Based on the abundance of insects in the fossil record and analogies with modern settings, Pennsylvanian coal swamps teemed with insects and spiders. Many were delicately preserved in concretions in fine-grained sediments, like those at Mazon Creek we mentioned a few days ago. Even details such as wings and their veins are often preserved. The warm, moist climate in at least the tropical and sub-tropical Pennsylvanian world might have stimulated the evolution of insects, or it might have been more a co-evolution with the plants that dominated those swamps.
Did the presence of insects as food promote the development of land life? Or did the pressure from evolving land predators stimulate the diversification of insects? Most likely, it was a combination of both and more. There is actually quite a gap in the fossil record between the first insects in the Devonian and the proliferation of insects in the Pennsylvanian, so their evolutionary history is not all that well known. Insects had evolved the ability to fly by Pennsylvanian time, but the details of that evolution are obscure.
|Meganeura - Dragonfly photo by Hcrepin via Wikipedia under GDFL.|
Pennsylvanian swamps had cockroaches four inches long, and the air buzzed with relatives of dragonflies with wingspreads of 25 inches or more. Gerarus was a bug with a wingspan of 10 centimeters – 4 inches – and a body covered with spikes.
It’s not certain why Pennsylvanian insects grew so large. A common idea is that there was more oxygen in the Pennsylvanian atmosphere, giving both more energy for growth and more lift to winged insects. There was more oxygen – possibly as much as 35% vs. today’s 21%, probably largely because of all the plants taking up carbon dioxide and giving off oxygen. But one problem with that is that gigantic insects survived and even became larger into the Permian, when oxygen levels were considerably lower than in the Pennsylvanian. It might have been that large insects were better able to manage oxygen usage than small ones. It has also been suggested that the gigantism was a result of evolutionary pressure, an arms race, among competitors for food resources.
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Florence Bascomb was born July 14, 1862, in Williamstown, Massachusetts. She was educated at the University of Wisconsin and Johns Hopkins, where she earned her PhD in geology in 1893 – the second American woman to do so. She was the first woman hired as a professional by the U.S. Geological Survey, and she also established the geology department at Bryn Mawr College. Her research focused on the igneous and metamorphic rocks of the mid-Atlantic states, including both seminal field work and state-of-the-art petrographic analysis.
Today is also Woody Guthrie’s birthday.
—Richard I. Gibson
Insect fossils in Kansas
Oxygen and giant insects
Oxygen and giant insects – another view
Dragonfly photo by Hcrepin via Wikipedia under GDFL.