Prototherians differ from all other mammals by the
Prototherians have a cloaca. The anus and urinogenital aperture are separate in metatherians and eutherians. Pinna is absent in prototherians. The mammary glands and the diaphragm are present in all the mammals.
Hence, the prototherian, duck billed platypus is considered a connecting link between the reptiles and mammals.