Monotremata

Abstract

Abstract 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.