Having survived the devastation of the late Mesozoic Era until modern times, a group of primitive mammals still hides in the "paradise" of Australia: the platypus (and the echidna on Irian Jaya). They possess fur for insulation; their body temperature is not high and varies, but remains within a certain range; they lack nipples but have mammary glands to nurse their young; therefore, they are undoubtedly mammals. However, their reproductive method is oviparous rather than viviparous, which gives them a distinct primitiveness derived from their reptilian ancestors. They are classified into a separate subclass—the only order within the Prototheria—Monothrix.
platypus
Echidna
Evidence suggests that monotremes diverged early from columnodonts and have survived to this day while retaining their primitive characteristics. They are living fossils reflecting early mammals.
