ABSTRACT

ATMOSPHERIC OXYGEN INFLUENCES ON THE SIZE OF MODERN AND FOSSIL INSECTS (0746352)

Jon Harrison, PI

Recent geological models have suggested that oxygen levels varied significantly over the past 500 million years. These variations in atmospheric oxygen have been invoked to explain a vast array of evolutionary events, including major extinctions, changes in animal diversity, and the conquest of land by animals (reviewed by Berner, VandenBrooks and Ward, 2007). One of the most oft-cited and dramatic proposed effects of this oxygen variation is that increased oxygen levels during the Paleozoic allowed for increases in body size of vertebrates and insects. However, the intriguing hypothesis that hyperoxia was responsible for insect gigantism is weakened by two major problems: 1) the lack of statistical evidence that body size correlates with oxygen levels in the groups of animals that were present during these times of oxygen fluctuation and 2) a lack of modern physiological studies to support this hypothesis. Therefore, the PI will carry out a multi-faceted approach to the question of the impact of oxygen on insect development and evolution. He will measure body sizes of the three major fossil insect groups for which museum fossils are most abundant from the Carboniferous period through the Triassic period [Odonatoptera ? ancestors of modern dragonflies, Blattodea ? ancestors of modern cockroaches, and Palaeodictyopterida ? an extinct group], and perform the first statistical analysis of whether average or maximal insect size correlates with oxygen across this period. He will subsequently rear two species descended from these ancient taxa across the range of oxygen that occurred in the past - the dragonfly Anax junius, and the cockroach, Blatella germanica ? to better understand the effect of varying oxygen on modern physiology. The PI also will attempt to develop the first ever proxy for paleo-oxygen levels: the ratio of leg tracheal diameter to leg length in fossil insects. Recent studies using phase contrast x-ray synchrotron imaging at Argonne National Labs have shown that insect leg tracheal diameter depends on two factors: body size and atmospheric oxygen level. The PI will attempt to visualize leg tracheae in fossil insects preserved in amber, and then use the ratio of leg tracheal diameter to leg length for modern cockroaches reared in different oxygen levels to predict what the atmospheric oxygen concentration was when the fossilized insect lived. Development of an independent technique for assessing geologic atmospheric oxygen levels would be a major advance in geology and climate science and could help resolve current competing models.
Insect gigantism is of broad interest to the general public, and serves as a focal point to draw them into learning about fundamental processes of paleontology, evolution and physiology. The PI will leverage this interest to highlight ongoing research in paleontology and evolution by partnering with ASU Ask-A-Biologist web site (http://askabiologist.asu.edu/) and the Arizona Science Center to produce a new K-12 teaching resource. The PI is currently working closely with museums to develop the first exhibit on past and present climate change. He plans on incorporating this research on insect gigantism into an exhibit which will be of interest to museum goers of all ages.

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