June 6th, 2012 by James Ayre
Giant insects with wingspans of more than two feet used to rule the skies from 300-150 million years ago. New research is suggesting that they were killed off by the emergence of birds, not by declining oxygen levels as had previously been believed.
When birds first appeared 150 million years ago, insect sizes shrunk rapidly despite rising oxygen levels, which had previously correlated with insect size.
Insects reached their largest sizes about 300 million years ago, during the Carboniferous and early Permian periods. Large, predatory, dragonfly-like insects called griffenflies ruled the skies, reaching wingspans of over two feet. Accepted theory attributes their size to the high oxygen levels in the atmosphere at the time, which allowed them to get enough oxygen through the breathing tubes that they have instead of lungs.
For the new study, researchers created a huge database of wing lengths from published records of fossil insects. They then analyzed insect size in relation to atmospheric oxygen levels over a period of a couple hundred million years.
“Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years,” Matthew Clapham, lead author of the study and an assistant professor of Earth and planetary sciences at UC Santa Cruz, said. “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.”
With the emergence of birds, the need for maneuverability and agility became a driving force in their evolution, favoring smaller body sizes.
The researchers compiled a data set of more than 10,500 fossil insect wing lengths. For the atmospheric oxygen levels, the researchers used the widely used “Geocarbsulf” model developed by Yale geologist Robert Berner. They also repeated the analysis using a different model and got the same results.
The study also suggests that the flying reptiles that emerged about 230 million years ago, pterosaurs, didn’t exhibit much influence over insect sizes. There were actually larger insects after they appeared than before.
Another large drop in insect sizes occurred around 90-65 million years ago, long after the emergence of birds. The researchers think this was likely caused by the increased specialization of birds, but also possibly the emergence of bats, or the end-Cretaceous mass extinction event.
“I suspect it’s from the continuing specialization of birds,” Clapham said. “The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds.”
The researchers also emphasize that their research focused on maximum insect size, not average size. Average insect size would be impossible to estimate accurately because of biases in the fossil record for larger organisms.
“There have always been small insects,” he said. “Even in the Permian when you had these giant insects, there were lots with wings a couple of millimeters 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.”
Source and Image: University of California-Santa Cruz
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