Student Notebook/Journal Page I learned that plate movements have affected how animals have evolved. I thought this was curious so I decided to do more research on the topic of evolution. I made some interesting discoveries. 1. Mammals maintain several modes of reproduction. The two dominant forms are placental and marsupial. Both forms evolved from an early mammal ancestor. Using genetic data, scientists Koalas are one example of a marsupial that lives in Australia. date the divergence of these forms to 90 million years ago. Marsupials derive their names from the Latin word marsupium, which means “pouch.” This is a perfect description because a marsupial youth reaches maturity in its parent’s pouch.
These animals maintain a shorter period of gestation. The underdeveloped young move to their parent’s pouch for the remainder of development, which may take weeks or months. Placental mammals gestate internally and give birth to well-developed young. 2. Marsupials were common on the supercontinent Pangaea. Today, most marsupials are found on the island continent of Australia. In fact, the opossum is the only naturally occurring marsupial in North America. (The North American opossum is not the same thing as the Australian possum, though people often confuse the two. In fact, the term “playing possum” refers to the opossum’s habit of pretending to be dead when threatened.) The lack of marsupials on most continents can be traced to the breakup of Pangaea, about 240 million years ago.
Opossums are the only naturally occurring marsupials in North America.
3. Pangaea broke into several large landmasses. One large section consisted of the modern continents of South America, Australia, Africa, India, and Antarctica. About 120 million years ago, this landmass broke into three pieces. One piece consisted of the modern continents of South America and Africa. One piece contained the modern continent of India. One piece consisted of the modern continents of Australia and Antarctica. At this time, marsupials lived on the landmasses consisting of Australia and Antarctica. Marsupials also lived on South America.
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Evolution of Marsupials
4. By five million years ago, the continents of North America and South America became joined at the isthmus (or “land bridge”) of Panama. The isthmus provided a pathway for placental mammals from North America to migrate south. The marsupials on South America could not compete with the placental mammals, and they became extinct. Antarctica rifted from Australia 84 million years ago. As Antarctica moved over the South Pole and gradually froze, the marsupials and other animals living on the continent became extinct as well. Australia remains the location where the majority of marsupials continue to thrive and evolve. Today, the marsupial diversity on Australia spans 140 species. Common marsupials include kangaroos, koalas, Tasmanian devils, and wombats. As Pangaea broke apart, marsupials became isolated on Australia.
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