(Marsh, 1894) and Omorhamphus storchii (Sinclair, 1928). O. storchii was described based on fossils from lower Eocene rocks of Wyoming. The species was named in honor of T. C. von Storch, who found the fossils remains in Princeton 1927 Expedition. The fossil bones originally described as Omorhamphus storchii are now considered to be the remains of a juvenile Gastornis gigantea. Specimen YPM PU 13258 from lower Eocene Willwood Formation rocks of Park County, Wyoming also seems to be a juvenile – perhaps also of G. gigantea, in which case it would be an even younger individual. Gastornis xichuanensis, from the early Eocene of Henan, China, is known only from a tibiotarsus (upper foot bone). It was originally described in 1980 as the only species in the distinct genus Zhongyuanus. However, a re-evaluation of the fossil published in 2013 concluded that the differences between this specimen and the same bone in Gastornis species were minor, and that it should be considered an asian species of Gastornis. Paleobiology Diet A long-standing debate surrounding Gastornis is the interpretation of its diet. It has often been depicted as a predator of contemporary small mammals, which famously included the early horse Eohippus. However, with the size of Gastornis legs, the bird would have had to have been more agile to catch fast-moving prey than the fossils suggest it to have been. Consequently, Gastornis has been suspected to have been an ambush hunter and/or used pack hunting techniques to pursue or ambush prey; if Gastornis was a predator, it would have certainly needed some other means of hunting prey through the dense forest. Alternatively, it could have used its strong beak for eating large or strong vegetation. The skull of Gastornis is massive in comparison to those of living ratites of similar body size. Biomechanical analysis of the skull suggests that the jaw-closing musculature was enormous. The lower jaw is very deep, resulting in a lengthened moment arm of the jaw muscles. Both features strongly suggest that Gastornis could generate a powerful bite. Some scientists have proposed that the skull of Gastornis was ‘overbuilt’ for a herbivorous diet and support the traditional interpretation of Gastornis as a carnivore that used its powerfully constructed beak to subdue struggling prey and crack open bones to extract marrow. Others have noted the apparent lack of predatory features in the skull, such as a prominently hooked beak, as evidence that