If you look around Waukesha County in Wisconsin today, it can be difficult to imagine a tropical coastline teeming with trilobites, the oldest known scorpions and jawless vertebrates. But 437 million years ago, during the Silurian period, these creatures lived and died there, some getting washed into a salty cove and entombed under mats of microbes. Soft-bodied species were preserved in remarkable detail, with some retaining traces of eyes, guts and even hearts.
A team of scientists recently described the region’s latest fossil find: the oldest known species of leech. In a paper published Wednesday in the journal PeerJ, scientists say the parasitic worm pushes back the origins of these organisms by at least 200 million years. They also highlighted its remarkable powers of suction and surprising appetite for prey.
Leeches today are both friend and foe, helping to draw blood in traditional medicine and menacing journeys through wetlands. But the evolutionary origins of leeches remain enigmatic because their soft bodies rarely fossilize.
“Leeches are an entire branch of the tree of life for which we basically have no evidence in the fossil record at all,” said Karma Nanglu, a paleontologist at the University of California, Riverside, and one of the authors of the paper.
Dr. Nanglu was reading a scientific paper on fossils unearthed in quarries in southeastern Wisconsin when he was struck by fossils of wormlike animals that had been labeled potential leeches. He shared it with Danielle de Carle, an expert on modern leeches at the University of Toronto and the lead author of the new study.
Dr. de Carle was intrigued, especially when she came across photos of a two-inch-long fossil in the University of Wisconsin Geology Museum in Madison. “This one looked the most leech-like to us,” she said. “The second we saw it, we knew we had something.”
The team analyzed the fossil, which is shaped like a cowbell, and identified telltale features. The animal’s body is segmented and ends in a wide sucker that resembles the adhesive structure seen in modern leeches. The team named the animal Macromyzon (“large sucker”) siluricus.
Macromyzon pushes the origin of leeches back into the Silurian and calls into question theories about the group as a whole. Today, most leeches reside in freshwater environments, which were thought to be the group’s ancestral home. But Macromyzon lived in a salty marine environment.
Scientists have also long thought that the earliest leeches were bloodsuckers. All modern leeches, including species that do not consume blood, possess a suite of anticoagulants. But Macromyzon lived in an environment where vertebrates were scarce, making a blood-only diet difficult to sustain.
Instead, the researchers proposed that Macromyzon was either engulfing invertebrate prey whole or vacuuming out the insides of its prey.
“There are some modern leeches that attach to arthropods and suck out their bodily fluids from the soft membranes between their plates,” Dr. de Carle said. “There are lots of trilobites at Waukesha, so we think that might have been a pretty viable source of food.”
Kenneth Gass, a paleontologist at the Milwaukee Public Museum who was not involved in the study, thinks it is difficult to conclusively link Macromyzon with leeches because the front end of the animal is not intact. In 2023, Dr. Gass and his colleagues published a study that found another suspected leech from Waukesha was really a worm that had been fossilized midmolt. The animal’s sucker turned out to be skin that had sloughed off.
However, Dr. Gass is not ready to rule out that ancestral leeches terrorized trilobites in prehistoric Wisconsin.
“If leeches go back that far, it would not be surprising that their fossils would turn up in the Waukesha Biota,” he said, referring to the Wisconsin fossil site.
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