What are Monotremes, Marsupials and Eutherian Mammals?

Echidna Tachyglossus aculeatus in southern Australia — Image Peter Miles.

Platypus and echidna are classified as monotremes; kangaroos, koala and bandicoots among others are marsupials; over 4,000 other animal species, including ourselves, are eutherian mammals. Of the Monotremes and Marsupials many are found in the Australian region.

Scientific nomenclature (naming) of organisms has a varying number of taxonomic (classification) ranks but…