The platypus is placed with the echidnas in the Order called Monotremata (meaning 'single hole' because of the common external opening for urogenital and digestive systems).
Traditionally, the Monotremata are considered to belong to a subclass Prototheria, which diverged from the therapsid line to the Theria and subsequently split into the marsupials and eutherians (Placentalia).
The divergence of monotremes and therians falls into the large gap in the amniote phylogeny between:
Estimates of the monotreme-theria divergence time range between 160 and 210 million years ago, recently estimated from fossil and molecular data (Warren et al., 2008).
Although known to have had several fossil relatives, Ornithorhynchus anatinus is the only living species of platypus.
The platypus genome, as well as the animal, is an amalgam of ancestral reptilian and derived mammalian characteristics.
The platypus karyotype comprises 52 chromosomes in both sexes, with a few large and many small chromosomes, reminiscent of reptilian macro- and microchromosomes.
The Ornithorhynchus anatinus whole-genome shotgun project has been deposited in DDBJ/EMBL/GenBank under the project accession AAPN00000000 (Warren et al., 2008).