GEOLOGIC TIME SCALEPRECAMBRIAN (PROTEROZOIC, HADEAN, ARCHEAN)-Notice that the detail of thegeologic time scale does not beginuntil about 540 mya, the date for thebeginning of the Cambrian Period. The Precambrian is defined as thetime from when the Earth formed(about 4.5 billion years ago, theearliest-known rocks) until thebeginning of the Cambrian period(540 million years ago). ThePrecambrian used to be defined asthe time from when the Earth'srocks formed until the earliest lifeforms evolved, but as the date of theearliest fossils gets earlier andearlier, this definition has also changed.PHANEROZOIC EON1. PALEOZOIC ERAThe Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped outapproximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy.The Paleozoic is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous(in the U.S., this is divided into the Mississippian and Pennsylvanian Periods), and Permian. Most of these names derive from locations where rocks of these ages were first studied. The Paleozoic Era is bracketed by the times of global super-continents. The era opened with the breakup of the world-continent Pannotia and closed with the formation of Pangea, as the Earth's continents came togetheronce again.A. CAMBRIAN PERIOD (540-500 mya)AGE OF TRILOBITES Cambria was the Latin name for Wales…“Age of Trilobites" -The Cambrian Explosion of life occurs; all existent phyla develop. Many marine
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- Summer '17
- Extinction event, Gondwana, Carboniferous