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Biology 4218A

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, Gondwana.  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 reached their greatest diversity, forming “meadows” on the sea floor.  THE PERMIAN (297-251 MYA) o Continents coalesced into the supercontinent Pangaea. o Near the end, reptiles greatly outnumbered the amphibians. o Late in the period, the lineage leading to mammals diverged from one reptilian group. o In fresh waters, the Permian period was a time of extensive diversification of ray-finned fishes. o Massive volcanic eruptions resulted in outpourings of lava that covered large areas of Earth; ashes produced blocked the sunlight and cooled the climate  forming largest glaciers in history. o Atmospheric temperature dropped; about half of the Permian land would have been uninhabitable. o Resulted in the most drastic mass extinction even in Earth’s history. Geographic differentiation increased during the Mesozoic era  Few organisms that survived found themselves in a relatively empty world at the start of the Mesozoic era (251 mya).  Pangaea slowly separated into individual continents; oceans rose and reflooded continent shelves, forming huge, shallow inland seas.  Atmospheric oxygen concentrations gradually rose to their former levels.  Different groups of organisms came to dominate the Earth: o Three groups of phytoplankton---dinoflagellates, cocccolithophores, and diatoms— became ecologically important at this time.  Earth’s biota became increasingly provincialized –distinct terrestrial biotas evolved on each continent.  Biotas of the shallow waters bordering the continents also diverged from one another.  By the end of the era, the continents were close to their present positions and many organisms looked similar to those living today. 
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