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Steropodon Galmani
Steropodon galmani is the oldest known monotreme. It is known only from some jaw fragments that were found in New South Wales, Australia. The jaw fragments were dated to the early Cretaceous period. The teeth in the jaw are similar to the teeth found in juvenile platypus. Due to the size of the teeth, scientists estimate the creature to be about the size of a cat.
Obdurodon Insignis
Obdurodon insignis is known from a jaw, a few teeth, and a pelvis. This animal lived in the Tertiary period during the Miocene era, approximately 23.8 to 5.3 million years ago. Unlike the modern platypus, O. insignis had canine teeth.
Obdurodon Dicksoni
Obdurodon dicksoni is known from a single skull found in 1992. Modern platypus lose their teeth before they mature, but the O. dicksoni skull shows teeth as an adult. The snout is straighter and narrower than the modern platypus, and it may or may not have been aquatic.
Living Relatives
The closest living relative of the platypus is the echidna. It is in the same order as the platypus, but a different family. Platypus belong to the family Ornithorhynchidae, echidnas belong to the family Tachyglossidae. They do share several important features. Both animals lay eggs, store their young in a pouch, and feed their young with milk. Unlike other mammals, neither the echidna nor the platypus have nipples. Both also have electrosensors in their beaks, though the platypus has more than the echidna. These sensors help them find food.