Living monotremes (the platypus and echidnas) are highly specialised egg‐laying mammals found only in Australia and New Guinea. However, their fossil history extends back to the Mesozoic Era. The past distribution of platypuses included Antarctica and southern South America, and their history extends back to the Mesozoic. Monotremes are, therefore, the oldest living mammalian group known. Platypuses are semi‐aquatic, feeding on benthic invertebrates in rivers, streams and lakes of eastern Australia. Echidnas are spine‐covered, terrestrial insectivores. There is just a single species of living platypus (Ornithorhynchus anatinus), but four living species of echidna: the short‐beaked echidna (Tachyglossus aculeatus) from Australia and New Guinea, and three species of long‐beaked echidna (genus Zaglossus), all of which live only in New Guinea. All monotreme species face challenges from climate change, development and human pressure. The platypus and long‐beaked echidnas are especially vulnerable, and there is much concern for their future.

Key Concepts

  • Monotremes are mammals that lay eggs, like the earliest mammals, instead of bearing live young like all other living mammals.
  • Living monotremes are found only in Australia and New Guinea, but platypuses were once distributed across southern Gondwana to the southern tip of South America.
  • Many anatomical features in monotremes have an ancient heritage, such as the retention of certain skull bones and additional bones in the shoulder girdle.
  • Both living monotreme types – platypuses and echidnas – are highly specialised for particular niches.
  • Monotremes have a ‘sixth sense’ that helps them find cryptic prey: uniquely sensitive beaks or bills with both touch receptors and electroreceptors.
  • Platypuses are extremely well adapted for life in water, with a wide bill, streamlined bodies, webbed feet and thick, waterproof fur.
  • Echidnas are toothless, spine‐covered ant and termite specialists with long tubular beaks.
  • There are two types of echidnas, the short‐beaked echidna from Australia and New Guinea and three very rare long‐beaked echidna species known only from New Guinea.
  • Although the fossil record for monotremes is over 100 million years old, they are now threatened by climate change, hunting and development pressures, and their ongoing survival is not assured.

Keywords: Monotremata; platypus; echidna; Australia; New Guinea

Figure 1. Skeleton of the short‐beaked echidna, T. aculeatus. The posttemporal canal (ptc); cervical ribs (cr); epipubic bones (eb) (a feature shared with marsupials); and archaic shoulder girdle are considered to be primitive characteristics retained by monotremes, while ossified sternal ribs (sr) occur in birds but not most mammals. The solid build of the skeleton is attributed to the echidna's fossorial lifestyle. Source: Courtesy of Anne Musser.
Figure 2. The platypus, Ornithorhynchus anatinus. The platypus is beautifully adapted to its aquatic lifestyle, with a thick, waterproof pelt; dorsally placed eyes; streamlined body and extensively webbed forefeet. Its extraordinarily sensitive, wide bill is its primary navigational tool when feeding underwater. Source: Courtesy of Anne Musser.
Figure 3. The short‐beaked echidna, Tachyglossus aculeatus. The short‐beaked echidna is characterised by a short, straight beak and by a heavy coat of stout, sharp spines. T. aculeatus uses its beak, massive forearms and spatulate claws to break into ant and termite mounds or rotting logs in search of insect prey. Source: Courtesy of Anne Musser.
Figure 4. The eastern long‐beaked echidna, Zaglossus bartoni. Its long beak has a marked downward curve, and its spines are smaller and less conspicuous than in T. aculeatus. Z. bartoni differs from the other large New Guinea species, Z. bruijnii, in having five (rather than three) claws on its forefeet and hindfeet. Source: Courtesy of Anne Musser.
Figure 5. The reconstructed skull and lower jaw in side view of the Miocene platypus Obdurodon dicksoni, illustrating the permanent dentition of archaic platypuses. The living Platypus, Ornithorhynchus anatinus, has lost the permanent dentition in adults, although juvenile platypuses have vestigial teeth. The teeth of Obdurodon have enabled comparisons with the teeth of Mesozoic mammals, a study that will help place monotremes in a phylogeny of early mammals. Source: Courtesy of Anne Musser.


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Further Reading

Augee ML (ed.) (1992) Platypus and Echidnas. The Royal Zoological Society of New South Wales: Sydney.

Burrell H (1927) The Platypus: Its Discovery, Zoological Position, form and Characteristics, Habits, Life History, etc, 227 pp. Angus and Robertson: Sydney.

Flannery TF (1995) Mammals of New Guinea, 568 pp. Australian Museum/Reed Books: Sydney.

Moyal A (2001) Platypus: The Extraordinary Story of How a Curious Creature Baffled the World, 226 pp. Allen & Unwin: Sydney.

Musser AM and Temple‐Smith P (2009) Trouble in Paradise: challenges facing monotremes in an era of climate change. Australian Institute of Biology Newsletter.

Nicol SC (ed.) (2003) Monotreme biology, contributions from a satellite symposium of the ICCPB, Tasmania, Australia 2003. Comparative Biochemistry and Physiology Part A 136: 795–963.

Parer‐Cook E and Parer D (2003) Platypus – World's Strangest Animal, 72 pp. Zaurora Books.

Pian R, Archer M and Hand SJ (2013) A new, giant platypus, Obdurodon tharalkooschild, sp. nov. (Monotremata, Ornithorhynchidae), from the Riversleigh World Heritage Area, Australia. Journal of Vertebrate Paleontology 33 (6): 1255–1259.

Rismiller P (1999) The Echidna: Australia's Enigma, 120 pp. Hugh Lauter Levin Associates.

Rose RW (ed.) (1998) Special issue: proceedings of a platypus symposium. Australian Mammalogy 20 (2): 147–314.

Walton DW and Richardson BJ (1989) Fauna of Australia 1B, pp 401–1227. AGPS Canberra.

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Musser, Anne M(Dec 2020) Monotremata. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029231]