Biomechanics of the jaw apparatus of the gigantic Eocene birdDiatryma: implications for diet and mode of life

Abstract

Discovery of several new specimens of the gigantic Eocene ground birdDiatryma giganteafrom the Willwood Formation of northwestern Wyoming, has prompted an analysis of its feeding apparatus and an assessment of the mode of life of this unusual bird.Diatrymaexhibits many of the features predicted by biomechanical models to occur in animals delivering large dorsoventral bite forces. Similarly, the mandible ofDiatryma,which was modeled as a curved beam, appears well equipped to withstand such forces, especially if they were applied asymmetrically. Interpretation of these size-independent biomechanical properties in light of the large absolute skull size ofDiatrymasuggests a formidable feeding apparatus. The absence of modern analogues complicates the determination of just how this unique skull morphology correlates with diet. Suggestions thatDiatrymawas an herbivore seem improbable in that they require the postulation of excessively high safety factors in the construction of the skull. The traditional hypothesis ofDiatrymaas a carnivorous bird accords as well or better with the data at hand. Carnivory raises the probability of “accidental” encounter with bones, thus explaining the high safety factors. In fact, the skull and mandible ofDiatrymaare so massive that bone crushing may have been an important behavior.Diatrymacould have been a scavenger. However, limb allometry and phylogenetic interpretation of limb proportions call into question the picture ofDiatrymaas a slow, plodding graviportal animal, suggesting that active predation was within its behavioral repertoire.