Mammals are very diverse group of animals. Mammals first appeared over 200 million years ago, but they didn’t start to become the diverse group they are today until after the extinction of dinosaurs, approximately 65 million years ago. In the 10 million years that followed, the relatively small group grew to 4,000 species through adaptive radiation. The result was mammals of all types and sizes: fliers, swimmers, carnivore, herbivores--the list goes on. The mammals can be divided into egg-laying monotremes, pouched marsupials and placental mammals.

Australia and its surrounding islands are interesting places to study mammalian diversity. Australia is home to nearly all the marsupial species and monotreme species that still exist on earth. In fact, the only placental mammals that exist on Australia today that didn’t fly or swim there were introduced by humans. Australia’s extinct mega-fauna included 7-foot tall kangaroos, the 6,000 pound Diprotodom (think giant wombat) and most recently extinct, the Tasmanian tiger. In fact, the Tasmanian tiger did not go extinct until the 1900s when competition from the non-native dingo and conflict with European settlers led to its demise.

Monotremes are fascinating and rare. Once upon a time there were hundreds of monotreme species that roamed the earth, such as the extinct spoon-billed platypus, but only 5 species remain. The duck-billed platypus is by far the most famous monotreme but echidnas have the most living species.

Echidnas somewhat resemble porcupines and hedgehogs, both placental mammals, but in fact, these three species have a very unique evolutionary lineage. The spines on these three species are analogous structures, structures that have separate evolutionary origins but are superficially similar because of the similar forces of natural selection that shaped them. The wings of birds, bats and butterflies are another example of analogous structures.

And what about belly buttons? Belly buttons, more formally known as navels, is the point of attachment for the umbilical cord, which connects a developing fetus to the mother’s placenta. The belly button is what is left after the baby loses it umbilical cord after it is born. Most placental mammals have a belly button, even whales!

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