue of journal Nature.