Marsupials

Four legs.

Marsupial, native to Australia.

What I'm trying to tell you is it doesn't hop along.

Well, we know that.

First, we will go over the history of mammal classification, then provide some additional information on modern mammalian evolution before reviewing the monotremes.

Previously, it was widely accepted that there were only eleven orders of mammals, Rodentia, Chiroptera, Cetacea, Carnivora, Primates, Artiodactyla, Perissodactyla, Lagomorpha, Insectivora, Marsupialia, and Monotremata.

Though this system has gone through numerous iterations over the years, with the most recent rewrite being in twenty twenty-three, there are aspects of it that have remained firm, while others have changed substantially.

For example, the monotremes are so different that they are still considered their own order, while the marsupials have been found to be so diverse that their single order, Marsupialia, has been raised to the level of infraclass and placed as a sister taxon to the placental mammals, whose total order count has increased dramatically.

Meanwhile, some, like Insectivora, have been determined to be polyphyletic and thus are no longer used.

In this tutorial, we are not concerned with the marsupials or the placentals.

In order to better understand these animals, let's first go over a more detailed comparison of marsupial and placental evolution, followed by their reproduction, before finally reviewing the extant marsupial orders.

Early marsupial evolution originated between one hundred and sixty and one hundred and twenty-five million years ago during the early Cretaceous or late Jurassic period.

Soon after the diversification of their egg-laying ancestors, marsupials diverged from the placental mammals on the ancient supercontinent Gondwana.

Fossil evidence indicates their presence in South America and Antarctica during the late Cretaceous period.