Fossil of segmented animal preserves three-part central nervous system
Rusty red stains on the head of a fossilized segmented creature found in southwestern China are a paleontological record-breaker: They are the remains of the oldest arthropod brain ever found. The imprint of the 520-million-year-old critter’s three-part brain indicates that complex nervous systems evolved fairly early in animal evolution, among the ancestors of insects, centipedes and crustaceans.
The roughly 7-centimeter-long specimen includes the entire body of Fuxianhuia protensa. The species lived during the Cambrian period, before modern arthropod lineages evolved. The fossil shows F. protensa had a brain composed of three sections that sat in front of the animal’s gut. That’s the same setup seen today in insects, crabs, lobsters and many other arthropods, researchers report in the Oct. 11 Nature.
“It was very fascinating and very exciting,” says study coauthor Nicholas Strausfeld, a neuroscientist at the University of Arizona. “It suggests that the organization we see in the modern [arthropod] brains is very ancient.”
Scientists had thought early arthropods had simpler brains like those of modern water fleas, fairy shrimp and other tiny freshwater crustaceans called branchiopods. The branchiopod brain consists of two connected parts with a third mass of nervous tissue sitting behind the stomach. Sometime after the branchiopod lineage split from the other arthropods, scientists had assumed, the nervous tissue behind the gut migrated up and connected with the other parts of the brain, Strausfeld says.
“With this complex Cambrian brain, we have to rethink our current interpretation,” says Steffen Harzsch of the University of Greifswald in Germany. The ancestors of branchiopods probably had a more complicated brain originally and later did some evolutionary backpedaling, he says. “Most likely the brain of branchiopods … many of which are filter feeders, was simplified in response to their lifestyle.”
Alternatively, F. protensa’s style of brain may not have been the norm for early arthropods. The animal may have independently evolved a complex brain that resembles those of modern species, making it a case of convergent evolution. Strausfeld and his colleagues are now looking for remnants of brains in other Cambrian fossils to see if other ancient arthropods also had sophisticated nervous systems.
X. Ma et al. Complex brain and optic lobes in an early Cambrian arthropod. Nature, Vol. 490, October 11, 2012, p. 258. doi: 10.1038/nature11495
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