What is the difference between marsupials and placental mammals?

Answer 1

Most people think the difference between placentals and marsupials are simply the placenta and the pouch (marsupium), however all marsupials have a placenta (in the inter-uterine stage) and not all of them have a pouch! Early embryonic development of the foetus of the two types of mammals are similar and embryologists have investigated what changes happen during this early stage to discover what triggered the differences between the placentals and marsupials.

It appears to have began with the bladder!

Fossil and genetic evidence has been extrapolated together indicating that placentals and marsupials mammals diverged from a common ancestor around 130 million years ago in current day Asia (due to tectonic plate activity the world looked very different!). But what caused the divergence? One embryonic difference appears to have been caused by the ureters (ducts from the kidneys) and how they migrate to the bladder. In marsupials they migrate from the back (dorsal) of the urogenital sinus to the front (ventral) towards the bladder, over the top between the reproductive tract (oviducts), where as placental ureters migrated underneath around the outside.

This migration of ureters had a profound implications, causing their differences in reproduction. Placental females are able to fuse their reproductive tracts together having one uterus, one vagina and one cervix where as marsupials have the ureters "in the way" and are unable to fuse like the placentals and have two uteri, two vaginae two cervices. In Placental males, when their testes "dropped" to below the penis, the ureters got in the way and now the vas deferens ducts that delivers the sperm to the penis from the testes has to travel way back up into the body over the top of the ureters and back down to the penis. This inefficient system does not happen with marsupials as the ureters went between the reproductive tract, and the testes remained above the penis!

This split womb prompts difficulties to have a large fetus for marsupials who adapted to form a separate birth canal that can only fit a small embryonic neonate (newborn). This birth canal usually closes up after birth except for macropods (e.g. kangaroos, wallabies, quokkas). Macropods are also the only marsupials also to not have a bifurcated (split) penis however all the other marsupials do (except for the Honey possum who hardly has a gland at all!). The split penis is believed to be able to utilise both vaginae during mating. A hypotheses is indeed inplace theorising that macropods potentially may be able to deliver sperm through the permanently open birth canal thus having two lateral and one medial vaginae though this has not been proven!

Placental mammals are able to develop a more complex, longer lasting placenta that transfers material between the mother and young. Marsupials do have a less developed yolk like placenta in early uterine stage, some like the bandicoots have a more complex developed one and are born still with umbilical chords (in only 12 days!).

Marsupials develop the penis behind the testes, placentals in front. Not all female marsupials have pouches like the numbat and a couple males have (had) pouches i.e. yapok(water opossum) and thylacine. Since these two mammal groups evolved from their common ancestor, many other differences have developed including dentition (four molars in marsupials, three in placentals), jaw and bone structure, like the marsupial epipubic bone (though not found in "marsupial lion" or marsupial mole). Marsupials do not have a corpus collosum and have a lower basal body temperature. Marsupial males don't have nipples.

Marsupial fossils have been found in all continents, with the oldest "metatherian" ancestor in the country of China, NE of Beijing, in the same fossil bed as the oldest "eutherian" ancestor of Placentals . Even Europe's oldest mammal fossil is a marsupial recently found in in SW France. Marsupials are now living naturally in Indonesia, Papua New Guinea, throughout Americas and Australia. Marsupial dispersal to Australia from Asia began over 100 million years ago believed to be via North America, South America, Antarctica and then maybe only one shrew like marsupial onto Australia around 55-60 million years ago. As we drifted away from Antarctica splitting in isolation we spawned a host of different species of varying sizes and shapes to cover the huge new range of ecological niches that came available as Australia's plate drifted north and its changing cooling drying climate. South America did the same and they have today over 70 closely related species except for one very interesting and enigmatic marsupial known as the Monito del Monte or "little mountain monkey", who is more closely related to the Aussie marsupials. One bizarre example of similarity between the Australasian marsupials and the Monito del Monte is that the sperm swim individually where as the sperm of all American marsupials are paired up at the head when they swim. Genomics also shows it is much closer linked to the Aussies.

The placentals are believed to have completely replaced the original Northern hemisphere marsupials. When, however, South America rejoined with North America only five million years ago, this allowed one marsupial species to "sneak" into the northern hemisphere via the newly formed Ismuth of Panama (one strategy this marsupial used to survive is by fainting and thus appearing to be dead). The Algonquin Indians gave it the name possum (hence the phrase "playing possum"). The Algonquin people often pronounced by the word with an "oh" first - like the scots say "ah" laddie, so it was recorded as opossum. Its scientific genus name is Didelphis (greek meaning "two wombs") - one of the defining differences between placental and marsupial mammals.

Answer 2

Marsupials give birth to relatively undeveloped young that continue to develop in a pouch, while placental mammals have young that fully develop within the mother's uterus connected by a placenta. Placental mammals have longer gestation periods and typically have more complex reproductive systems compared to marsupials.

Answer 3

marsupials have a pouch (marsupium )outside their body to keep their young ones while as placentals have placenta inside their body for development of offsprings.