Phallic worm shared its home with bristly squatters in ancient symbiosis

Scientists have uncovered rare fossil evidence of a close inter-species relationship more than 500 million years ago in what is now the Canadian Rockies.

The two kinds of animals routinely shared a home under the sea floor during the Cambrian period, fossils from the Burgess Shale deposit in B.C.'s Yoho National Park show.

It's one of the oldest examples in the fossil record of symbiosis, a close, long-term interaction between two species, Canadian scientists have reported in a new study.

All about acorn worms

The two marine worms are a bit of an odd couple whose relatives still exist today.

The larger of the two is an acorn worm, with an acorn-shaped head, a collar at the base of that, and a worm-like body that together give it a phallic shape. The fossil specimens are about five centimetres long and a centimetre wide, similar in size to the cap of a drawing marker.

They're not related to earthworms or parasitic worms such as roundworms. Instead, they're "hemichordates" more closely related to sea urchins and to chordates, the group of animals that includes humans and other animals with a backbone.

The acorn worm's appearance has barely changed in more than 500 million years.

"If I had a modern day species next to it, the two look almost exactly the same," said Karma Nanglu, lead author of the new study published in the journal Proceedings of the Royal Society B. 

Nanglu, a postdoctoral researcher at the Smithsonian National Museum of Natural History in Washington, D.C., worked on the study with Jean-Bernard Caron, curator of invertebrate paleontology at the Royal Ontario Museum in Toronto. 

In a previous study, they found that while modern acorn worms dig simple burrows lined with mucus, their ancient Cambrian relatives built sturdy, elaborate tubes out of a protein called collagen in the sea floor.

Naglu said the tubes were likely built as a defence strategy to protect against predators that existed 506 million to 508 million years ago in the shallow seas that formed the Burgess Shale. 

At a Burgess Shale location called Raymond Quarry, large acorn worm tubes contain multiple acorn worms, suggesting they may have collaborated on construction.

Researchers proposed they could also be juveniles living in a home constructed by an adult, but there was no obvious evidence for that. 

All about bristle worms

The tubes also contained tiny roommates of another species — bristle worms about the size of a sewing needle (not including its bristles).

Bristle worms, known to scientists as polychaetes, actually are relatives of earthworms and are ringed with segments, each with a pair of bristles. There are more than 10,000 modern species in all shapes and sizes, including some that resemble the tiny fossil worms found in the ancient acorn worm burrows.

Interestingly, modern day bristle worms are notorious squatters, living in the burrows, tubes, shells or even on the bodies of many other animals — and have even been reported living with acorn worms, Nanglu said.

Taking advantage of a dwelling built by another animal is, he said, a "pretty common sort of symbiosis." 

It's not clear what the acorn worms get out of the relationship. The two species don't have any specialized body parts that suggest they relied on one another, as some symbiotic animals do.

That led researchers to conclude that it's likely a "commensal" relationship, where only the bristle worms benefit, but the acorn worms aren't affected. 

'Beyond lucky' fossil find

Nanglu said the discovery is exciting as evidence of behaviour is rare in the fossil record.

"An interaction between species preserved perfectly in the rock is beyond lucky," Nanglu said. "And then the second thing is … it's not an association between two fossils that have readily fossilized. The parts are two of the most rare groups of animals to find preserved in this kind of exquisite detail."

That's because fossils tend to preserve hard things like bones and shells, and fossils of soft-bodied animals are rare.

The new study was funded by the Peter Buck Deep Time post-doctoral fellowship at the Smithsonian National Museum of Natural History and the Natural Sciences and Engineering Council of Canada.

Prof. Leif Tapanila, director of the Idaho Museum of Natural History, studies fossil evidence of animal interactions. He called the worm fossils "a remarkable find" and said the evidence that they were cohabiting is "pretty darn convincing."

Even though they're rare, this isn't the oldest or only example of symbiosis ever found in Cambrian fossils. In 2017, scientists from the Natural History Museum in London reported 520-million-year-old marine worm fossils from China with smaller worms attached to them.

Tapanila has also found trace fossils left by worms living on the skeletons of coral from a slightly later period, the Ordovician. But he said the kind of cohabitation in the new study is different and provides evidence for the types of interactions he would have expected in an ecosystem as complex as this one appears to be.

"Seeing animals in close association, living in a common space … suggests a kind of co-operation between two different species that I think is still mysterious," he said.

"Are they helping, you know, generate resources for each other, as many co-operative symbioses are? And maybe in the future someone will come across a fossil … that will allow us to explore that mystery further."