What are Monotremes, Marsupials and Eutherian Mammals?
Platypus and echidna are classified as monotremes; kangaroos, koala and bandicoots among others are marsupials; over 4,000 other animal species, including ourselves, are eutherian mammals. Of the Monotremes and Marsupials many are found in the Australian region.
Scientific nomenclature (naming) of organisms has a varying number of taxonomic (classification) ranks but includes, in a hierarchy of names from the largest group to the individual species, the following: kingdom, phylum, class, order, Family, Genus and species.
Australia’s first settlers probably looked on in wonder at the sight of the local kangaroos standing on their hind legs, then bounding off in great two-legged leaps using a big tail for balance. The wonder probably included wondering how they tasted, the people being hungry after a long sea voyage from somewhere in Indonesia. Around 50,000 years ago, during an ice age when sea levels had dropped, distances of approximately 80kilometres between islands still needed to be crossed on the journey from Indonesia to Australia for the first settlers (Migration Heritage Centre 2010).
It has long been debated whether humans took the southern route through the Indonesian Lesser Sunda Islands, including Bali to Timor islands then to northwest Australia with the Timor Sea area being mostly dry because of an ice age.
Or the northern route through Sulawesi to New Guinea which was connected by land to Australia at Cape Yorke also because of an ice age. Kealy, Louys and O’Connor (2018) have found much archaeological research support for the northern route and suggest Misool Island, New Guinea as a landing place.
Much later, about 250 years ago, a new wave of settlers came to Australia and also looked on in wonder at the strange kangaroos. In 1770 Joseph Banks, while the HMS Endeavour vessel with Captain Cook was repaired at Cooktown in Queensland, wrote in his diary on the 22nd June that year “…. Myself employd all day in laying in Plants. The People who were sent to the other side of the water in order to shoot Pigeons saw an animal as large as a grey hound, of a mouse colour and very swift; they also saw many Indian houses and a brook of fresh water.” On the 25th June “In gathering plants today I myself had the good fortune to see the beast so much talkd of, tho but imperfectly; he was not only like a grey hound in size and running but had a long tail, as long as any grey hounds; what to liken him to I could not tell, nothing certainly that I have seen at all resembles him.” The 14th July “Our second lieutenant who was a shooting today had the good fortune to kill the animal that had so long been the subject of our speculations.” The 15th July “The Beast which was killd yesterday was today Dressd for our dinners and provd excellent meat.” [sic] (Sandford 1998).
A lot of the wonderful Australian fauna is classified as either Monotremes, Marsupials or Eutherian Mammals and form parts of the class of Mammalia, mammals.
They are amniotes and produce an amnion membrane for reproduction, this encloses the embryo in a fluid filled sack or an egg, similar to conditions in the sea or a pond. This is a major adaptation to dry terrestrial life.
Mammals have epidermal hair at some stage of their life cycle, their newly born young are suckled by milk secreted from mammary glands and they are endothermic, that is they can maintain their own high body temperature.
Mammals are thought to be evolved from mammal-like reptiles, the therapsids.
Mammalia has two living sub classes Prototheria and Theria.
Prototheria are oviparous — egg laying, contains one order, Monotremata, monotremes, the echidnas and the platypus.
Theria are viviparous — producing live young, contains the order Metatheria, marsupials eg bilbies and kangaroos, and the Eutheria, placental mammals, which include over 4,000 species from elephants to rodents to humans.
Monotremes include only three species, the duck billed platypus and two echidna species, one of which is found in New Guinea, and they are well adapted to their habitats.
Platypus have thick fur, a flattened muzzle and webbed feet making it suitable to its aquatic environment which is also free of predators.
Echidnas have strong spines for protection, have a long muzzle and long tongue for catching ants and well-developed claws for digging and are able to rapidly bury themselves. They are also able to go into a state of torpor if food isn’t available — a state of low body temperature and low metabolic rate, as in hibernation.
Females monotremes have two separate uteri and the male has a bifurcate penis. The eggs are large, without a hard shell and are incubated by the parent. Young feed on milky secretion but the mammary glands secrete from patches rather than nipples. The males have a venomous spur on the hind feet and as adults lose their teeth and use horny plates as substitutes.
Monotremes are the most ancient species of mammals and retain some basic features of amniotes. Some reptilian bones in the pectoral girdles (forelimbs); the female reproductive tract has some shell glands; and a cloaca (one opening for excretion).
Marsupials have a pouch and there are about 330 species, most of which occur in the Australian region and in the Americas. They include kangaroos, koalas, bandicoots, the bilby, the eastern quoll and opossums in America.
Marsupial females like the monotremes also have two uteri, left and right and the males have a bifurcate penis. Females have short gestation periods of 8–42 days, with the embryo nourished by a yolk sac placenta and the small, newly born young appears as if at the early fetal stage. The young are active and quickly find the nipple of the mammary gland within the pouch and remain attached until more developed.
This group of mammals spread across continents over 100 million years ago during the Cretaceous period, allowing time for many different adaptations and species to evolve. They can be diurnal or nocturnal, terrestrial or arboreal, can be herbivores, insectivores or carnivores. Their habitats range through decreasing rainfall zones, that is, tropical rainforests, tropical wet dry savanna grasslands and deserts. Marsupials have been very successful at adapting to different environments and in Australia are the dominant mammalian fauna.
Eutherian mammals are the dominant mammalian fauna on other continents and include many species we are familiar with, such as rabbits, hippopotamus and camels. In Australia they include seals, dugongs, rodents and dingoes. They evolved by the Late Cretaceous and Paleogene periods about 70 to 45 million years ago, from small insectivorous mammals.
Eutheria produce live young. Embryos are fed through attachment to a placenta and the young are feed through nipples on the mammary gland.
The eutheria are divided into various groups according to how they have evolved, for example the whale and hippopotamus are closely related as are the hedgehogs and bats.
Differing evolution can be seen in a species’ teeth, for example browsers have incisors (the teeth at the front upper jaw and on front lower jaw) to grip and cut off foliage and molars (large flat teeth at the back) to grind the leaves. Carnivores have canine teeth (teeth either side of the incisors, like Count Dracula’s vampire teeth) to tear flesh. Omnivores eat some of both, plants and prey, and have teeth that can rip and some teeth that can grind, humans are in this group.
Monotremes, Marsupials and Eutherian Mammals comprise the class of Mammalia, the mammals which together with the reptiles, birds, fish and amphibians are all part of the phylum Chordata, the vertebrates.
References:
Kealy, S., Louys, J., & O’Connor, S. (2018). Least-cost pathway models indicate northern human dispersal from Sunda to Sahul. Journal of human evolution, 125, 59–70.
Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.
Migration Heritage Centre (2010) http://www.migrationheritage.nsw.gov.au/objects-through-time/essays/50000-years-before-present/index.html
Sandford B.P. (1998) From the transcript of The Endeavour Journal of Joseph Banks, 25 Aug 1768–12 Jul 1771, 2 Vols downloaded from the web site of the Mitchell Library, State Library of New South Wales, Sydney, Australia. https://detoursdesmondes.typepad.com/files/banksvo2.pdf
