Giant insects that ruled prehistoric skies for millions of years may have met their end due to the evolution of predatory birds, researchers say.
Gigantic insects once dominated the Earth. About 300 million years ago, during the late Carboniferous and early Permian periods, the largest flying insects known, the predatory dragonfly-like griffinflies, had wingspans of up to 28 inches, about the same as the modern wood duck.
The leading theory of how flying insects reached such stupendous sizes has to do with past periods of high oxygen concentrations in the atmosphere, reaching up to some 50 percent richer than today. All this extra oxygen is thought to have supported the energy-hungry metabolisms of flying insects, helping them grow to titanic maximum sizes.
To test this theory linking oxygen with body size, paleontologists Matthew Clapham and Jered Kerr at the University of California, Santa Cruz, compiled 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 oxygen levels as they varied up and down for the first 150 million years of insect evolution.
Clapham and Kerr detailed their findings online June 4 in the Proceedings of the National Academy of Sciences.
"Before, we only had anecdotal reports that there were giant insect fossils in the late Carboniferous and early Permian when oxygen levels were high, but no one had looked systematically at the maximal size of insects over a broad range of evolutionary time," said evolutionary physiologist Jon Harrison at Arizona State University, who did not take part in this research.
However, Clapham and Kerr found this pattern changed dramatically about 150 million years ago, with insect size shrinking despite rising oxygen levels. They note this shift coincided with the first appearance of birds in the fossil record. They suggest that given the aerial threat posed by feathered predators, the driving force in the evolution of flying insects became the need for maneuverability, thus favoring smaller body size.
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.
Ancient insects contended with other aerial predators, including the flying reptiles known as pterosaurs, which appeared about 230 million years ago and perished with the K-T mass extinction. The researchers found weak evidence that insect size shrunk after pterosaurs evolved, although a 20-million-year gap in the insect fossil record and a drop in oxygen level at about the same time has made it challenging to see how much of an effect pterosaurs had.
"Although there is evidence from pterosaur teeth that some may have been insectivorous, I think they probably weren't as agile or maneuverable fliers as birds so it would make sense that they had a lesser effect on size evolution," Clapham said.
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.
"Our results describe changes in the maximum size of the largest insects, not changes in the size of all insects," Clapham said. "Even though there were giant insects in the past, most insects have always been small like they are today."
The problem with studying if atmospheric oxygen levels influenced average insect wing size is that the wings of small insects may not always get preserved very well -- they may be too small to make enough of an impression on surrounding sediment that survives to get fossilized.
"For example, insects preserved in marine rocks are dominated by dragonflies and other powerful flying insects, which tend to be large, and lack a lot of the small things you would find in a more continental deposit," Clapham said.
Further studies can also investigate how different kinds of insects may have changed in size depending on atmospheric oxygen levels over time, Harrison said. 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, he noted.