Researchers from the University of Cambridge recently uncovered a 520 million year old fossil with a special designation– it’s the most detailed Cambrian period nervous system ever found.
The fossil is that of an ancient crustacean-like animal named Chengjiangocaris kunmingensis, which was recovered from southern China. C. kunmingensis, which belonged to a group of animals called fuxianhuiids, was an early ancestor of modern arthropods. It had a broad, nearly heart-shaped head shield, a long body, and pairs of legs that grew smaller in size as they got farther from the head.
The fossil contains not only a preserved nerve cord, but individual nerves—the first time scientists have ever come across such a minute level of detail in a fossil of this age. Which means this fossil (also known as specimen YKLP 12026) gives researchers a one-of-a-kind insight into how the nervous system of arthropods—animals with segmented bodies, exoskeletons, and jointed appendages, otherwise known very scientifically as “creepy crawlies”—evolved.
Chengjiangocaris kunmingensis itself lived through an extremely important evolutionary event known as the Cambrian explosion about half a billion years ago, during which most of the major animal groups made their first appearance in the fossil record.
“This is a unique glimpse into what the ancestral nervous system looked like,” said study co-author Dr Javier Ortega-Hernández, of the University of Cambridge’s Department of Zoology, in a statement. “It’s the most complete example of a central nervous system from the Cambrian period.”
The brains of the operation
Finding any preserved soft tissue is extremely rare; the vast majority of recovered fossils consist of bone or other hard body parts, like the exoskeleton—since soft tissue tends to degrade before it can be preserved.
Up until now, though, researchers have come some soft-tissue nervous system specimens from animals roughly contemporaneous with C. kunmingensis—but most of these have been fossilized brains, or more often than not just the profiles of fossilized brains. The Cambridge researchers, however, had discovered five specimens lacking brains, but with preserved ventral nerve cords (like a spinal cord, only running down the belly of the beast).
And, after careful preparation of specimen YKLP 12026—which involved chipping away at the rock surrounding the fossil using a fine needle—a close examination revealed that not only was its nerve cord preserved, but “delicate nerve roots” about one five-thousandth of a millimeter in length as well, according to the paper in Proceedings of the National Academy of Sciences.
“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-Hernández.
“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.”
Perhaps the most exciting part of this discovery to the Cambridge researchers, however, is what this find means in regards to the evolution of certain animals.
For example, there seem to be some modern animals with similar nervous system structure to C. kunmingensis: Namely, priapulids* (penis worms) and onychophorans (velvet worms).
However, other animals down the line aren’t quite so similar. For example, dozens of nerves were lost over time in tardigrades (“water bears,” which turn into glass when dehydrated) and modern arthropods—which suggests that evolutionarily, simplification played a major role in the progression of the nervous system.
But perhaps the most striking part of the discovery is what is different between modern animals and 520-million-year-old C. kunmingensis: Its particular preserved nerve cord is a structure that is otherwise unknown in living organisms.
“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-Hernández.
Image credit: Jie Yang (Yunnan University, China).
*Named for the Greek and Roman god Priapus, who had an eternal erection. Read a bit more about him here (NSFW–penises.)