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| Monotreme Mammals |
Monotremes (Division Monotremata) represent the most primitive mammalian group alive today. Like all mammals monotremes have a single bone making up their lower jaw and three middle ear bones; however, they also possess many primitive traits. Monotremes are egg-laying mammals. They have mammary glands without nipples and exhibit poor thermoregulation. Monotremes have a more primitive shoulder girdle and walk with limbs splade outward. Monotremes have a cloaca, a common opening for both the urethra and reproductive tract. The cloaca represents a primitive amniote urogenital tract. The female monotreme has two uteri that fuse into a short vagina. Monotreme embryos form in a pea-sized, soft-shelled egg that is retained and nourished inside the female. The eggs possess a sticky surface and are carried in a slit on the female’s belly. Monotreme females do not suckle their young, instead the hatchlings lap up milk that is extruded from mammary glands on the chest. Monotremes first appear in the Cretaceous. Paleontologists think that monotremes arose sometime in the Late Jurassic or Early Cretaceous in the southern continents. Today monotremes are restricted to Australia (Duckbilled platypus) and New Guinea (Echidna). Monotreme Fossils & Classification Extant (living) mammals are traditionally divided into two subclasses based upon reproductive strategies. The subclass Prototheria includes the egg-laying mammals, while the subclass Theria includes marsupials and placentals, which bear young live. The subclass Prototheria unites monotremes with many ancient Mesozoic mammal groups, but is now no longer in use. Monotremes were thought to be related to basal mammals with a linear arrangement of cusps such as morgonocodontids, triconodonts, and multituberculates. Determining relationships among mammals requires teeth and monotremes do not have teeth as adults. Finally, in 1985 a fossil monotreme was found in which teeth were retained into adulthood. The Cretaceous aged Steropodon was found in the famous opal mine of Lightning Ridge, New South Wales (Kemp, 2005, p. 176). Steropodon was the first Mesozoic mammal fossil found in Australia and is an ancestor to the platypus. However, unlike the living platypus the adult Steropodon had cheek teeth that exhibit a primitive triangular cusp arrangement (tribosphenic) similar to young monotremes and an extinct southern hemisphere mammal family Ausktribosphenidae. Monotremes are now grouped with these extinct organisms into the superdivision Australosphenida (Benton, 2005, p. 399). Tribosphenic molars help to define the subclass Theria (marsupials and placental mammals). Debate continues over whether this triangular cusp pattern evolved independently through convergent evolution or if it indicates that monotremes evolved from primitive therian mammals. New fossil finds will hopefully shed more light upon the relationship between monotremes, marsupials, and placental mammals. Currently, the fossil record of monotremes is poor. Obdurodon is another fossil relative of the platypus from the Miocene of Australia. Monotrematum, another fossil relative of the platypus, is from the Paleocene of Patagonia and is the first monotreme to be found outside Australia. |
Bibliography |
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Benton, M.J. (2005). Vertebrate Palaeontology [3rd edition]. Main: Blackwell Publishing. Kemp, T.S. (2005). The Origin and Evolution of Mammals. New York: Oxford University Press. |