COMPARING THE EFFECTS OF TRANSPORT-INDUCED FLUVIAL ABRASION ON AVIAN AND NONAVIAN VERTEBRATE SKELETAL REMAINS

Avian bones are relatively rare in the fossil record compared to those of other vertebrates. One generally accepted hypothesis explaining this disparity is that the hollow, thin-walled bone structure makes them relatively fragile, and thus they are frequently destroyed by taphonomic processes (e.g., fluvial abrasion) before fossilization can occur. However, avian bones are mechanically strong enough to endure the stresses of powered, flapping flight, and thus may not be as fragile as many researchers assume. Modern, unweathered, relatively small bones (3.81cm-16.51cm) of mammals, reptiles, and birds were exposed to simulated fluvial abrasion during a 192 hour period. Experiments varied sediment grain size and sediment to water ratio in order to compare the type(s) and amount of degradation recorded among taxa. For all taxa, experiments using fine-grained sediment (0.1-4.0mm) resulted in a polished surface on the compact outer bone with slight to moderate damage to any exposed cancellous bone; no rounding of the bones was observed. Coarser sediment (0.1-15.0mm), reflecting the bedload of the Middle Saluda River near Cleveland, South Carolina, produced more severe damage to both the cancellous and compact bone. Only mammalian and reptilian bones experienced significant rounding of condyles and processes, but only the mammalian bones experienced cracking of the outer cancellous layer. Increasing the water to sediment ratio did not affect either the type(s) or the amount of damage the bones accrued. In all experiments, mammalian and reptilian bones consistently exhibited a higher rate of degradation compared to avian bones. These results indicate that avian bones are not more susceptible to fluvial abrasion than bones of other vertebrates. Thus, another taphonomic process may produce the paucity of fossilized avian remains.