2 families, 3 genera and 3 species currently represent the monotreme order. The fossil history of monotremes goes back from early Cretaceous to Recent in New Guinea and Australia, and early Pliocene in South America. In Australia, two diverse families are represented from the early Cretaceous, which indicates that the order may actually predate this period. Because modern monotremes are only found in Australia and New Guinea, the presence of fossil evidence in South America may be indicative of a dispersal of monotremes via Antarctica to South America when the two landmasses were joined.

There has been much discussion about the taxonomical placement of these unusual species. Some taxonomists have categorized them in a subclass with extinct lineages separate from all other extant mammals. Others have placed them as living therapsid reptiles. The source of the confusion is the fact that monotremes possess characteristics of reptiles, mammals and marsupials.

It is the reptilian characteristics that set monotremes furthest apart from the other mammals. Among the most noteworthy is that female monotremes lay eggs. The embryos develop inside leathery shells that are incubated and hatched externally from the mother. Monotremes also possess a single external opening for the reproductive, excretory and digestive systems called the cloaca. Another reptilian resemblance is the lack of a corpus callosum bridging the two hemispheres of the monotreme brain. This connection of nervous tissue is present in all other mammals but not in reptiles or marsupials.

Monotremes possess a potentially significant, yet puzzling marsupial characteristic in the presence of marsupiam bones in the pelvis. These bones are usually associated with support of the marsupial pouch, however the purpose in the monotreme anatomy is more obscure because the bones are present in both sexes. Scientists have proposed that the bones may have functioned at one time as support to a pouch structure or, perhaps be remnants of skeletal and muscular structures supporting the large hindquarters of a reptilian ancestry.

Even with all of these reptilian characteristics, the current consensus is that monotremes are in more ways mammalian. They have fur, they are warm-blooded, they nurse their young and their heart is four-chambered (however with some differences in the atrioventricular valve and left aortic arch). The skeletal system also features significant mammalian characteristics, such as the existence of one bone on each side of the lower jaw and three middle ear bones. As mentioned, aspects of the monotreme brain are reptilian, however a notable mammalian characteristic is a more developed pallial region than striatal region. The opposite is true for reptiles and birds.