protothériens

Protothériens is a former taxonomic group that encompassed the monotremes, a unique order of mammals characterized by their reproductive and anatomical distinctiveness. This group was historically used to classify animals that laid eggs but possessed mammary glands and fur, traits typically associated with live-bearing mammals. The term "protothérien" literally translates to "first beast," reflecting the early understanding of these animals as a transitional form between reptiles and other mammals.

The living representatives of what were once considered prototherians are the echidnas (Tachyglossidae) and the platypus (Ornithorhynchus anatinus). These species are found exclusively in Australia and New Guinea. They share a number of key characteristics, including the presence of a cloaca, a single opening for the urinary, defecatory, and reproductive tracts, a trait also found in reptiles and birds. Unlike other mammals, they lay leathery eggs. After hatching, the young are nourished by milk secreted from pores on the mother's abdomen, rather than from nipples.

Modern evolutionary biology and genetics have refined the classification of these animals. While the term "protothérien" is now largely considered obsolete in formal scientific classification, it remains historically significant in understanding the development of mammalian taxonomy. The monotremes are now universally recognized as a distinct and basal lineage within the class Mammalia, representing an ancient divergence from the evolutionary paths leading to marsupials and placental mammals. Their unique biological features provide invaluable insights into mammalian evolution.