The rise of insects was a factor in the downfall of dinosaurs, a new book argues.

It wasn't just the bugs themselves, but also the diseases they carried and their impact on vegetation that led to the long decline and eventual disappearance of the animals, authors George and Roberta Poinar said.

Researchers found evidence of diseases in ancient bugs preserved in amber, suggesting that the insects could have infected dinosaurs.
(Oregonstate.edu)

"The largest of the land animals, the dinosaurs, would have been locked in a life-or-death struggle with [insects] for survival," they wrote in What Bugged the Dinosaurs? Insects, Disease and Death in the Cretaceous.

It's widely accepted that a giant meteorite caused the dinosaur's extinction after it smashed into the Yucatan peninsula in Mexico and caused environmental chaos. Other suggested causes include climate change and volcanic activity, or some combination of all three.

But the authors said that another explanation is required because dinosaurs didn't die off suddenly, as a traumatic event — like a meteorite hit — would suggest.

Current explanations insufficient

"By themselves, such events do not explain a process that in reality took a very, very long time, perhaps millions of years. Insects and diseases do provide that explanation," George Poinar said in a release Thursday.

Dinosaurs declined over the period known as the K-T boundary, the line between the Cretaceous and Tertiary periods about 65 million years ago.

There is evidence that catastrophic events, such as a major asteroid hit or lava flows, occurred at that time. But those events don't fully explain the gradual decline of dinosaur populations, some of which lived for thousands of years past the boundary, the release said.

"That time frame is just not consistent with the effects of an asteroid impact," George Poinar said.

A courtesy professor of zoology at Oregon State University, Poinar has, along with his wife Roberta, extensively studied plant and animal life forms preserved in amber.

Amber holds clues

It was those preserved bits of ancient history that prompted the idea that insects "were an extremely significant force in the decline of the dinosaurs," George Poinar said.

The gut of a biting insect from the late Cretaceous period, preserved in amber, contained the pathogen that causes the disease leishmaniasis, which is  
potentially fatal. Another insect had organisms that cause malaria.

Reptiles can get both diseases from biting insects.

The researchers also found evidence of organisms that could cause dysentery in dinosaur feces. The parasites are carried by insects, he said.

The authors argued that a warm late Cretaceous world swarming with insects carrying diseases and parasites could have led to local epidemics that wiped out small dinosaur populations.

There was no natural immunity of the kind that mammals and reptiles have built up since then, so "massive outbreaks causing death and localized extinctions would have occurred," he said.

Insects spread flowering plants

At the same time, insects were changing the nature of the Earth's vegetation, reducing the dinosaur's food supply. Their usual food sources, such as pre-flowering plants, were being displaced by flowering plants.

Insects helped the flowering plants spread by pollinating them. They may also have spread diseases that attacked the dinosaur's food plants.

The combination of factors created by insects, "could all have provided a lingering, debilitating condition that dinosaurs were ultimately unable to overcome" as they also faced a changing environment, meteor impacts and massive lava flows, the researchers said.