In one short section of his book Parasite Rex, science writer Carl Zimmer asked a simple question: "Did tapeworms live in dinosaurs?" There is no reason to think they didn't. Both the living descendants of dinosaurs (birds) and their crocodylian cousins harbor tapeworms, Zimmer pointed out, and so it isn't unreasonable to imagine monstrous, prehistoric tapeworms winding their way through dinosaurian intestines.
The thought of a hundred-foot-long prehistoric tapeworm is fascinatingly repulsive—and even formed the basis of the carrion-eating monster Carnictis in Peter Jackson's revival of King Kong—but so far, no one has found a trace of one. This doesn't mean that dinosaurs were parasite-free.
To find a dinosaur parasite, you need to know what to look for and where to look for it. The soft bodies of dinosaur parasites were more likely to decay after the death of their host than become preserved, but like living parasites today, dwellers in a dinosaur's digestive tract probably produced cysts or eggs that were passed out of the body in the dinosaur's excrement. Since fossilized feces, called coprolites, have been found for a variety of dinosaurs, they are the best places to look for signs of infestation.
In 2006, George Poinar and Arthur Boucot announced that they had discovered the tell-tale signs of intestinal parasites in an Early Cretaceous dinosaur coprolite found in Belgium. The fossil had come from a coal shaft where over 30 individual Iguanodon skeletons had been found in the late 19th century, although the fossilized scat was probably not left by a herbivore. After studying 280 of the site's coprolites in 1903, C.E. Bertrand concluded that the trace fossils represented a carnivorous dinosaur, which was supported by fragments of a large theropod also found at the locality.
Analysis of the fossil required its destruction. It was scrubbed, ground down into grains, suspended in hydrochloric acid solution, spun in a centrifuge, placed in hydrous hydrofluoric acid, centrifuged again, and so on, leaving the scientists with a concentrated dino-poo residue that could be readily viewed under the microscope. Traces of a protozoan and three helminth worms were visible.
The parasites appeared to be very similar to organisms that still infest animals today. Tiny protozoan cysts found in the coprolite residue were proposed to be a prehistoric species of Entamoeba—a widespread microorganism that can be harmless or disease depending on species—and eggs from both trematode and nematode worms were recovered that resembe those of still-living varieties. In terms of cyst and egg anatomy, at least, these types of parasites have not changed very much in 125 million years.
Just how these organisms affected their host dinosaur is unclear. Some may have caused illness, while others carried out their life cycles without triggering observable symptoms. Barred from studying the dinosaur that left the scat behind, there's no way to tell. In another case of dinosaur parasitism, though, the infesting organism had clear, harmful effects on its host.
When large theropod dinosaurs fought, they often bit each other on the face. We know this because of damaged skulls with healed bite marks on them. But some tyrannosaurs had lower jaws perforated by a series of smooth-edged holes that were inconsistent with the damage a bite would have produced. Instead, a 2009 paper by Ewan Wolff, Steven Salisbury, Jack Horner and David Varricchio argued, these holes were created by a microorganism similar to Trichomonas gallinae—a protozoan that causes throat ulcers and opens the same kinds of lesions in the jaws of modern birds.
Once the paleontologists recognized the jaw lesions, they found them in numerous tyrannosaur specimens. Of 61 examined skulls, about 15 percent showed evidence of the pathology, so the microorganism certainly got around. (The Tyrannosaurus known as "Sue" had one of the worst infestations.) How it found new hosts is unknown. Perhaps the microorganism lived in herbivorous dinosaurs, too, and entered the tyrannosaurs when they fed on infested prey. Then again, the behavior of the tyrannosaurs themselves probably made things worse. Face-biting may have transmitted the microorganisms from one individual to another, and some tyrannosaurs may have picked up the parasites when they scavenged their own dead.
There were certainly many more dinosaur parasites than those recognized so far. Finding them is hard work, but there are ways to detect their presence. Who knows? Maybe someone will eventually find Zimmer's tyrannosaur tapeworm.
POINAR, G., & BOUCOT, A. (2006). Evidence of intestinal parasites of dinosaurs Parasitology, 133 (02) DOI: 10.1017/S0031182006000138
Wolff, E., Salisbury, S., Horner, J., & Varricchio, D. (2009). Common Avian Infection Plagued the Tyrant Dinosaurs PLoS ONE, 4 (9) DOI: 10.1371/journal.pone.0007288