Trove of Fossils Uncovered in the Grand Canyon Offers a Rare Glimpse Into Cambrian Life, With Toothy Worms and Slug-Like Mollusks

Rocks found along the Colorado River in Arizona turned out to contain fossilized fragments of soft-bodied creatures, suggesting the site may have been an “evolutionary hotbed”

Sara Hashemi | Daily Correspondent

July 29, 2025

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More than half a billion years ago, strange worms and mollusks teemed in the shallow waters that covered where the Grand Canyon is today.

That discovery came after Giovanni Mussini, a paleontologist at the University of Cambridge in England, and other scientists came across a trove of dozens of fossils along the Colorado River in Arizona back in 2023. The team then brought the rocks back to the United Kingdom for further research, and their results were published in the journal Science Advances last week.

To study the fossils, Mussini and his team dissolved the rocks they were encased in using hydrofluoric acid, then passed the resulting sediment through multiple sieves. “It’s very time-consuming,” Mussini tells Kenneth Chang at the New York Times. “There’s no guarantee of success, and few people have used this technique to explore Cambrian ecosystems.”

Thankfully for the team, the filtering revealed thousands of tiny fossils that paint a picture of life along the Grand Canyon during the Cambrian Period, an era that saw the rapid emergence of animal groups that have descendants today. Though none of the fossils were complete, they retained structures that allowed the researchers to identify them. Among the collection: slug-like mollusks, crustaceans with molar teeth and a new species of cactus worm—a group also known as penis worms.

The fossils, which date to between 507 million and 502 million years ago, suggest the Grand Canyon’s prehistoric ecosystem was an “evolutionary cradle,” Mussini tells Leo Sands at the Washington Post. “It sharpens our understanding of the economics of early animal evolution.”

The new species of cactus worm—named Kraytdraco spectatus after the fictional krayt dragons from the Star Wars franchise because of its rows of teeth—exemplifies that impressive rate of evolution. The toothy worm would have been about an inch long and could extend a tube-like mouth to catch prey.

“Some of these look like fossilized woolly bear caterpillars in some way,” says study co-author James Hagadorn, a curator of geology at the Denver Museum of Nature and Science, to the New York Times. “Clearly, they’re not caterpillars, but they have every little hair and fiber preserved on them.”

The researchers also found burrows dug by some of the animals, as well as scrapes on rocks where the mollusks had raked their teeth to gather up algae or bacteria for food. “By combining these fossils with traces of their burrowing, walking and feeding—which are found all over the Grand Canyon—we’re able to piece together an entire ancient ecosystem,” Mussini adds in a statement.

Typically, it’s oxygen-poor conditions that might preserve a large trove of Cambrian fossils, like at the Maotianshan Shales in China and Burgess Shale in the Rockies. But areas with lots of oxygen are more likely to be hotbeds of life. The Grand Canyon’s oxygen-rich waters would have been more conducive to evolutionary experimentation.

“Animals needed to keep ahead of the competition through complex, costly innovations, but the environment allowed them to do that,” Mussini explains in a statement.

The newly described fossils also represent soft-bodied creatures, making them unique among other Cambrian finds in the Grand Canyon.

Karma Nanglu, a paleontologist at Harvard University who was not involved in the work, tells the Washington Post that incomplete fossils like these can often be unappreciated. However, he adds that statistical comparisons with other sites are needed to confirm whether the ecosystem described in the study was truly home to more diverse forms of life than other areas were.

“I wouldn’t say it incontrovertibly shows a higher level of diversity of functional modes when compared to other places,” he explains to the outlet. “This site does show quite a unique assemblage of animals. Whether or not it is uniquely escalated is pretty hard to validate.”

Still, the study highlights the fact that places like the Grand Canyon, which aren’t well-known for their fossils, can have much to offer. Further research might unearth even more secrets about ancient life in the region.

Sara Hashemi | Read More

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.