About 425 million years ago, a sneaky wormlike parasite invaded a crustacean before the two were fossilized together in the limestone of modern-day England, a new study finds.

The parasite, a newly identified species, is an ancient type of tongue worm, an arthropod that has a wormlike body with a head and two pairs of limbs, the researchers said. It is the first adult tongue worm to be found in the fossil record, they added.

"This is the most important fossil evidence yet discovered of the origins of this type of parasitism," study co-author Derek Briggs, a professor of geology and geophysics at Yale University and curator of invertebrate paleontology at the Yale Peabody Museum of Natural History, both in New Haven, Connecticut, said in a statement. [The 10 Most Diabolical and Disgusting Parasites]

The new species was dubbed Invavita piratica, meaning "ancient intruder," as a nod to maritime piracy. It was discoveredin a crustacean host, called Nymphatelina gravida, a marine creature with two shells connected by a hinge. In fact, the researchers found several specimens of I. piratica in the fossil, which the scientists found in limestone rocks in Herefordshire, England. 

The I. piratica specimens are "exceptionally preserved," and range in size from about 0.04 to 0.16 inches long, the researchers wrote in the study.

"This discovery is important not only because examples of parasites are exceptionally rare in the fossil record, but also because the possible host of fossil tongue worms — and the origin of the lifestyle of tongue worms — has been the subject of much debate," said the study's lead author, David Siveter, a paleontologist at the University of Leicester, in the United Kingdom.

Researchers found several of the parasites inside the crustacean's shell, near a bundle of eggs that may have belonged to the host, the researchers said. Other parasites were anchored to the outside of the shell, a position that is unique even among living tongue worms, the researchers said.

Nowadays, there are about 140 known species of parasitic tongue worms, known as pentastomids, which mostly prey on vertebrates, especially fish and reptiles.

However, these parasitic worms are known sometimes to live in the gut-respiratory system of humans, which the worms infect after a person eats an intermediate host, such as a fish, the researchers said.

The study was published online May 21 in the journal Current Biology.

 

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