Noneutherian

Noneutherian is an informal term used in mammalian biology to refer to all mammals that are not placental mammals (not Eutheria). In most classifications, the noneutherian component comprises two lineages: Monotremata, the egg-laying mammals such as the platypus and echidnas; and Metatheria (marsupials), a diverse group that includes kangaroos, opossums, and many other species. The term is not a formal clade in modern cladistics because it excludes placentals while not defining a single common ancestor to all its members; accordingly it is usually described as paraphyletic with respect to placental mammals.

Evolutionarily, monotremes and therians diverged early in mammalian history, with monotremes retaining several ancestral traits. Monotremes are today restricted to Australia and New Guinea; marsupials occupy Australia and nearby islands, with some groups found in the Americas; placental mammals (Eutheria) are distributed worldwide. Marsupials generally give birth to immature young that complete development in a pouch or on external surfaces, whereas monotremes lay eggs and produce milk without nipples.

Characteristics common to noneutherians include mammalian traits such as fur, mammary glands, and endothermy, but monotremes differ markedly in reproductive biology and skull morphology from therians. The noneutherian concept is primarily used in comparative anatomy and paleontology to discuss non-placental mammalian evolution; in modern molecular phylogenies, discussions of non-placental groupings are often avoided in favor of explicit clades.

Related terms include Monotremata (monotremes), Metatheria (marsupials), and Eutheria (placental mammals).