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M82A1
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- #21
Gondwana:
Near the end of the Precambrian, roughly 700 million years ago, all landmasses assembled into a supercontinent. The continental collisions resulted in environmental changes that had a profound influence on the evolution of life. No broad oceans or extreme differences in temperatures existed to prevent species from migrating to various parts of the world. Between 630 and 560 million years ago, the supercontinent rifted apart and four or five continents rapidly drifted away. Evidence for the breakup exists in a long belt of volcanism near the present Appalachians.
Most of the continents were near the equator, which explains the existence of warm Cambrian seas. The continental breakup caused sea levels to rise and flood large portions of the land at the beginning of the Cambrian. The extended shoreline might have spurred the explosion of new species, with twice as many phyla living during the Cambrian than before or since.
Many organisms were in existence, none of which have any modern counterparts. One example is helicoplacus, whose body parts were configured in a manner not found in any living organism. It was about two inches long and shaped like a spindle covered with a spiraling system of armored plates. It emerged during the transition from the Precambrian to the Cambrian, when more types of body plans arose than at any other time. Helicoplacus, like most species of the early Cambrian, was unsuccessful in the long run and became extinct about 510 million years ago, just 20 million years after it first appeared.
During the Cambrian, continental motions assembled the present continents of South America, Africa, India, Australia, and Antarctica into Gondwana, named for an ancient region of east-central India. Evidence for Gondwana exists in geologic provinces with similar rock types from the late Precambrian to the early Cambrian; these show matches between Brazil and west Africa; eastern South America, South Africa, west Antarctica, and east Australia; and east Africa, India, east Antarctica, and west Australia. A great mountain-building episode deformed areas between all pre-Gondwana continents, indicating their collision during this interval.
Much of Gondwana was in the south polar region from the Cambrian to the Silurian. The present continent of Australia was at the northern edge of Gondwana and located on the Antarctic Circle. A later collision between North America and Gondwana near the end of the Cambrian about 500 million years ago created an ancestral Appalachian range that continued into western South America long before the Andes formed. North America then broke away from Gondwana and linked with Greenland and Eurasia to form Laurasia about 400 million years ago. Eurasia, the largest modern continent, assembled with about a dozen individual continental plates that welded together at the end of the Proterozoic.
A preponderance of evidence for the existence of Gondwana includes fossilized finds of a mammal-like reptile called lystrosaurus in the Transantarctic Range of Antarctica, which indicates a link with southern Africa and India, the only other known sources of lystrosaurus fossils. A fossil of a South American marsupial in Antarctica, which acted as a land bridge between the southern tip of South America and Australia, lends additional support to the existence of Gondwana.
Further evidence for Gondwana includes fossils of a reptile called mesosaurus in eastern South America and South Africa. Fossils of the late Paleozoic fern glossopteris, from the Greek word meaning featherlike and whose leaf impressions actually look like feathers, exist in coal beds on the southern continents of India. However, the plant is suspiciously absent on the northern continents, suggesting the existence of two large continents, one located in the Southern Hemisphere and another in the Northern Hemisphere, separated by a large open sea.
Near the end of the Precambrian, roughly 700 million years ago, all landmasses assembled into a supercontinent. The continental collisions resulted in environmental changes that had a profound influence on the evolution of life. No broad oceans or extreme differences in temperatures existed to prevent species from migrating to various parts of the world. Between 630 and 560 million years ago, the supercontinent rifted apart and four or five continents rapidly drifted away. Evidence for the breakup exists in a long belt of volcanism near the present Appalachians.
Most of the continents were near the equator, which explains the existence of warm Cambrian seas. The continental breakup caused sea levels to rise and flood large portions of the land at the beginning of the Cambrian. The extended shoreline might have spurred the explosion of new species, with twice as many phyla living during the Cambrian than before or since.
Many organisms were in existence, none of which have any modern counterparts. One example is helicoplacus, whose body parts were configured in a manner not found in any living organism. It was about two inches long and shaped like a spindle covered with a spiraling system of armored plates. It emerged during the transition from the Precambrian to the Cambrian, when more types of body plans arose than at any other time. Helicoplacus, like most species of the early Cambrian, was unsuccessful in the long run and became extinct about 510 million years ago, just 20 million years after it first appeared.
During the Cambrian, continental motions assembled the present continents of South America, Africa, India, Australia, and Antarctica into Gondwana, named for an ancient region of east-central India. Evidence for Gondwana exists in geologic provinces with similar rock types from the late Precambrian to the early Cambrian; these show matches between Brazil and west Africa; eastern South America, South Africa, west Antarctica, and east Australia; and east Africa, India, east Antarctica, and west Australia. A great mountain-building episode deformed areas between all pre-Gondwana continents, indicating their collision during this interval.
Much of Gondwana was in the south polar region from the Cambrian to the Silurian. The present continent of Australia was at the northern edge of Gondwana and located on the Antarctic Circle. A later collision between North America and Gondwana near the end of the Cambrian about 500 million years ago created an ancestral Appalachian range that continued into western South America long before the Andes formed. North America then broke away from Gondwana and linked with Greenland and Eurasia to form Laurasia about 400 million years ago. Eurasia, the largest modern continent, assembled with about a dozen individual continental plates that welded together at the end of the Proterozoic.
A preponderance of evidence for the existence of Gondwana includes fossilized finds of a mammal-like reptile called lystrosaurus in the Transantarctic Range of Antarctica, which indicates a link with southern Africa and India, the only other known sources of lystrosaurus fossils. A fossil of a South American marsupial in Antarctica, which acted as a land bridge between the southern tip of South America and Australia, lends additional support to the existence of Gondwana.
Further evidence for Gondwana includes fossils of a reptile called mesosaurus in eastern South America and South Africa. Fossils of the late Paleozoic fern glossopteris, from the Greek word meaning featherlike and whose leaf impressions actually look like feathers, exist in coal beds on the southern continents of India. However, the plant is suspiciously absent on the northern continents, suggesting the existence of two large continents, one located in the Southern Hemisphere and another in the Northern Hemisphere, separated by a large open sea.