Dinosaurs were dying off long before asteroid hit: study

Dinosaurs were in trouble long before the massive asteroid - believed to be the primary cause of their extinction - hit the Earth some 65 million years ago, a study suggests.

Researchers from the University at Albany in the US found that the emergence of toxic plants combined with dinosaurs' inability to associate the taste of certain dangerous foods had them already drastically decreasing in population.

"Learned taste aversion" is an evolutional defence seen in many species, in which the animal learns to associate the consumption of a plant or other food with negative consequences, such as feeling ill, researchers said.

"A reason why most attempts to eliminate rats have not been successful is because they, like many other species, have evolved to cope with plant toxicity," said Gordon Gallup, a professor at University at Albany.

"When rats encounter a new food, they typically sample only a small amount; and if they get sick, they show a remarkable ability to avoid that food again because they associate the taste and smell of it with the negative reaction," said Gallup, who led the study published in journal Ideas in Ecology and Evolution.

The first flowering plants, called angiosperms, appear in the fossil record well before the asteroid impact and right before the dinosaurs began to gradually disappear.

Researchers claim that as plants were evolving and developing toxic defences, dinosaurs continued eating them despite gastrointestinal distress.

Although there is uncertainty about exactly when flowering plants developed toxicity and exactly how long it took them to proliferate, the team notes that their appearance coincides with the gradual disappearance of dinosaurs.

In addition to studying the proliferation of toxic plants while dinosaurs were alive, researchers examined whether or not birds (descendants of dinosaurs) and crocodilians (also descendants of dinosaurs) could develop taste aversions.

They found that the birds, rather than forming aversions to taste, developed aversions to the visual features of whatever made them sick.

Still, they knew what they should not eat in order to survive.

In a previous study in which 10 crocodilians were fed different types of meat, some slightly toxic, Gallup discovered that like dinosaurs, crocodilians did not develop learned taste aversions.

"Though the asteroid certainly played a factor, the psychological deficit which rendered dinosaurs incapable of learning to refrain from eating certain plants had already placed severe strain on the species," said Gallup.

He said the prevailing view of dinosaur extinction based on the asteroid impact implies that the disappearance of dinosaurs should have been sudden and the effects should have been widespread, but the evidence clearly shows just the opposite.

"Dinosaurs began to disappear long before the asteroid impact and continued to gradually disappear for millions of years afterwards," said Gallup.