Finding fossilized soft tissues and nerve fibers is quite rare, but an international team of scientists have hit the jackpot by unearthing one of the most intricate and oldest fossils of the nervous system ever found.
Dug up from Southern China, the fossil is that of a 520 million-year-old crustacean-like animal. It has been so well-preserved that its individual nerves are noticeable, marking the first time that this level of detail is observed in a fossil of its age.
Unprecedented Level Of Detail
The crustacean-like animal, which is called Chengjiangocaris kunmingensis, existed during the Cambrian Explosion about half a billion years ago. It was a time of rapid evolutionary progression when most major animal groups emerged in the fossil record.
C. kunmingenis is part of a group of animals called fuxianhuiids. It was an early ancestor of modern arthropods such as crustaceans, spiders, and insects.
Dr. Javier Ortega-Hernández, co-author of the study, said the C. kunmingenis fossil is the most complete sample of a nervous system from the Cambrian period, offering the most unprecedented level of detail.
With that, he said the fossil provides a glimpse into what the ancient nervous system looked like, and how the nervous system of joint-legged creepy-crawlies had evolved.
Brains And Bones
Throughout the last five years, researchers have distinguished partially-fossilized nervous systems in various species from the Cambrian period, but most of them were fossilized brains.
In most of these fossils, the details of the profile of the brain were the only ones preserved. The amount of information available was considered limited.
Meanwhile, the vast majority of fossils that scientists possess are mostly bone and other hard parts of the body such as teeth or exoskeletons. The nervous system and soft tissues are essentially comprised of fatty-like substances, so finding them preserved as fossils is extremely rare.
The team of researchers in the study, most of them from China, the United Kingdom and Germany, first identified a fossilized central nervous system in 2013. Now, the new material they discovered allowed them to investigate the significance of these findings.
Examining C. kunmingenis
C. kunmingensis appeared like a crustacean with a broad, nearly heart-shaped head shield. It also had a long body with pairs of legs of different sizes.
Researchers carefully prepared the fossils by chipping away the surrounding rock with a fine needle. They were able to view not only the hard parts of the animal's body, but the fossilized soft tissues as well.
Vertebrates such as fish, mammals, primates, and birds possess a brain and a spinal cord, while arthropods have a condensed brain and a chain-like series of interconnected nervous tissues called ganglia. These tissues resemble a string of beads.
Like modern arthropods, C. kunmingensis possessed a nerve cord - similar to the spinal cord of vertebrates - running across its body. Each one of the ganglia controlled a single pair of walking legs.
The team studied the exceptionally conserved ganglia of the specimen, and found dozens of fibers that measured five thousandths of a millimeter in length.
Ortega-Hernández said the delicate fibers exhibited a highly regular distribution pattern. They wanted to find out if the fibers were comprised of the same material as the ganglia. And so he and his colleagues used fluorescence microscopy to confirm that the fibers were in fact individual nerves that were fossilized as carbon films.
"These fossils greatly improve our understanding of how the nervous system evolved," he said.
Further analysis of the specimen revealed that some aspects of the nervous system of the C. kunmingensis seemed to be structured similarly to that of onychophorans or velvet worms, with nerves coming out from the ventral nerve cord.
On the other hand, these nerve fibers were lost in modern arthropods, indicating that simplification played a role in the evolution of the nervous system.
Meanwhile, the C. kunmingensis fossil's contribution to the record offers understanding toward the early understanding 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," added Ortega-Hernández.
The team's findings are to be featured in the Proceedings of the National Academy of Sciences.