Systematics
Due to its fragmentary nature – the bones of pseudotooth bords are very thin-walled and light and notoriously easily broken and crushed when fossilizing – it was often allied with the enigmatic Cladornis, and though placed in the order Pelecaniformes (as pseudotooth birds often were) separated in a suborder Cladornithes. But the slightly older (Late Oligocene) Cladornis from the Argentinian part of Patagonia is known from a distal right tarsometatarsus only, and thus not directly comparable to Cyphornis. The two genera were allied simply because of their size and because they both vaguely reminded of the tarsometatarsus of pelicans. Today however, Cladornis is more generally held to be a terrestrial bird rather than a seabird. Other authors had been more conservative all along, and considered Cyphornis quite close to pelicans, uniting these as a superfamily Pelecanides in suborder Pelecanae, or later on (after the endings of taxonomic ranks were fixed to today's standard) Pelecanoidea in suborder Pelecani.
To set it apart from its alleged relatives, Cyphornis was early on separated in a family Cyphornithidae together with Palaeochenoides mioceanus and eventually also Tympanonesiotes wetmorei which are also little-known pseudotooth birds but inhabited the Atlantic. But these are probably all closely related to the better-known Pelagornis, type genus of the family Pelagornithidae. And even if Cyphornis is the senior synonym of all the later-described genera (which is not very likely), according to the rules of zoological nomenclature the family name Pelagornithidae would not change. Thus Cyphornithidae would almost certainly be a junior synonym of Pelagornithidae even if the pseudotooth birds are (as some have proposed) divided into several families – rather than being all placed in the Pelagornithidae as is usual nowadays –, as Cyphornis, Osteodontornis, Palaeochenoides, Pelagornis and perhaps the smaller Tympanoneisiotes appear to be very closely related and are probably all part of a monophyletic lineage of (usually) giant pseudotooth birds. Only if the Pacific lineage is sufficiently distinct, the Cyphornithidae would remain valid, but in this case they would presumably not include the Atlantic forms.
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