Late paleozoic atmospheres and biotic evolution

Abstract

The latter half of the Paleozoic era is marked by notable evolutionary advances, followed by the greatest of all mass extinctions and the subsequent establishment of Mesozoic‐Cenozoic faunas of very different aspect. Current models suggest marked changes in concentration of oxygen and carbon dioxide in the Paleozoic atmosphere. Atmospheric oxygen is thought to have increased from 15% in the mid‐Devonian to near 35% by the end of the Carboniferous, followed by a decline to 17% near the end of the Permian. Atmospheric carbon dioxide was near 0.5% in the early Paleozoic, declining to less than 0.3% in the Devonian, and then more steeply downward to a minimum near 0.04% at the end of the Carboniferous. The principal causes of these changes were the advent and expansion of land plants, deposition of carbonates and continental weathering. Notwithstanding quantitative uncertainties, it seems clear that a major pulse of high oxygen concentration and associated shifts in carbon dioxide characterized the late Paleozoic atmosphere. Atmospheres with such different compositions have markedly different physical characteristics. These changes have major implications for the physiologies of contemporary organisms. The fossil records of various taxa indicate dramatic changes in the biosphere that coincide in time with the inferred changes in composition of the atmosphere. Major changes in phenotype observed in numerous lineages of animals and plants, including accelerated radiations in fresh water and on the land, are inferred to have occurred in response to these changes in the atmosphere. The morphologies, physiologies, and inferred behavior of many organisms preserved in the fossil record are in good accord with expectations based on hyperoxic, low carbon dioxide conditions of the Carboniferous atmosphere and with a return to lower oxygen levels by the end of the Permian.