Additional reconstruction through shared genes in sex chromosomes supports this hypothesis of independent evolution. The sequencing of the platypus genome has also provided insight into the evolution of a number of monotreme traits, such as venom and electroreception, as well as showing some new unique features, such as monotremes possessing 5 pairs of sex chromosomes and that one of the X chromosomes resembles the Z chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved after the split from the monotreme lineage. The external opening of the ear still lies at the base of the jaw. Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in cynodonts and other premammalian synapsids this feature, too, is now claimed to have evolved independently in monotremes and therians, although, as with the analogous evolution of the tribosphenic molar, this hypothesis is disputed.
Monotreme jaws are constructed somewhat differently from those of other mammals, and the jaw opening muscle is different. Tooth loss in modern monotremes might be related to their development of electrolocation. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this hypothesis remains disputed. Fossil forms and modern platypus young have a "tribosphenic" form of molars (with the occlusal surface formed by three cusps arranged in a triangle), which is one of the hallmarks of extant mammals. (1) Shell (2) Yolk (3) Yolk Sac (4) Allantois (5) Embryo (6) Amniotic Fluid (7) Amniotic Membrane and (8) MembraneĮxtant monotremes lack teeth as adults. The name monotreme derives from the Greek words μονός ( monós 'single') and τρῆμα ( trêma 'hole'), referring to the cloaca.ĭiagram of a monotreme egg. There is currently some debate regarding monotreme taxonomy. The extant monotreme species are the platypus and four species of echidnas. The only surviving examples of monotremes are all indigenous to Australia and New Guinea although there is evidence that they were once more widespread, as Monotrematum is known from the Paleocene of South America. Monotremes are a subgroup of the Australosphenida, a clade which contains other related extinct mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia. Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right)