and even some species, of plants were shared among these widely separated lands, suggesting a common origin.
Later in the 1800s, evidence of stratigraphic congruence of various plant and animal groups among the southern continents supported a hypothetical separation of northern and southern supercontinents. Wegener (1924) was the first to outline a hypothetical geologic history of drift for all the continents, concentrated during Cenozoic time. Wegener's continental drift hypothesis was criticized because this history appeared to be incompatible with nonmarine paleontology. However, a growing body of geologic and biological evidence, including strati-graphic congruence, rift valleys, uplift and subsidence zones, and distributions of both extinct and extant flora and fauna, eventually was unified into the theory of plate tectonics.
According to this theory, a single landmass (Pangaea) split about 200 million years ago and separated into northern (Laurasia) and southern (Gondwanaland) supercontinents that moved apart as a result of volcanic upwelling in the rift zone. About 135 million years ago India separated from Gondwanaland, moved northward, and eventually collided with Asia to form the Himalaya Mountains. Africa and South America separated about 65 million years ago, prior to the adaptive radiation of angiosperms and mammalian herbivores. South America eventually rejoined North America at the Isthmus of Panama, permitting the placental mammals that evolved in North America to invade and displace the marsupials (other than the generalized opossum) that had continued to dominate South America. Marsupials largely disappeared from the other continents as well, except for Australia, where they survived by virtue of continued isolation. South American flora and fauna moved northward through tropical Central America. This process of continental movement explains the similarity of fossil flora and fauna among the Gondwanaland-derived continents and differences among bio-geographic realms (e.g., Nothofagus forests in southern continents vs. Quercus forests in northern continents).
Continental movements result from the stresses placed on the Earth's crust by planetary motion. Fractures appear along lines of greatest stress and are the basis for volcanic and seismic activity, two powerful forces that lead to displacement of crustal masses. The mid-oceanic ridges and associated volcanism mark the original locations of the continents and preserve evidence of the direction and rate of continental movements. Rift valleys and fault lines usually provide depressions for development of aquatic ecosystems. Mountain ranges develop along lines of collision and subsidence between plates and create elevational gradients and boundaries to dispersal. Volcanic and seismic activity represents a continuing disturbance in many ecosystems.
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