Animal evolution and atmosphericpO2: is there a link between gradual animal adaptation toterrain elevation due to Ural orogeny and survival of subsequent hypoxicperiods?

摘要

Considering evolution of terrestrial animals as something happening only on flat continental plains seems wrong. Many mountains have arisen and disappeared over the geologic time scale, so in all periods some areas of high altitude existed, with reduced oxygen pressure (pO2) and increased aridity. During orogeny, animal species of the raising terrain can slowly adapt to reduced oxygen levels.This review proposes that animal evolution was often driven by atmospheric oxygen availability. Transitions of insect ancestors and amphibians out of water are here interpreted as events forced by the lack of oxygen in shallow and warm water during Devonian. Hyperoxia during early Carboniferous allowed giant insects to be predators of lowlands, forcing small amphibians to move to higher terrains, unsuitable to large insects due to reduced pO2. In arid mountainous habitats, ascended animals evolved in early reptiles with more efficient lungs and improved circulation. Animals with alveolar lungs became the mammalian ancestors, while those with respiratory duct lungs developed in archosaurs. In this interpretation, limb precursors of wings and pneumatised bones might have been adaptations for moving on steep slopes.Ural mountains have risen to an estimated height of 3000 m between 318 and 251 Mya. The earliest archosaurs have been found on the European Ural side, estimated275 Myr old. It is proposed that Ural orogeny slowly elevated several highlandhabitats within the modern Ural region to heights above 2500 m. Since this processtook near 60 Myr, animals in these habitats fully to adapted to hypoxia.The protracted P-Tr hypoxic extinction event killed many aquatic and terrestrialanimals. Devastated lowland areas were repopulated by mammaliaformes that came downfrom mountainous areas. Archosaurs were better adapted to very lowpO2, so they were forced to descend to the sea level laterwhen the lack of oxygen became severe. During the Triassic period, when the relativecontent of O2 reduced to near 12%, archosaurs prevailed asonly animals that could cope with profound hypoxia at the sea level. Their diversedescendants has become dominant terrestrial animals, until the K-Pg extinction dueto meteor impact.Electronic supplementary materialThe online version of this article (doi:10.1186/1742-4682-11-47) contains supplementary material, which is available to authorizedusers.