The great horned owl is part of the genus Bubo, which may include as many as 25 other extant taxa.[6] Almost all Bubo species are large and powerful; even the mighty great horned would rank as a medium-large species amongst the many species distributed predominately in Africa and temperate Asia. Among all the world's owls, there are 10 species heavier in mean mass (9-10 of which are in Bubo depending on the placement of the fishing owls), 13 species longer at average total length (8-9 of which are in Bubo) and 11 species wider in wingspan at average (6-7 of which are in Bubo) than the great horned owl's. The maximum weight of the great horned owl has only been topped by 6 other species but this may have to do with smaller sample sizes in other large species than the full range variation of size in this species.[4][6][7][14] The great horned owl represents one of the one to two radiations of this genus across the Bering Land Bridge to the Americas. Whereas the Magellanic horned owl clearly divided once the owl had spread through the Americas, the consensus seems to be that the snowy owl and the great horned owl divided back in Eurasia and the snowy then spread back over the Arctic through northernmost North America separately from the radiation of the horned owl.[7][32] Some writers have proposed that the great horned and Eurasian eagle owls may in fact be conspecifics, pointing out their similarities in life history, geographic distribution and appearance and arguing that about as much superficially separating them as what delineates the American and Eurasian representations of the long-eared owl or great grey owl.[4] In one case, a zoo-kept male great horned owl and female Eurasian eagle owl even produced an apparently healthy hybrid, although its large size favored the mother. However, a case where a great grey owl hybridized with a great horned owl in a zoo was also recorded and they are certainly not closely related.[33] Furthermore, genetic testing have indicated the snowy, not the Eurasian eagle owl, is the most closely related living species to the great and Magellanic horned owls.[7] The Eurasian eagle owl, as the world's second most massive living owl on average, is considerably larger than the great horned owl, with an average weight approximately twice that of the horned owl with linear measurements often at least 30-40% greater. When compared side-to-side, the eagle owl has an even bulkier frame than the great horned owl, with orange eyes rather than the yellow typical of the horned owl. The eagle owl has a similar or perhaps even larger east-to-west distribution in Eurasia than the north-to-south distribution of the horned owl and a similar number of subspecies are known but the eagle owl shows less overall variation in color, especially towards the darker end of the spectrum. The eagle owl has vertical, not horizontal, streaking on its underside and lacks the horned owl's strong dark brackets on the sides of its facial disc.[6] Pleistocene Era fossils have been found of Bubo owls in North America, which may either be distinct species or paleosubspecies, from as far east as Georgia but predominantly in the Rocky Mountains and to the west of them.[10][34] Almost all fossils indicate these owls were larger than modern great horned owls, with Bubo sinclairi from Californian cave deposits being around the size of a Eurasian eagle owl and the unnamed specimen from Georgia being even larger. The reasons for the size decrease to the modern great horned owl, considering the similarities of their life history and prey to the larger Old World species, is not known.[35][36]