Sea creature's half-billion-year-old petrified brain ignites a debate about brain evolution

The fossil measures 1.5 cm in length and was discovered in 1984.

The fossil of a tiny marine creature that died more than half a billion years ago could force a rewrite of how the brain evolved in science textbooks. A new study published in the journal Science provides the first detailed description of Cardiodictyon catenulum, a worm-like animal preserved in rocks in southern China’s Yunnan province.

The 1.5 cm long fossil discovered in 1984 hides a hitherto important secret: a well-preserved nervous system, including the brain.

“As far as we know, this is the oldest brain fossil we know of,” said Nicholas Strausfeld, Regent Professor in the University of Arizona Department of Neuroscience.

Cardiodictyon belongs to an extinct group of animals known as armored lobopodians, which were abundant in the Early Cambrian Period, when nearly all major animal lineages appeared in the very short period between 540 million and 500 million years ago.

The study also states that “lobopodians probably moved on the seabed using several pairs of short, soft legs that lacked the joints of their descendants, the euarthropods, which is Greek for ‘true arthropods.'” The closest living relatives of the lobopodians today are the velvet worms which live mainly in Australia, New Zealand and South America.

The cardiodictyon fossil shows an animal with segmented trunks and arrangements of nerve structures known as ganglia. This was in stark contrast to the head and brain, which showed no signs of segmentation.

Straussfeld continued: “This anatomy is wholly unexpected in that for over a hundred years the heads and brains of modern arthropods, and some of their fossil ancestors, have remained fragmentary.”

This discovery, according to the authors, resolves a long-standing and controversial debate about the origin and formation of the cephalothorax in arthropods which includes insects, crustaceans, spiders and other spiders in addition to centipedes and centipedes.

Study co-author Frank Hirth said: ‘Since 1880, biologists have noted the appearance of a clearly segmented trunk characteristic of arthropods, extrapolating it primarily to the head.’

“But Cardiodictyon showed that the early head was not segmented, and neither was the brain, suggesting that the brain and nerve stem most likely evolved separately,” Strausfeld added.

According to the authors, their findings also provide a message of continuity at a time when the planet is changing dramatically due to climate change.

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