Scientists have discovered one of the oldest fossils of the central nervous system yet distinguished, from a crustacean-like animal that lived more than 500 million years back. The fossil, from southern China, has been so well preserved that individual nerves are noticeable, this the first time that details of this kind have been seen in a fossil so old.
The discoveries, appearing in the Proceedings of the National Academy of Sciences, are offering assistance to researchers with understanding how the nervous system of arthropods -creatures with joined legs came into being. Finding any fossilized tissue is uncommon, yet this specific discovery, by scientists in the UK, China and Germany, speaks volumes.
The animal, called Chengjiangocaris kunmingensis, lived during the Cambrian ‘explosion’. C. Kunmingensis belongs to a group of animals called Fuxianhuiids, and was an early ancestor of modern arthropods – the diverse group that includes insects, spiders and crustaceans.
This is a unique glimpse into what the ancestral nervous system looked like, said study co-author Dr Javier Ortega-Hern ndez, of the University of Cambridge’s Department of Zoology. It’s the most complete example of a central nervous system from the Cambrian period.
Over the past five years, researchers have identified partially-fossilised nervous systems in several different species from the period, but these have mostly been fossilised brains. And in most of those specimens, the fossils only preserved details of the profile of the brain, meaning the amount of information available is limited.
The vast majority of fossils we have are mostly bone and other hard body parts such as teeth or exoskeletons. Since the nervous system and soft tissues are essentially made of fatty-like substances, finding them preserved as fossils is extremely rare.
Like modern arthropods, C. 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 exceptionally preserved ganglia revealed dozens of spindly fibres, each measuring about five thousandths of a millimetre in length. These delicate fibres 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, said Ortega-Hern ndez. Using fluorescence microscopy, we confirmed that the fibres were in fact individual nerves, fossilised as carbon films, offering an unprecedented level of detail. These fossils greatly improve our understanding of how the nervous system evolved.
For Ortega-Hern ndez and his colleagues, a key question is what this discovery tells us about the evolution of early animals, since the nervous system contains so much information. Further analysis revealed that some aspects of the nervous system in C. 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.
Possibly one of the most striking implications of the study is that the exceptionally preserved nerve cord of C. kunmingensis represents 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. The more of these fossils we find, the more we will be able to understand how the nervous system – and how early animals – evolved, said Ortega-Hern ndez.