Biting insects 'killed dinosaurs'

Biting insects might have brought down the mighty dinosaurs rather than a cataclysmic meteor impact, a new theory claims.

Disease spread by ancient mosquitoes, mites and ticks was probably the major factor that finished off the extinct reptiles, say scientists.

Insects could have also made it harder for dinosaurs to survive by changing the nature of plant life on Earth.

Bees and other pollinators helped to promote the rapid spread of flowering plants, leading to the loss of vegetarian dinosaurs' traditional food sources. As the plant-eating dinosaurs declined, so would their predators.

The theory helps explain why dinosaurs took so long to die off, according to husband and wife team George and Roberta Poinar.

According to the most widely accepted explanation, the dinosaurs were wiped out by an asteroid or comet that smashed into the Earth off the coast of Mexico 65 million years ago.

Another theory is that they were driven to extinction by massive volcanic eruptions in India which led to extreme climate change.

The time at which the dinosaurs disappeared, between the Cretaceous and Tertiary periods, is known as the K-T Boundary.

But George Poinar, a courtesy professor of zoology at Oregon State University in Corvallis, US, points out that they did not vanish immediately. Their extinction was drawn out over hundreds of thousands or even millions of years.

An impact scenario should have led to an abrupt extinction, and volcanism-induced climate change would probably also have wiped them out in a relatively short time.

On the other hand, emerging new diseases spread by biting insects, combined with the spread of flowering plants, and competition with insects for plant resources, was "perfectly compatible" with a lengthy process of extinction.

Insects date back as far as the Carboniferous period, around 300 million years ago.

During the later part of the dinosaurs' reign insect numbers and species exploded.

Mosquitoes evolved in the early Cretaceous. The oldest example known, from Burma, was trapped in amber 100 million years ago.

Prof Poinar and his wife have carried out an in-depth study of plants and creatures preserved amber, the fossilised tree that is used for ornaments and jewellery.

They outline their dinosaur extinction theory in a new book published by Princeton University Press, What Bugged The Dinosaurs? Insects, Disease And Death In The Cretaceous."

Prof Poinar said: "During the late Cretaceous Period, the associations between insects, microbes and disease transmission were just emerging.

"We found in the gut of one biting insect, preserved in amber from that era, the pathogen that causes leishmania - a serious disease still today, one that can infect both reptiles and humans. In another biting insect, we discovered organisms that cause malaria, a type that infects birds and lizards today.

"In dinosaur faeces we found nematodes, trematodes and even protozoa that could have caused dysentery and other abdominal disturbances. The infective stages of these intestinal parasites are carried by filth-visiting insects."

He said that during this period the world was covered with warm-temperate to tropical areas swarming with blood-sucking insects. The infections they carried would have caused repeated epidemics that slowly wore down dinosaur populations.

"Smaller and separated populations of dinosaurs could have been repeatedly wiped out, just like when bird malaria was introduced into Hawaii, it killed off many of the honeycreepers," Prof Poinar added.

"After many millions of years of evolution, mammals, birds and reptiles have evolved some resistance to these diseases. But back in the Cretaceous, these diseases were new and invasive, and vertebrates had little or no natural or acquired immunity to them. Massive outbreaks causing death and localised extinctions would have occurred."

Largely thanks to insects, the traditional food sources of vegetarian dinosaurs - seed ferns, cycads, gingkoes and other gymnosperms - were displaced by flowering plants which began to dominate the landscape.

Insects would have destroyed large tracts of vegetation by spreading plant diseases. They would also have competed directly with dinosaurs for available plant food supplies.

"We don't suggest that the appearance of biting insects and the spread of disease are the only things that relate to dinosaur extinction," said Prof Poinar.

"Other geologic and catastrophic events certainly played a role. But 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."