The available fossil evidence does not support the once common belief that marsupials were a primitive forerunner to the placental mammals, but indicates that the two main branches of the mammalian tree (the marsupials and the placental) originated at around the same time, at the end of the Mesozoic era. This is believed to have occurred somewhere in the northern continents, probably in Asia, from whence they proceeded into North America and then into South America and eventually into Australia. Fossil remains have been found throughout these regions, however, marsupials later became completely extinct within the Northern continents, probably as a consequence of competition presented by the diversification of placentals in these areas. Indeed, where in competition, placentals have mostly dominated throughout the globe and it's history. For example, in Australia where placentals were absent, throughout much of the Tertiary, Marsupials dominate. Indeed, the morphology of many of the marsupials is incrediably similar to mammals in the same ecological environment for example, the Tazmanian tiger (right) has a remarkable similar morphology to the wolf. Those placentals that are now present in this continent, are relatively recent invaders from other continents.
Two Tazmanian Tigers. Last known individual of the species died in captivity in 1936. The animal is largly thought to be extinct, however, some people still believe that it my still survive in the Tazmanian forests. However, no conclusive evidence is avaliable.Images used, with permission, from Wikipedia
The Riversleigh Fossil Site
Prior to 1983 a total number of 70 fossilised mammals remains were known form the whole of the Australian continent. Obviously, for a continent of this size the low number of finds was incredibly frustrating to scientists. However, since the discovery of the Riversleigh fossil site, literally thousands of ancient Australian inhabitants have been found. Most surprisingly the vast majority of these creatures were found in the space of one hour! Consequently, the Riversleigh fossil site is of great palaeontological importance, as over half of what is known about the evolution of Australian mammals, over the last 30 million years, has been determined from fossils at this locality.
The Naracoorte Caves
During the Pleistocene Epoch (1.8 million years ago to 10 000 years ago, a series of limestone caves existed where in some cases the roof had collapsed forming dangerous obstacles for the Australian fauna of the time. Consequently many animals did fall into these pits and died leaving behind fossil remains. The caves at Naracoorte, Australia are an example. Many vertebrate animals have been excavated including extinct marsupial species, allowing palaeontologists to have a glimpse at the diverse fauna that existed at the time.
Many marsupias species, especially large ones, died out approximately 50 000 years ago. This time period coincided with the first arrival of humans on the Australian continent and the onset of climate change. It is unclear whether these creatures died out through alteration of their habitat, through these early hunters buring their natural habitat, or what extent climate change played.
When North and South America docked and formed their present geographical arrangement, great competition occured between the species on either continent. In the southern continent marsupial species had been evolving in isolation from placental mammals for many millions of years. However, North America supported a great diversity of placental creatures and consequently when the two faunas meet, as plate tectonics pushed the two continents together, great competition occurred between the two groups. In general, the marsupials lost out to their placental equivalents, although climate cahnge is believed to have also led to thier demise. However, many species of opossums are still successful in South America and one or two have now established themselves in North America too.
Additionally, many species, especially smaller ones, became extent with the arrival of Europeans. These people introduced foxes, rabbits and domestic cats, amoung others, onto the continent, all of which competed against the unique native fauna. This extinction process is contining to occur.
Author: Catrin Roberts
Last updated: 23.11.83
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