"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

Parasitism in Prehistory: what can fossils tell us about it?

Did dinosaurs (and other prehistoric animals) have parasites? Parasitism is a lifestyle which has evolved independently in all kind of plants, animals, fungi, and microbes, and almost all living things have some kind of parasites infecting them. Even parasites themselves can get parasitised. Given how common they are in modern ecosystems, they must have existed in prehistoric world too, right? Yet when you look back on the fossil record, fossils of parasites are few and far in between. Most parasites don't lend themselves well to being fossilised - unlike the thick tough shells of animals like ammonites or the massive bones of Tyrannosaurus or Triceratops, most parasites are small, squishy things that don't fossilise well.

Kabatarina pattersoni - a species of parasitic copepod described from the gills of some Early Cretaceous bony fish
(The larger individual is the female. From Figure 1 of this paper
While they are fairly rare comparing with seashells and animal bones, there are some fossils of parasites. So what kind of parasites did prehistoric animals have? As it turns out, pretty similar to the type that we find on modern animals. For example, the parasitoid wasps that lay their eggs in caterpillars and devours it from within have been around since the Jurassic. The zombie ant fungus would have been infecting ants in the same forests that harboured feathered dinosaurs during the Cretaceous. Such specimens are often found in amber, which are fossilised tree resin which sometimes entrap and preserve tiny invertebrates like insects and their parasites.

Other such amber-preserved specimens include blood-sucking insects - which was a major plot device in Jurassic Park. Unlike in Jurassic Park, those amber-preserved biting insects do not contain intact dinosaur DNA, but at least they do contain fossils of parasites that might have infected dinosaurs. Some biting midges from the Cretaceous were carrying around malaria-type parasites in their gut much like the types which infect modern birds and reptiles. So perhaps dinosaurs might have contract malaria and other such insect-borne diseases from those blood-suckers.

In addition to amber, another potential source of fossil parasites is coprolite. Coprolite is simply the technical term for fossilised poop, and examining coprolite can be a good way of finding out what the animal that dropped the dropping had been eating, as well as the kind of environment that it lived it. As any parasitologist or veterinarian would known, riffling through poop samples is a routine method for identifying what parasites are found in an animal. So why not do the same for dinosaur dung? After all, palaeoecologist are already sampling coprolite for fungal spores and many other things, and the same technique can be used to look for parasite eggs as well. It was through examining samples of fossilised dung that some scientists have found evidence for tapeworms in prehistoric sharks, pinworms in Triassic proto-mammals, and parasitic flukes and roundworms in dinosaurs.

Fossilised tapeworm eggs from shark coprolite
But what if the parasite itself or its eggs are not fossilised? Well, just like how a dinosaur may leave behind footprints on sand or mud which then get preserved as fossilised tracks, some parasites can leave tell-tale scars on their host's body which are preserved as fossils. For example, palaeontologists examining the jaw bones of tyrannosaurid dinosaurs have found pockmarks which are similar to those found in birds infected with a single-cell parasite call Trichomonas which can cause debilitating lesions in their mouth and throat. So the great Tyrannosaurus rex might have been afflicted and succumbed to parasites similar to the ones that plague pigeons today.
Reconstruction of a tyrannosaurid with canker sore (by Chris Glen)

It seems that prehistoric animals were mostly infected by parasites which are fairly similar to those of their modern equivalents. But some unexpected combinations of parasites and hosts have also been found in the fossil record. For example, a recently discovered fossil showed that tongue worms (Pentastomida) - which are weird parasitic crustaceans that live in the respiratory tract of terrestrial vertebrate animals such as reptiles - actually started out as external parasites that clung on the carapace of crustaceans during the Silurian period (about 425 million years ago).

But before we get to excited and start labelling every weird-looking fossil critters out there as parasites, we need to examine each examples critically as one should when hypothesising about the life style of any extinct organism. Otherwise, it will just muddle our understanding about the role that parasites played in prehistoric ecosystems, and how they might had evolved.

For example, earlier in 2014, there was a paper published about a Jurassic fly larva which the authors claimed was an ectoparasite of Jurassic amphibians. But there wasn't anything about that fossil that would specifically identify it as a parasite. It was not found in close association with hypothesised amphibian hosts, nor did it have any anatomical features which can only be explained by parasitism as opposed to many other equally plausible alternative explanation.

The authors claimed that the fossil insect's piercing straw-like sucking mouthpart and the possession of suckers were key features which made it a parasite. But piercing mouthparts are also found in non-parasites such as giant waterbugs and other predatory insects, and suckers are used by other non-parasitic aquatic animals for anchoring themselves to rocks and other substrate. Additionally, there are living relatives of that fossil fly which have larvae with similar anatomical features. But instead of being parasites, they are actually aquatic predators (which explains the mouthpart).

Pseudopulex is a Jurassic insect which has been interpreted as a
"flea" that parasitised dinosaurs, however some researchers
disagree with this interpretation
(Reconstruction Wang Cheng via Oregon State University)
Parasitism shouldn't simply be used as a convenient explanation for any organisms with unusual anatomical features. It is a lifestyle that requires some rather specific adaptations. We should be open to the potential of finding fossil parasites, but at the same time, stay grounded and compare any putative fossil parasites with the anatomy and ecology of living parasites. In a sense, the problem of "Crying Parasite" is the same problem when it comes to inferring the lifestyles of other prehistoric animals. On that note, I would suggest that next time someone finds what they think might be a fossil parasite, instead of coming up with a half-baked explanation based on some superficial ideas of what they think parasites are like, why not ask a parasitologist? Like this one, right here?

When analysed critically, fossils can provide valuable insight into how parasites evolved into what they are now, and what changes have taken place over their evolutionary histories, such as the kind of hosts they infected, and how they infected them. The fossil record seems to indicate that that the evolution of new groups of animals were often accompanied by the evolution of parasites that specialised on parasitising them. By looking at how parasites have evolved and lived in the past, we might be able to anticipate how they might evolve in the future in a changing world.

Given that parasites can often shape entire ecosystems through the effects they have on their hosts, when considering how prehistoric plants and animals might have interacted with each other, it is important to be mindful of their parasites as well. Velociraptor might had been buzzed by sandflies that carried dino-malaria in their gut, a T. rex chomping on the rump of a Triceratops might had swallowed a whole load of parasitic worm larvae with every mouthful. A Jurassic World full of dinosaurs is incomplete without its share of prehistoric parasites.

Leung, T. L. F. (2015) Fossils of parasites: what can the fossil record tell us about the evolution of parasitism? Biological Reviews DOI: 10.1111/brv.12238

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