POSSIBLE FILTER-FEEDING IN THE EXINCT PRESBYORNIS AND THE EVOLUTION OF ANSERIFORMES (AYES)

摘要

The fossil avian genus Presbyornis and its relatives (Family Presbyornithidae) are almost universally considered as one of the oldest known filter-feeding specialists among Anseriformes. Such an assumption is based almost entirely on the bill morphology which in Presbyornis is very similar to that of some modern ducks. However, the quadrate bone (key element of the kinetic apparatus of the avian skull) is different in Presbyornis as compared to that in other anseriforms, and shares plesiomorphic similarities with primitive non filter-feeding galliforms. This observation alone requires a functional explanation, as no modern (filter-feeding) anseriforms possess such quadrate bone. Here, we analyze the structural adaptations to filter-feeding in the feeding apparatus of modern ducks, as developed mainly by the late Prof. F.Ya. Dzerzhinsky, and assess their presence in the skull of Presbyornis. We show that a duck-like bill is one of the few duck-like features in the skull of Presbyornis, and most other functional units of its feeding apparatus are similar to those of Galliformes. Most importantly, Presbyornis lacks features indicative of the presence of the ligament-based system fixing the lower jaw against a rostral and caudal displacement. Such fixation is necessary for powerful piston-like tongue movements which in modern ducks provide water inflow and expulsion during filtration. Thus, Presbyornis could not have effectively filtrated very small particles using high-frequency jaw and tongue movements (as do modern ducks) and most likely fed on larger objects which it filtered with slower jaw movements. Presbyornis likely illustrates an important step in the evolution of Anserifomes, when these birds switched from feeding on fixed (likely underwater) to floating food items. The acquisition of specialized filter-feeding in anatids may be viewed as a gradual process initiating from a Presbyornis-like feeding apparatus. Our conclusions highlight a slow and stepwise evolution of the complex morphological traits and emphasize a mosaic nature of intermediate forms.