A small order of birds that contains a single family, the Opisthocomidae. Its one species, the hoatzin, is restricted to South America and has been frequently included in the Galliformes as a suborder; however, little evidence supports this conclusion. More recently, some researchers claimed that it was a typical member of the Cuculidae, the cuckoos, based only on evidence from deoxyribonucleic acid (DNA) hybridization studies. However, important differences in the morphology of the perching foot argue strongly against the inclusion of the hoatzin in Cuculidae. The hoatzin has an anisodactyl perching foot with three anterior toes and a well-developed hallux (first toe) that is completely different from the zygodactyl perching foot of the cuckoos (with the second and third toes anterior, and the fourth toe and hallux posterior). Selective demands for perching could not have resulted in the evolution of the anisodactyl foot of the hoatzin from the zygodactyl foot of the cuckoos. In the absence of definite evidence of its relationship to other avian orders, the Opisthocomiformes are best considered as a distinct order that evolved in South America since its isolation from the rest of Gondwanaland sometime in the late Mesozoic.