Wednesday, November 17, 2010

Big bugs



Dragonflies were fierce in the carboniferous period (roughly 300 million years ago), with a wingspan of - believe it or not - 2 1/2 feet! The 1st evidence was found in France in 1880. Meganeura (pictured as it would have appeared, and fossilized) preyed on amphibians and other insects. It is believed that these prehistoric bugs were able to attain such monstrous sizes because of higher oxygen levels in the atmosphere back then. New research supporting this theory has hit the weird news.

Paleobiologist John VanenBrooks of Arizona State University raised a total of 225 dragonflies in chambers emulating Earth’s oxygen concentrations approximately 275 million years ago (31%), 240 million years ago (12%), and today (21%). The insects were hand-fed for 6 months during the nymph stage, then their tracheal volume was measured when they molted into adults. VandenBrooks explains, “As you become a larger insect, more of your body is taken up by tracheal tubes. Eventually you reach a limit to how big you can be. The more oxygen that is available, the smaller that system needs to be and the bigger you can grow.” They correlated their results under controlled laboratory conditions with dragonflies preserved in amber and discovered that high oxygen concentrations lowered tracheal volume, while low oxygen concentrations boosted it. The dragonflies grew faster and bigger in a high-oxygen environment, suggesting that tracheal volume is tied to prehistoric dragonfly body size. Despite the sensationalized headlines and misleading photos, the test subjects did not approach carboniferous sizes, but they were larger by 15% - meaning that while the dragonflies in the modern habitat measured 3.5", those in the hyperoxic chamber had 4" wingspans.

While searching for images to illustrate this post, I learned of another type of preservation. Calcareous sinter, layering with calcium carbonate crystals, is considered a form of petrification rather than fossilization. Because they are unable to move if their body temperature is low, dragonflies are susceptible if they fly into and alight in a limestone cave.

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