prototherian /ˌpɹoʊtɵˈθɪəɹi.ən/adjective 1noun 1forms 3 adjective1 •Belonging or pertaining to the subclass Prototheria. noun1 •Any of the egg-laying mammals of the subclass Prototheria; a monotreme.Monotremes (prototherians), which include the duck-billed platypus, and short- and long-beaked echidnas, have a cloaca—a single opening for the fecal, urinary, and reproductive tracts. — 1999 formsprototherians · more prototherian · most prototherian