Platypus - Evolution and Conservation

Points covered:

A modern Platypus. Photo: Carl Bento, Australian Museum


Skull of Obdurodon (left) compared with modern Platypus (right) The holes in the upper jaw of Obdurodon are alveoli for true teeth - note the horny pads on the modern Platypus instead of teeth. Photo: Carl Bento, Australian Museum


Opalised jaw fragment of an extinct monotreme, Steropodon galmani, showing teeth. Photo: Carl Bento, Australian Museum

• What are Platypus?
• What do fossils tell us about Platypus?
• What is the evolutionary history of Platypus?
• What are the implications for conservation of Platypus?

Words to know:

• MONOTREMES - the group of mammals that lays eggs.
• MESOZOIC ERA - approximately 245 - 65 million years ago. (The three periods of Triassic, Jurassic and Cretaceous are included in this era).
• GONDWANA - the ancient southern supercontinent that included Australia, New Zealand, New Guinea, South America, Africa, India and Antarctica.

The Platypus (Ornithorhynchus anatinus) is a unique Australian species. Along with echidnas, Platypus are grouped in a separate order of mammals known as monotremes, which are distinguished from all other mammals because they lay eggs.

The fossil record for monotremes is poor in comparison to that of other groups of mammals, and until recently little was known about their evolutionary history. Several fossil discoveries since the early 1970s have shed some light on the origins of monotremes. We now know that monotremes were present in Australia during the Mesozoic Era, when Australia was still part of the supercontinent, Gondwana. The fossil evidence suggests that monotremes originated and diversified in the Australian/Antarctic section of Gondwana, and that there was only a single dispersal to South America before the break up of Gondwana.

Four species related to Platypus have been found in fossil deposits from Australia, including a complete skull of Obdurodon dicksoni and an opalised jaw fragment of Steropodon galmani. The latter is 110 million years old and represents one of Australia's oldest mammals. The only evidence that Platypus ancestors were once present outside Australia came in 1991, when a 61 - 63 million year old fossil tooth was found in Patagonia, in southern Argentina.

Studies of these fossils indicate that the one remaining living species of Platypus is more specialised than its predecessors. It is smaller, its functional teeth have been replaced by horny pads and other aspects of its anatomy appear simpler. It also appears to have a more restricted distribution, being confined to the river systems of eastern Australia. Although Platypus remains widespread and reasonably common, this trend towards increasing specialisation suggests that it may be moving out onto an evolutionary 'limb' and that its current status should not be taken for granted.

Sandy Ingleby
Mammals
Australian Museum