Body size evolution in palaeognath birds is consistent with Neogene cooling-linked gigantism

Abstract

Palaeognathae is an extant clade of birds with body masses ranging over several orders of magnitude of kilograms, including the largest-yet described terrestrial avian species. Most studies have suggested flight loss and increasing body size began early in its evolutionary history, prior to the divergence of the major extant lineages. However, recent phylogenetic work suggests body size increases occurred more recently and independently within many extant lineages. Here, we use linear regression equations to estimate the masses of extinct taxa from both the crown and stem of Palaeognathae to test these hypotheses. We allocate fragmentary specimens to discrete body mass categories to accommodate additional species for which more precise body mass estimation is not possible. The first fossil evidence for an increase in maximum palaeognath body size, from approximately that of a rhea (20 kg) or smaller to ostrich size (100 kg), is Miocene in age (approximately 20 Ma). The heaviest taxa, those exceeding 120 kg, all occur within the last 9 million years. Molecular divergence dating estimates for the crown ages of palaeognath subclades pre-date the oldest preserved material, often significantly; however, plotting the lightest member of each clade at the estimated crown ages does not change the observed pattern. These results may be affected by the predictive model used to estimate the mass of extinct taxa, but the observed trends in body mass do not change when four models from different studies are used. Both island-dwelling and continental clades show broad temporal congruence in the timing of their estimated gain of large body sizes, meaning geographical restriction on islands does not alone explain the observed trend. We suggest large and giant Palaeognathae body sizes may be more closely linked to global cooling which creates selective pressures on body size for thermoregulation, as well as causing landscape changes which alters dietary and locomotory requirements. Global cooling has received extensive attention as a driver of mammalian body size extremes but has been relatively little discussed with respect to birds.