Ornithorhynchoidea is a superfamily of mammals containing the only living monotremes, the platypus and the echidnas, as well as their closest fossil relatives, to the exclusion of more primitive fossil monotremes of uncertain affinity.[1]
Opalios, the only described member of the extinct family Opalionidae, is considered the most basal ornithorhynchoid due to its unique combination of ancestral and derived traits.[1]
Although most members of this group—extinct and extant—are known from Australia, at least two ornithorhynchid-like forms reached southern South America during the Maastrichtian (Patagorhynchus) and early Paleocene (Monotrematum).[5][6]
The following genera are known:
Superfamily OrnithorhynchoideaFlannery et al., 2024
They can be distinguished from other fossil monotremes by their twisted-shaped dentaries, with the lingual surfaces being dorsoventrally flattened (aside from in echidnas).[1]