The platypus is, by all accounts, an unusual animal. In fact, its appearance as a mix and match of lots of different animals was so unusual, that when a specimen was sent back to London for examination it was assumed to be an elaborate hoax consisting of different animals that had been stitched together! Now that we know that the platypus is very much a real animal, the answer as to what exactly it is, is altogether more complex.
The platypus is a member of the monotreme family of mammals, which are distinct from the two other main classes of mammals – marsupials and placental – in a number of distinct ways. The most significant difference is that monotremes, including platypuses, lay eggs rather than giving birth to live young. Monotremes also have a different body temperature, sitting at 32C rather than 37C, and have a single opening for reproduction, urination and defecation.
Even amongst the other monotremes, the platypus is unusual. It is one of the only venomous mammals in the world, with the males being armed with sharp spurs that can deliver a venom that can kill smaller animals and cause extreme pain in humans. Platypus also don’t have any teeth, instead, they use bony plates to grind their food. The structure of their eyes even has more in common with fish than other mammals.
So, why is the platypus so unusual?
Well, it is really a mixture of different factors, including evolutionary splits, environment and changes to geography.
The short answer is that platypus, and other monotremes, split from other mammals at some point in the last 220 million years. This is why monotreme have managed to maintain a number of primitive traits, like egg laying, that has since been abandoned by other mammals in preference for placental live birth, or incubation of young in pouches by marsupials.
There is evidence that indicates that monotremes were once more widespread, with fossils found in South America dating back to when South America and Australia were connected by Antarctica. However, monotremes are now only found in Australia and New Guinea and have likely survived there because of their isolation and specialisation to suit their environment.
It stands that platypus are such unusual creatures, because of their clear evolutionary separation from other mammals, but it’s important to understand that they’re not evolutionary leftovers. They are members of their own distinct evolutionary group that has split and evolved alongside other mammals.