Prehistoric parasites

Our understanding of very early life is constantly developing. Carolyn Lewis, research technician in the Museum’s Earth collections, describes a recent discovery.

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Invavita piratica is a new species of fossil parasite, recently discovered at the Museum in a 425 million year old rock from Herefordshire. This tiny creature, which belongs to an unusual group of parasitic arthropods called pentastomids, is particularly exciting because it was found attached to its host, an ostracod crustacean. A paper published last week in the journal Current Biology by Professor Derek Siveter, Senior Research Fellow at the Museum, and his co-researchers describes how this discovery sheds new light on the evolution of pentastomids.

The pentastomid Invavita piratica is the latest new species from the Silurian Herefordshire Lagerstätte, a deposit of exceptionally well preserved marine invertebrate fossils ranging from less than a millimetre up to a few centimetres in length. We investigate the fine structure of the Herefordshire fossils by a process of serial grinding and photography followed by painstaking editing and 3D digital reconstruction of the specimens as ‘virtual’ fossils.

The many arthropods so far described from the Herefordshire Lagerstätte include four new species of ostracod, tiny bivalved crustaceans that are widespread in the oceans of today. The Herefordshire ostracod fossils are preserved in exquisite detail including limbs, spines, eyes and in one species, Nymphatelina gravida, eggs.

It was while editing a specimen of Nymphatelina gravida, that we spotted three puzzling star-shaped objects: an overlapping pair attached externally to the shell and one inside the body of the ostracod. On further investigation these were identified as adult pentastomids, each with an elongated snout, two pairs of outstretched limbs and a long slender trunk, together forming the star-shape. The eggs of Nymphatelina gravida may have provided a source of nutrition for the internal parasite.

Fossil pentastomids are incredibly rare: Invavita piratica is the first adult fossil pentastomid to be discovered and the fossil pentastomid to be found attached to its host.  Apart from our Silurian specimens, just a few isolated juvenile pentastomid fossils are known from even older Upper Cambrian and Ordovician rocks.

Our discovery of a marine ostracod as the host of Invavita piratica shows that the parasitic lifestyle of pentastomids first evolved in the sea with invertebrates as early hosts. Pentastomids like Invavita piratica may have been transferred to marine vertebrates when their ostracod hosts were eaten by fish or conodonts. The timing of the terrestrialisation of pentastomids is unknown but it may have been in parallel with the subsequent vertebrate invasion of the land.

Living pentastomid species almost exclusively infest the respiratory tract of land-dwelling vertebrates, particularly reptiles but also birds and mammals. Because all known fossil pentastomids lived long before land vertebrates evolved, the identity of these early hosts were something of a puzzle.

Carolyn Lewis – Research Technician