A fossil of a 520-million-year-old animal is so well preserved that its individual nerve fibers are still visible, according to a new study on the crustacean-like creature that once lived in southern China.

The fossil represents the oldest and most detailed central nervous system ever found, reports the study, which is published in the journal Proceedings of the National Academy of Sciences.

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The vast majority of fossils only reveal bone and other hard body parts, such as teeth, since the nervous system and soft tissues are essentially made of fatty-like substances that degrade over time.

"This is a unique glimpse into what the ancestral nervous system looked like," study co-author Javier Ortega-Hernández of the University of Cambridge's Department of Zoology said of the fossil in a press release. "It's the most complete example of a central nervous system from the Cambrian period."

The animal, Chengjiangocaris kunmingensis, lived during a key part of Earth’s history known as the Cambrian explosion, a time during which rapid evolutionary development took place. At this time about a half billion years ago, most major animal groups first appear in the fossil record.

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C. kunmingensis was a type of animal known as a fuxianhuiid, and was an early ancestor of modern insects, spiders and crustaceans.

Its nervous system featured a nerve cord running throughout its body. The nerve cord was similar to the spinal cord that we and many other organisms have today. Bead-like ganglia, or bundles of nerve cells, controlled the animal's single pair of walking legs.

Ortega-Hernández and his team closely examined the exceptionally preserved ganglia and discovered dozens of spindly fibers, each measuring about five thousandths of a millimeter in length.

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“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,” he explained. “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.”

The researchers found that some aspects of the creature's nervous system are similar to that of modern velvet worms and penis worms, which really do resemble the male sexual organ. All of these animals have regularly spaced nerves that emanate from the central nerve cord.

Conversely, dozens of these nerves were independently lost over evolutionary time in many living animals including water bears, which are eight-legged segmented water-dwelling organisms that are so tiny that scientists refer to them as "micro-animals." The loss of the nerves over time in many species suggests that simplification played an important role in the evolution of the nervous system.

While the nerve cord is similar to today's spinal cord, it was a unique structure that is otherwise unknown in living organisms.

Ortega-Hernández 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."

Photo: Complete specimen of Chengjiangocaris kunmingensis from South China. Credit: Jie Yang, Yunnan University