The word monotreme comes from the Greek words mono- and trema, meaning one and hole. This name refers to the fact that monotremes have a single opening for the urinary, digestive and reproductive organs.
Like other mammals, monotremes
The only surviving examples are all indigenous to Australia and New Guinea:
Living monotremes lack teeth as adults. Fossil forms and the young of living monotremes have the "tribosphenic" molars which are one of the hallmarks of mammals. However, recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials. [1] The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. 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 pre-mammalian synapsid?s. However, the external opening of the ear still lies at the base of the jaw. The monotremes also have extra bones in the shoulder girdle, including an interclavicle, which are not found in other mammals. The monotreme leg bears a spur in the ankle region.
The physiology of monotremes is equally unique. Their metabolic rate is remarkably low by mammalian standards, and their internal temperature control mechanisms are less developed than in other mammals. Monotremes do lay eggs. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipples. All species are very long-lived, with low rates of reproduction and relatively prolonged parental care of infants.
[1] Luo, Z-X, RL Cifelli & Z Kielan-Jaworowska (2001), Dual origin of tribosphenic mammals. Nature 409: 53-57.
External link:
University of Tasmania's monotreme web site at http://www.healthsci.utas.edu.au/physiol/mono/Monotremata.html.
![[Home]](wikipedia.png)