B.B.C. Article
Researchers have discovered the 425-million-year-old remains of a new species of parasite - still clamped to the host animal it invaded.
The international team found the fossil at a site in Herefordshire.
Prof David Siveter, from the University of Leicester, said it had been "frozen in time" by a volcano that killed and preserved marine creatures at the site.
The discovery is published in the journal Current Biology.
The new species, which has been named Invavita piratica (meaning "ancient intruder" and "piracy"), belongs to a group of parasites known as tongue worms.
It is the ancient ancestor of 140 species of modern-day parasites that live within the respiratory system of their host.
"In fact, it's not a worm," said Prof Siveter. "It belongs to the broad group of animals we call arthropods."
This very large group includes insects, arachnids and crustaceans.

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Siveter et al, 2015 wrote:Highlights

•A 425-million-year-old pentastomid is the first adult example from the fossil record
•It is the only known fossil pentastomid preserved with its host
•Uniquely among pentastomids, some specimens are attached to the host externally
•Living pentastomids may have originated as ectoparasites on marine invertebrates

Summary

Pentastomids (tongue worms) are worm-like arthropods known today from ∼140 species [ 1 ]. All but four are parasitic on vertebrates. Their life cycle typically involves larval development in an intermediate host followed by maturation in the respiratory tract of a definitive terrestrial host. Fossil pentastomids are exceedingly rare and are known only from isolated juveniles [ 2–6 ]. The identity of the possible hosts of fossil pentastomids and the origin of their lifestyle have generated much debate. A new, exceptionally preserved species, described based on adults from 425-million-year-old marine rocks, is the only known fossil pentastomid associated with a host, in this case a species of ostracod crustacean. The pentastomids are preserved near eggs within the ostracod and also, uniquely for any fossil or living pentastomid, are attached externally to the host. This discovery affirms the origin of pentastomids as ectoparasitic on marine invertebrates. The terrestrialization of pentastomids may have occurred in parallel with the vertebrate invasion of land.