monotreme

/mon"euh treem'/, n.

any animal of the Monotremata, the most primitive order of mammals, characterized by certain birdlike and reptilian features, as hatching young from eggs, and having a single opening for the digestive, urinary, and genital organs, comprising only the duckbill and the echidnas of Australia and New Guinea.

[1825-35; < F monotrème < NL monotrema, assumed sing. of Monotremata, neut. pl. of MONOTREMATUS MONOTREMATOUS]

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Any of three living species of egg-laying mammals (order Monotremata): the platypus and two species of echidna.

Monotremes are found only in Australia, Tasmania, and New Guinea. Except for their egg laying, they have mammalian characteristics, such as mammary glands, hair, and a complete diaphragm. They lack teats; the young suck milk through pores on the mother's skin. The earliest fossil monotremes, found in Australia, are only about 2 million years old and differ little from present species. Monotremes probably originated from a line of mammal-like reptiles different from the line that gave rise to placental mammals and marsupials.

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▪ mammal

Introduction

 any member of the egg-laying (egg) mammalian order Monotremata, which includes the amphibious platypus (family Ornithorhynchidae) and the terrestrial echidnas (echidna) (family Tachyglossidae) of Australia, Tasmania, and New Guinea.

Natural history

      Monotremata is the most ancient living order of mammals (mammal). In addition to being egg layers (oviparous), members of this order share primitive skeletal features such as the shoulder girdle and skull characteristics that have been lost in other living mammals. Relationships to other mammalian groups have been difficult to determine because of the puzzling combination of primitive features and specialized characteristics, a phenomenon known as mosaic evolution. Certain features of the skull appear to link monotremes to the extinct early mammal groups. Other evidence, particularly genetic data, places Monotremata close to more-advanced mammals, namely marsupials (marsupial).

      Monotremes are very highly modified for their particular ecological niches. The platypus is a semiaquatic carnivore-insectivore, and the echidnas have a specialized diet of small invertebrates such as ants, termites, and worms. The name Monotremata means “one-holed,” in reference to the fact that both sexes have only one opening at the rear of the body, which is used for both reproduction and excretion.

Paleontology and classification

      Various mammals of the Mesozoic Era (248 to 65 million years ago) with more-advanced shoulder girdles (including a multituberculate) have recently been discovered, indicating that the monotreme shoulder girdle developed before the shoulder girdles of some other mammalian groups. None of the theories of monotreme relationships has been universally accepted, although the subject is of great interest to scientists who study mammalian evolution.

      Included within the monotreme order are two families known only from early in the Cretaceous Period (144 to 65 million years ago) in Australia: the platypus-like Steropodontidae, represented by a single species (Steropodon galmani), and the uniquely specialized Kollikodontidae, which is also represented by a single species (Kollikodon ritchiei). Both are known only from opalized jaw fragments. The strange rounded cusps on the molar teeth of K. ritchiei were a surprise to paleontologists, suggesting that Cretaceous monotremes may have been more diverse and widespread than previously thought and may in fact have been the dominant mammals in the Australian sector of the ancient supercontinent of Gondwanaland (Gondwana). The Cretaceous discoveries bring the number of families within the order to four.

      The oldest fossil platypus is recorded from early in the Tertiary Period (65 to 1.8 million years ago) of South America, providing evidence that during this time platypuses were distributed from Australia through Antarctica to South America.

      Although the fossil record extends back to the early Cretaceous, origins of the group undoubtedly lie much farther back in time. Anatomic evidence such as the shoulder girdle suggests an origin perhaps as long ago as the middle of the Jurassic Period (206 to 144 million years ago).

      Many researchers agree that the primitive features in monotremes reflect an ancient origin. Any attempt to determine the relationships of monotremes must take into account these primitive retentions. This is problematic in that either these archaic features reflect an origin far removed from that of living placental and marsupial mammals or else a mechanism must be found to explain how these features could be possessed by a group that is closely related to mammals without these traits.

Order Monotremata

 4 species in 2 families.

      Family Tachyglossidae (echidnas (echidna))

 3 species in 2 genera from Australia, Tasmania, and New Guinea.

      Family Ornithorhynchidae ( platypus)

 1 Australian species.

Anne Marie Musser

Additional Reading

Michael L. Augee (ed.), Platypus and Echidnas (1992), is a compilation of scientific papers presented at a 1991 symposium on monotremes. Mervyn Griffiths, The Biology of the Monotremes (1978), is a classic text by one of the first monotreme biologists.

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