How Do Scientists Date Ancient Events.docx

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University of Calgary
Biomedical Engineering
BMEN 515
William Huddleston

How Do Scientists Date Ancient Events? Life expanded rapidly during the Cambrian period  Cambrian period (452-488 mya) marks the beginning of the Palaeozoic era.  The O2concentration was approaching its current level; the continents had come together to form several large land masses.  The largest, Gondwan.  A rapid diversification of life took place  Cambrian explosion.  Most of the major groups of animals that have species living today appeared during this period.  THE ORDOVICIAN (488-444 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 changes.  SILURIAN (444-416 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 time.  DEVONIAN (416-359 MYA) o Rates of evolutionary change accelerated. o The northern land mass (Laurasia) and the southern land mass (Gondwana) moved slowly toward each other. o 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 era.  THE CARBONIFEROUS (359-297 MYA) o Large glaciers formed over high-latitude 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. o Amphibians became larger and better adapted to terrestrial existence after splitting from the lineage leading to the amniotes, vertebrates with well-protected eggs that can be laid in dry places. o In seas, crinoids rea
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