Gigantohierax is primarily known from fossils of the leg with bones of the femur and the tibiotarsus known. Based on their structure with the thigh having flared ends and twisted structure, these raptors may have been partially terrestrial but not necessarily entirely so, whereas few modern hawks and eagles travel in this manner. Furthermore, the osteology of Gigantohierax implied that it was robustly built, unlike some other large fossil buteonine fossil species.[2][1] The two species of Gigantohierax differed In size. G. itchei was around 29% smaller than G. suarezi. However, what is certain is that Gigantohierax was extremely large. In fact, they are regarded as the largest accipitrid ever known to exist in the Americas.[3] Furthermore, the fossil discovered of G. suarezi approach in size those of the Haast's eagle, which is considered the largest eagle and typical bird of prey known to have existed. G. suarezi was considerably larger than the harpy eagle, the largest extant eagle in the Americas (and one of the top two most massive living eagles). If Gigantohierax was nearly the size of Haast's eagle as projected, it may have been over 30 cm (12 in) greater in total length than the harpy eagle and around an estimated 40-50% heavier.[2] Gigantohierax was one of several examples of Island gigantism in Quaternary Cuba, especially several birds and mammals. Of note is that Gigantohierax co-existed with a similarly record largest giant owl, Ornimegalonyx, representing the nocturnal near ecological equivalent.[2][1][4]
