BAYESIAN REANALYSIS OF THE EVOLUTION OF PHORUSRHACIDAE (AVES, CARIAMIFORMES)

Phorusrhacids, commonly referred to as “Terror Birds”, are an extinct group of flightless carnivorous birds. The derived members of this group represent a unique example of predatory birds evolving to gigantic sizes. We present a revised evolutionary tree for the family Phorusrhacidae, with analyses on body size evolution.

Using previous studies as a framework, we formed a new composite matrix of diagnostic traits and constructed a Bayesian phylogeny. Mass correlated bone measurements were compiled and paired with their respective species. Due to the poor preservation record of Phorusrhacidae, multiple size proxies were evaluated. Those which reliably correlate to femur length within a phylogenetically normalized regression are presented here as possible alternative metrics for body size. Measurements were then used to test for the directional selection of gigantism and variability in body size within and between various lineages.

This new phylogeny indicates that gigantism occurred in multiple lineages within Phorusrhacidae, to varying degrees. Extreme examples, however, appear to have solely occurred in a newly defined crown group, composed of all phorusrhacids traditionally ascribed to the subfamilies Phorusrhacinae and Brontornithinae. Notably the largest individual species comprise two independent lineages with no biostratigraphic overlap, suggesting ecological succession. Overall, we observe a continuous trend of multiple co-occurring phorusrhacid species within the same ecosystem, each occupying a non-overlapping range of body sizes. Discrete size ranges associated with specific clades persisted even as ecosystems were restructured over geologic time, with no significant shifts observed within groups except when directly following the extinction of another lineage.

This revised phorusrhacid tree clarifies how this group evolved and with additional inquiry may have further implications regarding Phorusrhacid ecology. No directionality in body size evolution was detected, which we ascribe to the early divergence of size constrained clades. Speciation rather was associated with long-term stabilizing selection, or niche filling following extinction. Body size thus appears to be strongly associated with the maintenance of high phorusrhacid diversity and the partitionment of the terrestrial predator niche within mid-Cenozoic South American grasslands.