Scientists have found the fossil remains of a previously unknown species of parasite, clinging onto the animal it infected some 425 million years ago. This fascinating discovery, published in Current Biology, suggests that the "ancient intruder" was a type of tongue worm whose size ranged from 1 to 4 millimeters long.

"This is the most important fossil evidence yet discovered of the origins of this type of parasitism," said co-author Derek Briggs from Yale University in a statement.

The fossil remains, which were found in Herefordshire, England, were “exceptionally well-preserved,” according to an international team of researchers. The new parasite species has been aptly named Invavita piratica, which means "piracy" and "ancient intruder." This is the first time that researchers have found a fossilized tongue worm attached to its host. The tongue worm was found on an ostracod, which is a class of Crustacean that has two shells joined at a hinge.

"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 lead author David Siveter, from the University of Leicester, in a statement.

Tongue worms – called Pentastomida – are named after their worm-like bodies that in some species are shaped like tongues. These arthropods have a head and two pairs of limbs. The fossil specimens were found on the inside of the host animal’s shell and the external surface of the host's shell, which researchers say is unique for living or fossil tongue worms.

The fossil’s modern day relatives live within the respiratory system of the animals they invade. There are about 140 species, which are mainly parasitic on vertebrate animals. The vast majority of their hosts – around 70% – are snakes, but they are also known to infect humans. These parasites are known to have an "indirect" life cycle as they use an intermediate host, such as a fish, which is then consumed by the secondary host. 

Researchers suggest that the discovery of this fossil on an ostracod could mean that the lifecycle of this parasite evolved in a marine setting, which continued when they eventually moved to land onto the respiratory system of tetrapods. It’s unclear when this would have happened, but the study suggests that it could have occurred when vertebrates invaded land during the mid-to-late Palaeozoic period.