Macroevolutionary trends of avian ichnodisparity in Gondwana.

The concept of ichnodisparity is herein introduced for the study of fossil vertebrates, using Gondwanan avian footprints. We define ichnodisparity for trackways as the qualitative variation in morphological patterns of autopodial impressions, most likely reflecting functional or ecological thresholds. Foot architectural designs are defined based on anatomical features, including digit number and orientation and interdigital webbing, and size. Ichnodisparity in fossil bird tracks is proposed here as a useful proxy for analyzing evolutionary radiations and characterizing macroevolutionary patterns. Twenty-three geological formations from Gondwana were analyzed, spanning from the Early Cretaceous to the Late Pleistocene. The earliest morphotypes were predominantly large-sized, while small morphotypes became dominant over time. No zygodactyl footprints are recorded from Gondwana. Prior to the K/Pg boundary, ichnodisparity was high, consistent with a Cretaceous avian radiation. During the Eocene, morphological variety remained comparable to that of the Late Cretaceous - except for the large tridactyl design - suggesting ecological turnover rather than functional expansion; elevated ichnodisparity may be associated with the Early Eocene Climatic Optimum. The decline recorded in the Oligocene is likely attributable to sampling bias. The Miocene concentrates the highest ichnodisparity in the fossil record, with the reappearance of large pedal architectures linked to cursorial birds, supporting an expansion into open spaces. In the Pleistocene, small tridactyl forms disappeared and the large palmate architecture emerged for the first time, indicating a new episode of ecological turnover.