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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.

An engineering examination of allometric and analogical data on the flight of giant Permian insects (Protodonata, Meganeura or griffinflies) indicates that previous estimates of the body mass of these insects are too low and that the largest of these insects (wingspan of 70 cm or more) would have had a mass of 100-150 g, several times greater than previously thought. Here, the power needed to generate lift and fly at the speeds typical of modern large dragonflies is examined together with the metabolic rate and subsequent heat generated by the thoracic muscles. This evaluation agrees with previous work suggesting that the larger specimens would rapidly overheat in the high ambient temperatures assumed in the Permian. Various extant mechanisms of thermoregulation are modelled and quantified, including behaviour, radiation and the constraints on convective respiration and evaporation imposed by air flow through spiracles. However, the effects of these on cooling an overheated insect are found to be limited. Instead, an examination of the heat budget in the flight medium indicates that, at about 1.6 bar (160 kPa), thermoregulation supply enters into equilibrium and, even at high ambient temperatures, overheating can be avoided and enough oxygen sourced. This approach indicates how fossil biology can be used to examine past atmospheres.

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