The Ornithorhynchidae are one of the two extant families in the order Monotremata, and contains the platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas. Within Ornithorhynchidae are two genera, Obdurodon and Ornithorhynchus:Family Ornithorhynchidae Genus † Monotrematum †Monotrematum sudamericanum Genus †Obdurodon — an ancient branch of the platypus family †Obdurodon dicksoni †Obdurodon insignis †Obdurodon tharalkooschild Genus Ornithorhynchus Ornithorhynchus anatinus (the modern platypus)
Another two genera, Steropodon and Teinolophos, were originally thought to belong to Ornithorhynchidae. However, they were both placed into a new family, Steropodontidae. This decision was made based on differences in the dentary recovered from the Griman Creek Formation, Lightning Ridge, New South Wales, Australia. This dentary is the holotype for the genus Steropodon, and thus the lack of information is what led to the original misclassification. Further research on Teinolophos has indeed shown it to be an animal much different from ornithochrynchids, lacking a beak, possessing a more complete mammalian dentition and retaining primitive ears connected to the jaw as in more basal mammals.
The extinct Ornithorhynchus maximus was once included in Ornithorhynchus, but has now been determined to be from the echidna family as Zaglossus robustus.