A parasite so bizarre that scientists still cannot agree on how it evolved and who it’s related to has turned up in a 425 million-year-old fossil still clamped to its host.

The find is fortuitous not only because it is the first fossil of an adult form, but also because it is the first found with a host, say the authors of the study reporting the fossils in Current Biology. There has been much head-scratching over the identity this parasite’s ancient host.

Today, nearly all pentastomids, as the parasites are called, infect fish or terrestrial vertebrates. But in the Silurian when this fossil formed, modern bony fish were new on the scene and land vertebrates non-existent.

Instead, the new fossil parasite was found sponging off an ostracod. Also called seed shrimp, these tiny crustaceans are still with us and live inside shell-like hinged valves, often resembling pepper seeds.

Ostracod.JPG
"Ostracod" by Anna33 at English Wikipedia. Licensed under CC BY 2.5 via Wikimedia Commons.

But today, nary a pentastomid, as the fossil parasite is called, is found on an ostracod.

Modern pentastomids suck blood in the lungs and throats of reptiles, birds, mammals, and a few supremely unfortunate humans. They are certainly arthropods – joint-legged creatures with a hard exoskeleton and segmented bodies -- but what sort has long been unclear. They are wormy-looking, a few inches long, and have five short projections on their head. One bears the mouth. The others are two pairs of clawed legs. Their bodies are segmented and covered by a cuticle made of chitin – the usual construction material for arthropod exoskeletons.

They have a simple tubular gut, their only food being blood, and a simple nervous system similar to other arthropods. As with many parasites, they have dispensed with numerous structures essential to their free-living relatives, including such niceties as circulatory, respiratory, and excretory systems.

On this pentastomid, Linguatula serrata, you can see the five projections on the head are very small, on the right at the top. And you can see the simple tubular gut coiled inside the animal, and the fine segmentation of the body. This genus is also the source of pentastomids’ common name today – “tongue worms”.

Linguatula.jpg
"Linguatula" by Dennis Tappe & Dietrich W. Büttner - http://www.plosntds.org/article/slideshow.action?uri=info:doi/10.1371/journal.pntd.0000320&imageURI=info:doi/10.1371/journal.pntd.0000320.g001. Licensed under CC BY 2.5 via Commons.

In this tongue worm, Armillifer armillatus, collected in 1848, you can see the five projections are also tiny (that's probably them on the left end of the worm). Its segments are not nearly as small as in the previous example. You can also see that this thing was pulled “aus Python”.

Armillifer armillatus.jpg
"Armillifer armillatus" by José Grau de Puerto Montt at en.wikipedia. Licensed under CC BY-SA 3.0 via Commons.

Today, pentastomids require two hosts. Larvae develop in an intermediate host – today, often a fish or a small mammal -- who picks up the parasite by inadvertently swallowing its eggs along with its regular food. When the intermediate host is eaten by a “definitive” vertebrate terrestrial host like a snake, the parasite crawls out and makes its way to their respiratory tract. There it matures, mates, and makes eggs. These are either coughed up and out or find their way down the digestive tract to be excreted, and eventually once again re-ingested by an unwitting intermediate host.

Scientists have also argued about the evolution of these worms. Some see them as crustaceans (think crabs, lobsters, and shrimp) that took a parasitic U-turn. Others see them as arthropods that emerged very early in animal evolution, and as more closely related to nematodes, penis worms, and other worm-like groups. The new fossils do not resolve this debate; more DNA comparisons will likely be needed needed before scientists can resolve that question, the authors of the new study write.

The new fossil was found in England in Herefordshire’s “Wenlock Series” limestone, rocks laid down in the ocean during the Silurian, about 420 million years ago. In that time, most life on Earth remained in the sea but had grown remarkably complex. Jawed and bony fish evolved and diversified. On land, a few enterprising mossy plants and small arthropods had just hauled themselves, for the first time, out of the water. Whatever land existed no doubt remained a stark, harsh, terrifying place.

On this Earth, a place with a bounteous ocean but desolate continents, our little pentastomids lived. Just a few millimeters long (modern pentastomids are measured in centimeters and inches), they had the five appendages characteristic of their group, arranged in a star-shaped pattern not found among pentastomids found today. Their appendages were also longer relative to their bodies than modern pentastomids, but they possessed the same long worm-like profile. The appendages also tell us they were adults.

The pentastomid in the computer scanned and reconstructed fossil below is the orange thing at upper right. Its appearance could almost be described as … wispy. Scandanavian, perhaps.

Pentastomid riding an ostracod -- a fossil still life. Fig. 1A (bottom) from Siveter et al. 2015.

Perhaps its gaunt appearance has something to do with the stresses of fossilization or the meager dimensions of its ostracod meals, but it lacks the pudgy glow that its descendants, growing fat and happy on the riches of a snake or a sheep, seem to share today. It bears somewhat of a resemblance to modern long-legged pentastomids in the group Cephalobaenida, a few of whom you can see here or here. The describers of the new fossils -- of which there are more not seen in the above image -- have classified them as such.

They were found both outside the body on its shell, as you’ve seen, but also inside the ostracods – in a position that may have been nestled in the gills. Although the gills were unfortunately not preserved, gills have been found in the same position in similar ostracods. The internal parasite's location was also right next to the eggs as well.

Either location is unusual. Modern ostracods do not parasitize eggs, and are never found suctioned to the exteriors of their hosts. One modern isopod – crustaceans of which pillbugs are the most familiar example – does parasitize ostracods by attaching near their hearts and feeding on brooded eggs. It is possible this ancient pentastomid was doing the same.

Fossil parasites of any kind are rare. Fossil pentastomids are “exceedingly rare”, according to the authors of the report. But to find what is today an uncommon, if not obscure, organism (only about 140 pentastomid species are known) inside its host in a fossil formed on the better side of 400 million years ago is a spectacular find indeed. It also shows that the body form of pentastomids has been remarkably stable over the course of half a billion years -- assuming that whatever made the fossil isn’t an unrelated organism evolved convergently that resembles modern pentastomids by chance.

The fact that adults were found on the outside of the ostracod suggests a free-living life phase that modern pentastomids lack. It also suggests that the Silurian pentastomids may have possessed abilities like walking or crawling that would have helped them apprehend hosts in a way with which modern pentastomids no longer bother.

What was going on in the Silurian? As mentioned, jawed fish were the new hotness and land vertebrates a distant dream. But ostracods – well, they could be scooped up by the pound, and no doubt were by some Silurian filter feeder. Of course, they can still be today, so why pentastomids made the fateful leap probably first to fish and then to terrestrial vertebrates and have never looked back remains a mystery. It’s possible the transfer took place when fish consumed ostracods, and pentastomids discovered the figure-filling benefits of super-sizing their meals.

Reference

Siveter, David J., Derek EG Briggs, Derek J. Siveter, and Mark D. Sutton. "A 425-Million-Year-Old Silurian Pentastomid Parasitic on Ostracods." Current Biology (2015).