Incredible fossil find is the oldest known parasite

Artist's depiction of what this brachiopod—and its parasites—would have looked like.
Enlarge / Artist’s depiction of what this brachiopod—and its parasites—would have looked like.
Zhifei Zhang (Northwest University)

From the perspective of a legacy-seeking critter deep in Earth’s history, there’s little chance of you hitting the big time. The odds of getting fossilized are low enough. You need to die in the right kind of place, get buried before you are picked apart or decay, and encounter the right kind of chemistry underground that replaces your fleshy bits with enduring stone.

This unlikely chain makes capturing common life events like your last meal or developing embryos even more rare. But in the case of a newly published study, researchers were lucky enough to find what appear to be the earliest known parasites, still stuck to the hosts they targeted some 510 million years ago.

The find comes from Yunnan, China, where a sedimentary rock layer called the Wulongqing Formation is chock full of tiny fossil brachiopods of a species named (quite sensibly) Neobolus wulongqingensis. Back in the Cambrian Period, shortly after multicellular animal life bloomed into incredible variety, these creatures were living on the seafloor. A team led by Zhifei Zhang at China’s Northwest University discovered that N. wulongqingensis was not alone in the rock—many were adorned with whitish tubes on the exteriors of their shells.

One example of the brachiopod with the associated tube-shape creatures. The white scale bar at the bottom is 2 millimeters.
Enlarge / One example of the brachiopod with the associated tube-shape creatures. The white scale bar at the bottom is 2 millimeters.

Nothing is really preserved of these items beyond their tube-like shape. But the consistent pattern on the brachiopods suggests they were creatures themselves rather than “trace fossils” like burrows or something purely geological in origin. They stretch radially outward on the brachiopod shells, extending slightly beyond the edge of the shell.

To the researchers, they sure looked like parasites. But clear identification of parasites in the fossil record has been quite rare. Even if both organisms are preserved well enough to be found together, can you be sure the relationship was parasitic? That’s a zero-sum relationship where one organism extracts nourishment to the detriment of the other. There are also mutualist (both benefit) and commensalist (one benefits and the other doesn’t care) relationships that could well look the same when fossilized.

In this case, there were so many fossil specimens to work with that the researchers were able to do something pretty cool: compare the size of a couple hundred tube-hosting individuals to a couple hundred tube-free ones. They found a very clear difference. Individuals with one or more tubes stuck to them were about 26-percent smaller on average. There was also a correlation with how far back toward the base of the brachiopod the tube started from. It looks like tubes that attached earlier in the brachiopod’s life had a bigger impact on their size.

That’s pretty good evidence that these tubes were parasitic creatures rather than something else. Specifically, the researchers say they look like an example of what’s known as “kleptoparasitism,” meaning they pilfered the brachiopod’s lunch. Brachiopods are filter feeders, sucking in water and catching food particles caught in the stream. By extending just beyond the brachiopod’s open shell, the tube parasites could catch food on its way in. It would be a bit like strapping yourself to the nose of a baleen whale and holding a net in front of its mouth. You’d get free food, and the whale would get less nourishment for the effort expended.

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This video shows the rocks the fossils came from and an animation of what they might have looked like while alive.

This makes the tube creatures the oldest parasite identified in the fossil record. Their existence isn’t really surprising, as others have pointed to things like shell damage as possible hallmarks of (missing) parasites in other fossils this old. Studies examining the branches in the tree of life have also estimated that parasites must have had their evolutionary roots in the Cambrian. Still, it’s incredible to find one.

The researchers note that parasitism is thought to be a potent evolutionary force and possibly even an important driver in the rise of sexual reproduction. So from that perspective, it’s noteworthy to see parasitism in action during this critically important period in evolutionary history.

Nature Communications, 2020. DOI: 10.1038/s41467-020-16332-3 (About DOIs).

https://arstechnica.com/?p=1680590