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Disease has often been blamed for the extinction of the last dinosaurs, such as this Edmontosaurus at the Natural History Museum of Los Angeles. Photo by the author.

There are more than 100 hypotheses for the extinction of the dinosaurs. Asteroid impact is the most famous, and the effects of volcanic eruptions, sea level change and climate fluctuations remain debated, but other fantastic and weird ideas have been tossed around. Many of the discarded notions, proposed before we knew an extraterrestrial bolide struck the Yucatán Peninsula, cited pathologies as the deciding factor. Cataracts, slipped discs, epidemics, glandular problems and even a loss of sex drive have all been proposed as the reason non-avian dinosaurs perished about 66 million years ago. In fact, pioneering paleopathologist Roy Moodie suggested that a startling number of accidents and injuries could have killed Triceratops and kin.

Moodie wrote an initial report, Studies in Paleopathology, in 1917 and followed with a full book called Paleopathology in 1923. The books are surveys of fractures, infections, arthritis and other pathologies visible in fossils. And after examining these cases, he created a graph of injury and ailment incidence over time. Dinosaurs and their reptilian neighbors seemed to have a rough time. Bone breaks, infections and other pathologies “reached a maximum of development among the dinosaurs, mosasaurs, crocodiles, plesiosaurs and turtles,” and the curve dropped off only when the Mesozoic “Age of Reptiles” ended. The increasing occurrence of pathologies may have driven dinosaurs into extinction. “It seems quite probable,” Moodie wrote, “that many of the diseases which afflicted the dinosaurs and their associates became extinct with them.”

Dinosaurs really did suffer from a variety of ailments. Dinosaurs scratched at parasites, endured bone infections, and even developed cancer. But we now know that there wasn’t a dramatic uptick in dinosaur sickness between the Triassic and Cretaceous. There is no sign that pathologies did in the dinosaurs, and this hypothesis doesn’t explain why so many other creatures—from the seagoing lizards known as mosasaurs to coil-shelled ammonites—disappeared at the same time. Focusing on dinosaurs too narrowly hides the true pattern of extinction. Exactly what happened at the close of the Cretaceous will remain hotly debated for decades to come, but dinosaur disease no longer figures into the discussion.

A 44-million-year-old fossil of a feather louse. From Wappler et al., 2004.

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.

References:

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

The fused vertebrae of an archosaur with a diagram of where they fit on the living animal. From Cisneros et al., 2010.

When we envision prehistoric life, we often picture long-extinct animals in the most healthy state possible. Each restored individual is the acme of its particular species—be it Allosaurus or a woolly mammoth—but we know that things in the natural world are never so clean and neat. Not only do individual animals of any species vary from one another thanks to heredity, but injury and disease is ubiquitous. Just as animals break bones and contract ailments today, so did creatures during the past. (To their credit, some paleoartists such as Michael Skrepnick have deliberately illustrated individual animals with interesting pathologies.) A 245-million-year-old fossil from South Africa illustrates the long fossil record of a pathology that is still with us today.

There was not much left of the animal described by Juan Carlos Cisneros and colleagues; just a set of three vertebrae from the tail of an archosaur (the group containing crocodiles, pterosaurs, dinosaurs and their close relatives). The bones appear to have belonged to a crocodile-like creature that lived just a few million years after the mass extinction which marked the end of the Permian, and they had become fused together. This was not their normal condition—something must have happened to this animal to cause these three vertebrae to become fused.

When the scientists performed a neutron tomography scan—a type of 3-D scan similar to an X-ray—they found no evidence of fractures, traumas, or tumors which could account for the pathology on the bones. Neither did they find evidence consistent with the hypothesis that the animal had been born with this condition. After examining the list of potential causes, a particular kind of inflamed bone growth called spondarthritis appeared to be most consistent with the pathology seen on the vertebrae. If this identification is correct, it would be the oldest record of spondarthritis in the fossil record, with the next oldest cast being seen in an individual of the circa 147-million-year-old sauropod dinosaur Camarasaurus.

Just how the archosaur became afflicted with spondarthritis is unknown. As the authors point out in their description, even when doctors can examine people who suffer from this same condition, it can be difficult to uncover the cause. What can be determined, however, is that the pathology was probably painful for the archosaur and constrained the movement of its lower back and tail. Whether this pathology contributed to the death of this animal is unknown, but it certainly didn’t provide it with any benefits!

References:

Cisneros, J., Gomes Cabral, U., de Beer, F., Damiani, R., & Costa Fortier, D. (2010). Spondarthritis in the Triassic PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013425