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chapter 28.docx

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BIOL 4503

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Life expanded rapidly during the Cambrian periodCambrian period 452488 mya marks the beginning of the Palaeozoic era The O concentration was approaching its current level the continents had come together to 2form several large land massesThe largest GondwanaA rapid diversification of life took placeCambrian explosionMost of the major groups of animals that have species living today appeared during this periodTHE ORDOVICIAN 488444 MYA o The continents located primarily in the S Hemisphere still lacked multicellular plants o Evolutionary radiation of marine organisms during early stages o At the end massive glaciers formed over Gondawa sea levels were lowered about 50 meters and ocean temperatures dropped o About 75 percent of the animal species became extinct probably because of these major environmental changesSILURIAN 444416 MYA o Northernmost continents coalesced but the general positions did not change much o Marine life rebounded o Animals able to swim and feed above the ocean bottom appeared for the first time o No new major groups of marine life evolved o The tropical sea was uninterrupted by land barriers o Most marine organisms were widely distributed o First vascular plants appeared late in Silurian period less than 50cm tall and lacked roots and leaves o First terrestrial arthropods appeared at about the same timeDEVONIAN 416359 MYA o Rates of evolutionary change accelerated o The northern land mass Laurasia and the southern land mass Gondwana moved slowly toward each othero Great evolutionary radiations of coral and shelled squid like cephalopods o Fishes diversified as jawed forms replaced jawless ones and less rigid outer covering of modern fishes evolved o All current major groups of fishes were present by the end of the period o Terrestrial communities also changed dramatically club mosses horsetails and tree ferns became common o Their deep roots accelerated the weathering of rocks resulting in the development of the first forest soils o Ancestors of gymnosperms first plants to produce seeds appeared later in the era o Extinction of about 75 percent of all marine species marked the end of this eraTHE CARBONIFEROUS 359297 MYA o Large glaciers formed over highlatitude Gondwana but extensive swamp forests grew on the tropical continents o Dominated by giant tree ferns and horsetails with small leaves o Fossilized remains of those trees formed the coal o The diversity of terrestrial animals increased greatly o Insects evolved wings first animals to fly and gained access to tall plants
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