Biodiversity

Monday, May 01, 2006

CONTINENTAL DRIFT, MARSUPIALS AND PLACENTAL MAMMALS

Continental drift can be explained through the theory of plate tectonics. If one would cut the earth in half the interior can be roughly divided into the core, the mantle and the crust. But to understand plate tectonics, one has to understand the relationship of the outermost part of the mantle and crust and how they interact. The outermost part of the earth’s interior is made up of two layers. The lithosphere is made up of the crust and the upper solidified part of the mantle. This layer floats on the super-hot and liquid part of the mantle called the asthenosphere. The lithosphere is not a solid connected mass, but it is broken up into tectonic plates that can move relative to one another under the influence of convection currents within the asthenosphere. The movement of these floating tectonic plates explain the phenomenon of seafloor spreading and continental drift.(1)

It is about 180 million years ago that Pangea (2)(a super-continent that included all big land masses at that time) split to form the northern super-continent Laurasia (3)(today’s Europe, Asia and North America) and the southern super-continent Gondwana (today’s Africa, Madagascar, Antarctica, South America, India, Australia, New Zealand, New Guinea). (4) By that time in the animal kingdom, the mammals had arrived, although still overshadowed by the ruling dinosaurs.

We do not know for sure, if placental mammals evolved from marsupial mammals. It is more likely that they evolved side by side, but in geographic isolation. This brings us back to Gondwana. The super-continent began to break up in the late Jurrasic, with Africa first separating and drifting north, followed by India about 125 million years ago (which eventually collided with Asia and in the process formed the Himalayas) and then New Zealand about 80 million years ago. South America, Antarctica and Australia stayed connected, although only the tip of South America was connected to Antarctica that linked it to Australia, for another almost 50 million years.(4)

Coming back to the animal kingdom, it is only after the dinosaurs were wiped out less than 80 million years ago, that the mammals diverged to fill the empty ecological niches, left by the now extinct dinosaurs. From the fossil record, it seems that the placental mammals evolved in the northern hemisphere and the marsupials probably first evolved in the southern hemisphere, most likely in South America. Marsupials could migrate via the land bridge that connected South America to Antarctica and onwards to Australia, and vice versa. But then the three continents separated. First Australia separated and drifted north (45 million years ago) (4), then about 30 million years ago Antarctica and South America separated with Antarctica drifting south over the south pole and South America drifting north to eventually connect to North America (about 15 million years ago).(4)

The fact that Australia has been isolated since, explains the huge diversity of marsupials concentrated there. Without competition of the placental mammals, marsupials diverged to fill all the ecological niches offered by their environment. South America has the second largest diversity of marsupials, mostly opossums. Once South and North America became connected the placental mammals from the north started migrating south and they soon out-competed most marsupials. One very resilient marsupial however made it into North America and still exists there today, the Virginia opossum.

With the fossil records we have today, the correlation in plant species across different continents, geological similarities between continents, the study of tectonic plates and how continents have drifted over time and still do today; scientists have drawn a fairly complete picture that manages to explain the distribution of placental and marsupial mammals today. And so the mystery has been solved.


References:

Wikipedia contributors. Plate tectonics [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 1, 00:49 UTC [cited 2006 May 1]. Available from: http://en.wikipedia.org/w/index.php?title=Plate_tectonics&oldid=50972274.
2. Wikipedia contributors. Pangaea [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 1, 02:05 UTC [cited 2006 May 1]. Available from: http://en.wikipedia.org/w/index.php?title=Pangaea&oldid=50981883.
3. Wikipedia contributors. Laurasia [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 19, 12:44 UTC [cited 2006 May 1]. Available from: http://en.wikipedia.org/w/index.php?title=Laurasia&oldid=44497306.
4. Wikipedia contributors. Gondwana [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 26, 11:03 UTC [cited 2006 May 1]. Available from: http://en.wikipedia.org/w/index.php?title=Gondwana&oldid=50238306.


Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za

Web http://brit-journal.com/karen2006bcbnisl/