3D Fossilized Brain of 500 Million-Year-Old Worm Discovered

A worm brain dating back over 500 million years has been discovered by scientists, providing the first ever three-dimensional fossilized nervous system of this ancient scalidophoran.

The scientists hope this discovery will pave the way for future developments in the fossil record and in our understanding of animal evolution.

The study, published in Royal Society Open Science, provides details on the internal anatomy of the fossilized embryos of a prehistoric worm called Markuelia humanensis. The extinct creepy-crawly lived on Earth around 500 million years ago, when plants first started growing on land. Their closest living relatives, who are also members of the scalidophoran group, are the Loricifera, the mud dragons, and the affectionately named penis worms.

trilobite fossils
Stock image of trilobite fossils imprinted in the sediment. The scalidophoran fossils are special because they have been preserved in three dimensions.
Merlinus74/Getty

The species was first discovered in Hunan Province in South China. “It was a bit of a mystery what they were until we did some work about 10 years ago which showed that they belonged to the scalidophoran group,” Philip Donoghue, one of the study’s authors, told Newsweek.

Soft tissues are rarely preserved in fossil remains, especially those as delicate as the nervous system. “Prior to maybe five or six years ago it was simply non-existent,” Donoghue said. “Usually it’s all in the form of completely flattened fossils.”

However, the stunningly detailed specimens studied here were found to have been preserved in three dimensions.

“It’s preserved in a fundamentally different way,” Donoghue said. “Basically, as the fossil begins to decay, the microbes that are consuming it actually lithify themselves and turn the thing into rock. So, it turns into a piece of rock before the sediment [turns into stone], and that protects it from being collapsed into two dimensions.”

“I think it’s pretty awesome,” he said, comparing them to the worms he has worked with in the lab. “If [lab worms] die, they turn into something resembling snot in a matter of minutes or hours. How on earth can you turn this into something like rock?”

The donut-like structure of the worm’s brain is similar to that of nematodes and other closely related parasites. “It shows that [the donut] brain structure had evolved already 530 million years ago,” Donoghue said. “These tissues that we have described so far really have begun to make a material impact on our understanding of the gross anatomy of arthropods.”

Scalidophoran embryo fossils
Markuelia humanensis fossil embryos under the microscope. The curled up fossilized worm embryos display well preserved brains and nervous systems.
Dong et al./Royal Society Open Source

The fossils were dated back to the early Cambrian period, a time when conditions on Earth were just right to facilitate the biggest explosion of animal diversity in our planet’s history. This biological big bang is the period when most major animal groups begin to appear in the fossil record.

“[This discovery] suggests that there’s this whole other window of the preservation of brains and nervous tissues in 3D that’s just out there waiting for us to look for it,” Donoghue said. “It’s just that people haven’t thought that these kinds of deposits preserve tissues in this way.”

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