Humungous insects that ruled prehistoric skies may have been wiped out by evolutionary defaults and predatory birds, researchers say.
Around 300 million years ago, these gigantic insects that once dominated Earth, during the late Carboniferous and early Permian periods, had wingspans of up to 28 inches.
The leading theory of how flying insects reached such immaculate sizes has to do with the rich oxygen concentrations during the period, reaching up to some 50 percent higher than today. The extra oxygen is thought to have supported the energy-hungry metabolisms of flying insects.
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Tests have been conducted by paleontologists linking oxygen with body size by compiling a data set of insect wing lengths from more than 10,500 fossils collected from more than 1,000 published records. They next compared wing sizes with models of prehistoric atmospheric oxygen levels from data spanning the last 320 million years.
The researchers found that average insect wing size roughly matched atmospheric levels as they varied up and down for the first 150 million years of insect evolution.
The findings, conducted by Matthew Clapham and Jered Kerr at the University of California, are detailed online in the Proceedings of the National Academy of Sciences.
However, this patterned of such large insects would have continued if it hadn't been for predatory birds. The driving force in the evolution of these giant insects, the researchers say, became the need for maneuverability, thus creating a smaller body, for which these birds would then feast.
In addition, maximum insect size decreased further between 60 million and 90 million years ago. This change might be linked with how ancient birds got better at flying during this period, as they came to resemble modern birds in performance by then, Clapham said. Another factor could be the evolution of bats, or environmental collapses following the so-called K-T mass extinction that ended the age of dinosaurs about 65 million years ago. More insect fossils from this time are needed to judge which factors might be responsible.
Although these findings suggested the maximum sizes that insects reached were influenced by atmospheric oxygen, future research can seek to see if average insect sizes also rose and fell with oxygen levels.
For instance, experiments have shown that beetles are larger when raised in mildly high oxygen levels, while cockroaches are smaller and grasshoppers remain the same size,.