The fossil in question is of Chengjiangocaris kunmingensis, a crustacean-like animal that lived approximately 520 million years ago (Cambrian period) in what is now China.
This creature belongs to a group of animals called fuxianhuiids, and was an early ancestor of modern arthropods (insects, spiders and crustaceans).
The fossil has been so well preserved that individual nerves are visible, the first time this level of detail has been observed in a fossil of this age.
“This is a unique glimpse into what the ancestral nervous system looked like. It’s the most complete example of a central nervous system from the Cambrian period,” said team member Dr. Javier Ortega-Hernández, a paleobiologist at the University of Cambridge, UK, and a co-author of a paper in the Proceedings of the National Academy of Sciences.
Like modern arthropods, Chengjiangocaris kunmingensis had a nerve cord – which is analogous to a spinal cord in vertebrates – running throughout its body, with each one of the bead-like ganglia controlling a single pair of walking legs.
Closer examination of the ganglia revealed dozens of spindly fibers, each measuring about 1/5,000 mm in length.
“These delicate fibers displayed a highly regular distribution pattern, and so we wanted to figure out if they were made of the same material as the ganglia that form the nerve cord,” Dr. Ortega-Hernández said.
“Using fluorescence microscopy, we confirmed that the fibers were in fact individual nerves, fossilized as carbon films, offering an unprecedented level of detail,”
“These fossils greatly improve our understanding of how the nervous system evolved.”
Further analysis revealed that some aspects of the nervous system inChengjiangocaris kunmingensis appear to be structured similar to that of modern priapulids (penis worms) and onychophorans (velvet worms), with regularly-spaced nerves coming out from the ventral nerve cord.
In contrast, these dozens of nerves have been lost independently in the tardigrades (water bears) and modern arthropods, suggesting that simplification played an important role in the evolution of the nervous system.
Possibly one of the most striking implications of the study is that the exceptionally preserved nerve cord of Chengjiangocaris kunmingensisrepresents a unique structure that is otherwise unknown in living organisms.
“The specimen demonstrates the unique contribution of the fossil record towards understanding the early evolution of animals during the Cambrian period,” Dr. Zhang and co-authors said.
“The more of these fossils we find, the more we will be able to understand how the nervous system – and how early animals – evolved,” Dr. Ortega-Hernández said.