Poinar's studies convinced him that many deadly diseases were just emerging at the end of the Cretaceous period around 65 million years ago, shortly before the dinosaurs went extinct.
The effect on the dinosaurs, he said, would have been similar to the effect that European diseases had on Native Americans.
By some accounts, these diseases were responsible for as much as 95 percent of Native American casualties in the Americas during early exploration of the New World.
(Read: "Guns, Germs, and Steel: Jared Diamond on Geography as Power" [July 6, 2005].)
Similarly, the dinosaurs would have had no immunity to newly emerging infections, Poinar said.
"That's why they were so devastating."
In addition, he noted, dinosaurs were slow breeders, so they wouldn't easily have passed down immunity—although he added that no one really knows what dino immune systems were like.
Not that Poinar thinks that disease was the only factor. Climate change, an asteroid impact, or volcanic eruptions all could have put the dinosaurs under stress that would have made them more susceptible to illness.
But he and his wife believe that the famous Yucatán asteroid strike, known as the Chicxulub impact, is overrated as a singular dino-killing mechanism.
(Related news: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory" [October 30, 2006].)
"We don't feel that the effect [of the asteroid] was as great as was claimed previously," he said.
But not everyone agrees with Poinar's evidence.
"The thought is nice, but based on very thin air!" said Jan Smit, a sedimentologist at Vrije Universiteit in the Netherlands who studies the K-T extinction—the time between the Cretaceous and Tertiary periods when dinosaurs went extinct.
To begin with, Smit wrote by email, there is no evidence that the dinosaurs were already facing hard times when the asteroid struck.
"The only quantitative analysis, done by Peter Sheehan did not show such decline," he said.
In fact, other researchers have found an increase in dinosaur diversity just before the impact, due to the spread of duckbill dinosaurs, he added.
"My latest findings [released in October of last year] also show these critters were alive and kicking at the time of the Chicxulub impact."
Princeton University paleontologist Gerta Keller argues that climate change from volcanic eruptions played a bigger role than the Chicxulub asteroid in wiping out the dinosaurs.
She shares Smit's doubts about bugs as the main killer.
"Sure, insects carried disease in the Cretaceous, as they do now," she said by email. But that doesn't mean that diseases killed them all off.
"Malaria is a big problem for humans, but there is no danger of it decimating human populations globally, much less causing extinctions."
Furthermore, she noted, the K-T extinction also affected marine animals.
"None of these could have suffered death from insects," she said. "There must have been a common environmental cause that affected marine and terrestrial environments."
Poinar counters that not all of the events at the K-T boundary were necessarily related.
"A number of things were going on toward the end of the Cretaceous," he said.
Marine organisms, he said, would have been affected by changes in the sea, while dinosaurs would have been affected by all of the other stresses that were lowering their disease resistance.
The author also notes that, unlike modern humans, dinos would not have had access to treatments for parasitic diseases, which would have run rampant in the tropical environments where most of the land reptiles lived.
Still, Nathan Wolfe, an epidemiologist at the University of California, Los Angeles, is also cautious about Poinar's theory.
It's not hard to imagine insect-borne diseases spreading through the dinosaur world, he said.
"That they might have represented a burden on dinosaurs, I think that's quite likely," Wolfe said.
"Whether that contributed to the demise of the dinosaurs is much harder [to say for sure]," he added.
"I can't think of too many occasions where [such widespread] extinctions are related to infectious diseases."
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