Saturday, 9 June 2012

New Research Suggests That The Evolution Of Birds Ended the Dominance Of the Skies By The Insects

Travel back 300 million years ago to the ancient swamps which covered the Carboniferous Earth and you would find yourself in a world of giants. The increased oxygen levels in the atmosphere meant that arthropods were able to sustain larger bodies. Centipedes the length of a car slithered through undergrowth. Spiders and scorpions of nightmarish proportions lurked under logs.While massive dragonflies whirled and buzzed overhead; the first true aerial predators on Earth.

A fossil of Meganeura, one of the largest arthropods to have ever evolved
Oxygen levels continued to fluctuate after the Carboniferous, through the Permian and into the first half of the Mesozoic. Arthropods, insects in particular, still maintained incredible sizes. Yet as the Earth spun towards the Cretaceous period, they diminished incredibly quickly. The dropping point was sudden, at around 150 million years ago; an event which perplexed palaeontologists for years.

Now a theory connecting this fall to birds may solve this conundrum. 'Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years,' said Matthew Clapham, an assistant professor at UC Santa Cruz and leader of this study. 'Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds.'

Clapham and his research partner, Jared Karr, investigated the possibility of a link by first taking a data set of over 10,500 fossil insect wing lengths as a measure of overall size, and then plotting them against two models of changing oxygen levels in the atmosphere. Both comparisons gave similar results. Yet what made this data problematic was a 20 million year gap in the insect fossil record, combined with dropping oxygen levels at the time.

A fossil of Confuciusornis an early bird and proficient flyer
The researchers decided to take things further by examining more recent changes which occurred between 90 and 65 million years ago. While the insect fossil record is still scanty, it was reliable enough to be used. Again, there was a distinct drop in wing size. Yet Oxygen levels did not drop, so there must have been another explanation.

At this point in the Earth history, birds were proficient fliers and bats had evolved. Clapham and Karr concluded that insect wing size, and therefore body size, decreased in order to make them more manoeuvrable to avoid being eaten by newer, faster and larger aerial predators.

'There have always been small insects,' stated Clapham. 'Even in the Permian when you had these giant insects, there were lots with wings a couple of millimetres long. It's always a combination of ecological and environmental factors that determines body size, and there are plenty of ecological reasons why insects are small.' It is too easy to right off insects as weak hangers on in a mammal-dominated world, yet it is worth remembering that they outnumber us billions to one, and their overall body designs have not changed in over 410 million years. They are perfectly evolved to the Earth. We are, in comparison, quite crude at best.