As a rule parasites don’t fossilise well. They are usually small, soft bodied and many live inside other animals, making them even less likely to be preserved when their host dies and begins to break down. Occasionally eggs are found, mostly because they are adapted to be resilient, but their parents are usually conspicuous by their absence. Bearing this in mind the recent discovery of a beautifully fossilised ‘tongue worm’ is particularly spectacular.
The tiny organism is only a few millimetres long and is so far unique in being the only such parasite discovered along with its host. The host in question is a tiny arthropod from a group called the ‘ostracods’ which are highly successful animals with around 8000 existing species. They are also abundant in the fossil record. This particular individual is from 425 million year old rocks in Herefordshire, southern England, which were formed when this area was part of the sea. When the fossil was scanned and reconstructed using computer imaging it revealed two small parasites attached to the ostracod. The first, and most obvious from the reconstructions, clings to the outer carapace with its long, antenna-like body trailing, and there is another parasite of the same species clinging on underneath the ostracod near its eggs. Both parasites belong to a group today known as the ‘tongue worms’ but properly called the ‘pentastomids’.
Today the tongue worms live exclusively inside their hosts. They are ingested as eggs and then grow within their host’s body before the adult parasites move to take up residence in the lungs or nasal passage. They are most commonly found in snakes and crocodiles but some parasitise birds and mammals, including humans. This new fossil reveals that the first pentastomids actually evolved as external parasites before later evolving to move inside their hosts. It also proves that tongue worms evolved very early in the history or life, and likely moved onto land along with the first vertebrates. This is a fascinating and unique insight into the evolution of what is today a ubiquitous parasite, and hopefully as our ability to scan and reconstruct fossils improves more finds like this on will come to light.
Reference: Siveter, D.J. et al. A 425-Million-Year-Old Silurian Pentastomid Parasitic on Ostracods. Current Biology, 2015 DOI: 10.1016/j.cub.2015.04.035