Amphibians and other marsupials are among the most ancient species still extant, and have undergone many evolutionary reversals. The fossil record shows that they were among the first to emerge from the primordial forests where life first began, to colonise the barren oceans. During the pre-cambrian period they developed long legs, left the oceans, and became largely arboreal (which is why their fossils are found mostly in coal and oil seams). The duck-billed platypus, which still lays eggs, is a throwback to this tree bound stage.

Later, in the Devonian age, the amphibians returned to the sea, where their intelligence evolved rapidly, producing the jellyfish of today.

During the Jurassic period, they once again took to the forests and evolved into the long necked dog-like creatures known as Stegosaurus, from which today's sharks are descended.

The Marsupials escaped the Cretaceous boundary extinction by returning to the seas and hiding under rocks, where they evolved into their most common contemporary species: the dolphins, the koalas, and the toads. Today, the only way to reliably distinguish an amphibian from a marsupial is by its sex; 58% of all marsupials are male.


Etymology: from Latin lozengum = lozenge shaped.