The Ornithorhynchidae are one of the two extant families in the order Monotremata, and contain the platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas.

Within the Ornithorhynchidae are the main Cenozoic genera Ornithorhynchus and Obdurodon, and several potential stem-genera dating back to the Late Cretaceous, of which the oldest is possibly Dharragarra. Although fossil evidence suggests the presence of ornithorhynchids in the Cretaceous, phylogenetic evidence has largely found that they and the Tachyglossidae only diverged during the Cenozoic, although this varies based on the specific constraints used.

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