525-million-year-old fossil defies textbook explanation of brain evolution

Fossils of a tiny sea creature that died more than half a billion years ago could force a science textbook to rewrite how brains evolved.

a published study ScienceLed by Nicholas Strausfeld, Regent’s Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, Reader in Evolutionary Neuroscience at King’s College London – present the first detailed description of Cardiodictyon catenulum, a worm-like animal preserved in the rocks of southern China. Yunnan province. The half-inch (less than 1.5 centimeters) long fossil, first discovered in 1984, had until now hid an important secret: a delicately preserved nervous system, including the brain.

“To our knowledge, this is the oldest fossilized brain we’ve ever known,” Strausfeld said.

Cardiodicty belonged to an extinct group of animals known as armored lobopodians, which were abundant during a period known as the Cambrian, a period in which almost all the major animal lineages appeared in an extremely short period between 540 million and 500 million years ago. Lobopodians probably moved on the seafloor using multiple pairs of soft, slender legs that lacked the joints of their euarthropod descendants—Greek for “true jointed leg.” Today’s closest relatives of lobopodians are velvet worms, which live mainly in Australia, New Zealand and South America.

A debate going back to the 1800s

fossils Cardiodicty reveal an animal with a segmented body with repetitive arrangements of nerve structures known as ganglia. This contrasts strongly with his head and brain, both of which lack evidence of segmentation.

“This anatomy was completely unexpected because the heads and brains of modern arthropods and some of their fossilized ancestors had been thought to be segmented for over a hundred years,” Strausfeld said.

According to the authors, the finding resolves a long and heated debate about the origin and composition of the head in arthropods, the world’s most species-rich group of animals. Arthropods include insects, crustaceans, spiders, and other arachnids, plus some other genera such as millipedes and centipedes.

“Since the 1880s, biologists have noted the distinctly segmented appearance of the trunk characteristic of arthropods and extrapolated this largely to the head,” Hirth said. “That’s how the field came to think that the head is an anterior extension of the segmented body.”

“Boot Cardiodicty This suggests that the early head was not segmented, nor does it suggest that the cerebrum, the cerebrum, and the central nervous system developed separately,” Strausfeld said.

Brains fossilize

Cardiodicty It was part of the Chengjiang fauna, a famous fossil deposit discovered by paleontologist Xianguang Hou in Yunnan province. The soft, delicate bodies of lobopodians are well preserved the fossil recordbut otherwise Cardiodicty None have been examined for head and brain, possibly because lobopodia are generally small. The most prominent parts Cardiodicty were a series of triangular, saddle-shaped structures that defined each segment and served as points of attachment for pairs of legs. They were found in older rocks from the Cambrian period.

“This suggests to us that armored lobopodians may have been the earliest arthropods,” Strausfeld said, predating trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.

“Until recently, the common understanding was that ‘brains don’t fossilize,'” Hirth said. “So, first of all, you wouldn’t expect to find a fossil with a preserved brain. Second, this animal is so small that you wouldn’t even dare to look at it hoping to find a brain.”

However, most of the work in the last 10 years has been done by Strausfeld, who identified several cases of brain preservation in various fossil arthropods.

A common genetic blueprint for building the brain

In their new study, the authors not only identified the brain Cardiodicty but also compared it to known fossils and fossils of living arthropods, including spiders and centipedes. By combining detailed anatomical studies of lobopodian fossils with analysis of gene expression patterns in their living descendants, they conclude that a common blueprint of brain structure has been conserved from the Cambrian to the present day.

“By comparing known gene expression patterns in living species,” Hirth said, “we identified a common signature of all brains and how they are formed.”

In Cardiodictyeach of the three brain areas is associated with a characteristic pair of head appendages and one of the three parts of the anterior digestive system.

“We realized that regardless of the species we were looking at, each brain area and its corresponding characteristics were determined by the same combination of genes,” Hirth said. “This suggested a common genetic blueprint for creating the brain.”

Lessons for vertebrate brain evolution

Hirth and Strausfeld say the principles described in their research probably apply to other creatures outside of arthropods and their close relatives. This has important implications when comparing nervous system Arthropods, they said, share a similarly distinct architecture, with the forebrain and midbrain being genetically and developmentally distinct from the spinal cord.

Strausfeld said their findings also provide a message of resilience at a time when the planet is changing dramatically under the effects of climate change.

“At a time when major geological and climatic events were reshaping the planet, simple marine animals, e.g Cardiodicty led to the creation of the world’s most diverse group of organisms—the euarthropods—which eventually spread to every habitat on Earth, but are now threatened by our own ephemeral species.”

Paper, “Lower Cambrian Lobopodia Cardiodicty “Unraveling the origin of euarthropod brains” is co-authored by Xianguang Hou at the Yunnan Key Laboratory of Paleontology at Yunnan University in Kunming, China, and Marcel Sayre, who has appointments at Lund University and the Department of Biology in Lund, Sweden. Sciences at Macquarie University in Sydney.