The terror birds of the northern hemisphere are not all terror birds in their own right. The term terror bird is usually used in reference to the group of birds belonging to the family Phorusrhacidae, to which Gastornis does not belong. The family of Gastornis is actually Gastornithidae, but the order is Anseriformes, meaning that Gastornis is in fact a giant duck (or goose if you like those better). The order of the true terror birds is Cariamiformes, a group that includes the extant Seriemas; predatory South American terrestrial birds much like the African Secretary Bird. What all of this means is that the lineage of Gastornis evolved to look, act, and dominate its landscape convergently rather than as an offshoot genus of the true terror birds. This convergent evolution is seen throughout the anatomy of the bird (the fossil anatomy that we can look at that is of course). The true terror birds, represented by Titanis, have a hooked, stout bill consisting of the premaxilla and maxilla. The bill is highly akinetic and was likely used as a hatchet-like weapon in prey acquisition. The bills of Gastornis were also highly akinetic and composed of the premaxilla and maxilla. However, they lack the premaxillary hook seen in terror birds and, while they may have also been used as a hatchet-like weapon, the lack of a hook means that the bill could not be used to pierce prey. Instead the bill would have acted more as a hammer, bludgeoning prey. The synsacrum (fused sacral and sometimes lumbar vertebrae) of Gastornis was longer than that of the Titanis; however, this does not mean that Gastornis was more agile or that it used this elongated tail to run with better balance. The legs of Gastornis are actually more robust and less likely to have allowed for bursts of speed than the agile but powerful lower limbs of Titanis. These traits lead to the hypothesis that Gastornis may have been an ambush predator rather than an actively stalking and chasing predator. Gastornis also possessed traits that are reminiscent of their closest relatives in that it had fully formed, but reduced and weakened, wings and a sternum that was mostly flat. An extremely reduced ridge is somewhat present in the midline that is homologous to the fully keeled sternum of flying Anseriformes. Titanis and other Phorusrhacids have an absent keeling of the sternum or a completely flat (rounded as encloses the thorax) sternum.