Too big to fly? An engineering evaluation of the fossil biology of the giant birds of the Miocene in relation to their flight limitations, constraining the minimum air pressure at about 1.3 bar

Abstract

Abstract The fossil biology of flight is one of the few options available for obtaining information on the density of past atmospheres. The giant birds of the Miocene, the 70 kg teratorn Argentavis magnificens and the giant Pelagornithidae with wingspans of 6.5 m or more, have long intrigued bird specialists, leading one researcher, Colin Pennycuick, to hypothesize that a higher air density may have been necessary for these birds to fly. To test this hypothesis, previous mass estimates and wing shapes of these birds are reviewed and the revised values used in the Flight 1.25 bird simulation program to investigate and quantify in engineering terms the limitations of flight in the present atmosphere as well as in hypothetical higher densities. The results indicate that Pennycuick was probably right: the available takeoff power for a gorged teratorn in an atmosphere of 1 bar is too low and attempts at level takeoff could involve a high risk of injury; The giant Pelagornithidae would have had enough power for takeoff, but flapping bone stress at 1 bar would also have been critical. Simulations indicate that for both birds, power and bone stress constraints are overcome at about 1.3 bar, also enhancing the dynamic soaring of the Pelagornithidae. This level of atmospheric pressure would imply a different climate than the present, but is consistent with the data on average global temperatures for the Miocene before the Late Cooling Events, as well as with the polar to equator temperature gradient.