FURTHER EVIDENCE THAT THE ANTARCTIC LATEST CRETACEOUS FOSSIL AVIANS VEGAVIS IAAI AND POLARORNIS GREGORII WERE FOOT-PROPELLED DIVERS

A previous study on the utility of using Polar Section Modulus as a method to predict avian lifestyles produced a plot which shows different morphospaces indicating the lifestyles of sampled avian taxa. Measurements of the humerus and femur cross-sectional area and length were calculated, and then the values are plotted on a graph, which distributing points relative to a femur axis and a humerus axis to illustrate the resulting different morphospaces by taxon and lifestyle. The lifestyle morphospaces are plotted based on the bendability or rigidity of the bone. Many avian lifestyles are not represented on this plot, so non-represented lifestyles are being added to the dataset. To test if the current thought of Vegavis iaai and Polarornis gregorii representing foot-propelled divers remains supported, the two latest Cretaceous, Antarctic, fossil avians were measured and plotted on the Polar Section Modulus plot. The Vegavis iaai holotype included a right humerus and right femur. The measurements plotted Vegavis, which is thought to be an early duck/duck-like organism, close to both the flying cormorants morphospace and the Mergus merganser duck morphospace. This indicates that it is likely that Vegavis was a type of foot-propelled diver with some flight capability, possibly like ducks. No specimen of Polarornis gregorii includes a humerus, but the holotype does include a femur. A single-axis femur line was generated to help visualize where Polarornis falls along the line of femur values compared to the other avians regarding the bendability of its femur alone. Looking at this femora-only axis, Polarornis plotted close to the flightless cormorants. Polarornis is thought to be loon or loon-like, which is a specialized foot-propelled diver that is clumsy when trying to fly. It makes sense that Polarornis plotted close to other foot-propelled divers with the inability to fly. On this same single-axis femur line, Vegavis plotted close to all the foot-propelled divers, including the highly specialized Hesperornithiformes. The areas on the original study’s plot, where Vegavis and Polarornis plotted are consistent with and reinforce the idea that these latest Cretaceous fossil birds represent foot-propelled divers.