Into thin air: The loss of the pliocene giant volant birds

Four genera of distantly phylogenetically very large volant birds existed for most of the Pliocene: Pelagornithidae seabirds; the large North American Teratornithidae, the stork Leptoptilos falconeri in Africa and Asia, and the gigantic vulture Dryornis pampeanus in Argentina. All became extinct around 2 to 3 Ma. The reasons for their demise are puzzling, as the Pelagornithidae had a world-wide evolutionary history of more than 50 Ma, smaller teratorns were still extant in the Holocene, and smaller stork and vulture species continue to be successful today. Extant large birds have a common critical takeoff airspeed suggesting biomechanical constraints in terms of power, risk and launch speed. Atmospheric mass is not constant over time and estimates for Late Pliocene atmospheric density, based on the difference between marine and terrestrial derived pCO 2 and isotopes in amber, suggest a value equivalent to about 1.2 bar that dropped to the present level over the period from ∼3.3 to 2.0 Ma. Simulations of the flight of these extinct species suggest that in the present atmosphere at sea level (∼1) bar their takeoff airspeeds would have exceeded critical values; however, at 1.2 bar all the extinct species present takeoff airspeeds similar to those of large extant volant birds and which are within their muscle power and kinetic energy limits. A loss in atmospheric density may therefore have caused biomechanical and ecological stress contributing to their extinction and/or evolution of smaller forms.