April 12, 2011
Hunting dinosaurs is a dangerous business. Scores of fictional, time-traveling hunters have learned this lesson the hard way, but arguably the most unfortunate was the protagonist of Brian Aldiss’ short story “Poor Little Warrior.” All Claude Ford wanted to do was get away from his disappointing life and unhappy marriage by gunning down prehistoric monsters. Slaughtering a swamp-dwelling Brontosaurus briefly satisfied his escapist desires, but, unfortunately for Ford, the dinosaur had been home to scores of lobster-sized parasites that scurried off their dead host and onto the closest, warmest living thing.
Paleontologists have not yet found such monstrous Mesozoic parasites, but familiar pests did afflict dinosaurs. Tiny trematode and nematode worms lived in the guts of predatory dinosaurs, and Tyrannosaurus itself was plagued by a harmful microorganism commonly found among modern pigeons. But not all dinosaur parasites were internal. Although not as terrible as Aldiss’ creatures, prehistoric lice may have made the lives of many dinosaurs very itchy.
The prehistory of lice is poorly understood. Out of five supposed fossil lice scrutinized by entomologist Robert Dalgleish and colleagues in 2006, only one, a 44-million-year-old specimen described by Dalgleish, Torsten Wappler and Vincent Smith two years earlier, turned out to be the genuine article. Curiously, though, the single fossil specimen appeared to be a close relative to feather lice found on modern birds, and the researchers who described it suggest that birds may have “inherited [lice] from early-feathered theropod dinosaurs.”
(A 100-million-year-old relative of lice was announced in 2006, but it was a “booklouse” that was not an animal parasite.)
As yet, no feathered dinosaur specimen has been found with preserved lice, but a Biology Letters study just published by Smith and a different team of collaborators suggests that the pests might have taken up residence on some Cretaceous species. This hypothesis is based on comparisons of modern louse lineages. Since the prehistoric feather louse and the older “booklouse” remain the only finds close to the early history of lice, the scientists behind the new research used the genetics of living louse species to estimate when their respective lineages would have diverged from one another.
What the scientists came up with was a hypothetical tree of louse evolution. The genetic divergence estimates suggest that parasitic lice were diversifying just after 100 million years ago in a Late Cretaceous world teeming with hosts. Exactly which hosts these insects parasitized is unknown.
Even though news reports about the new study have focused on the likelihood that at least some dinosaurs were bothered by lice, the aim of the research was to use a fresh line of evidence to ascertain the timing of when lineages of modern birds and mammals began to appear. This is a subject of some dispute among scientists. Many paleontologists place the major radiation of modern bird and mammal groups after the end-Cretaceous mass extinction about 65 million years ago, but scientists using genetic and molecular techniques have suggested that these lineages originated deeper in the Cretaceous. Since lice are relatively host-specific and are associated with particular groups of birds and mammals, Smith and co-authors used the evolutionary pattern of lice to draw bird and mammal lineages back into the heyday of the dinosaurs. The lice appeared to track what was believed to be the early origins of modern groups.
But the tight connection between extant louse families and lineages of modern mammals and birds is an assumption. If the new study is correct, parasitic lice proliferated during the Late Cretaceous, when there were already plenty small mammals and feathered dinosaurs running around.
Smith and co-authors state that Archaeopteryx was the oldest-known feathered dinosaur at approximately 150 million years old, but Anchiornis may have pre-dated its more famous cousin by 10 million years or so. Either way, feathers and feather-like body coverings had already been present for over 50 million years before parasitic lice evolved. Smith and colleagues also cite the oldest known fossil hair as dating to about 55 million years ago, but paleontologists have found the exquisitely preserved bodies of much older mammals with intact fur, the approximately 125-million-year-old Eomaia being just one example. As with feathered dinosaurs, furry mammals were around for a long time before the first lice, and studies of fossil mammal evolution have also confirmed that there were many now-extinct groups of mammals present during the Late Cretaceous. Perhaps parasitic lice got their start on feathered dinosaurs and archaic mammals and were only inherited by lineages with living descendants later on.
Smith may have summed up the significance of the new findings best in a quote he gave to the New York Times: “The louse phylogeny adds one more piece of data to this puzzle. It says lice are old, predate the Cretaceous-Paleogene boundary, and must have been living on something.” What those “somethings” were remains unclear. Evolutionary estimates based on genetics make predictions about what may yet be found, and it will be up to paleontologists to test these hypotheses with the remains of long-dead creatures.
DALGLEISH, R., PALMA, R., PRICE, R., & SMITH, V. (2006). Fossil lice (Insecta: Phthiraptera) reconsidered Systematic Entomology, 31 (4), 648-651 DOI: 10.1111/j.1365-3113.2006.00342.x
Smith, V., Ford, T., Johnson, K., Johnson, P., Yoshizawa, K., & Light, J. (2011). Multiple lineages of lice pass through the K-Pg boundary Biology Letters DOI: 10.1098/rsbl.2011.0105
Wappler, T., Smith, V., & Dalgleish, R. (2004). Scratching an ancient itch: an Eocene bird louse fossil Proceedings of the Royal Society B: Biological Sciences, 271 (Suppl_5) DOI: 10.1098/rsbl.2003.0158
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