Past Eras of Time
The Paleozoic Era (543 to 251 million years ago)
This era, which lasted some 300 million years, is the first and longest part of the Phanerozoic Eon. Throughout the course of the Paleozoic, the climate was generally humid and temperate, though ice ages did take place from time to time.
|The Paleozoic Era is studied under six distinct periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian:|
The Cambrian Period (543-490 million years ago)
The Ordovician Period (490 to 443 million years ago)
The Silurian Period (443 to 417 million years ago)
The Devonian Period (417 to 354 million years ago)
The Carboniferous Period (354 to 290 million years ago)Also known as the Coal Age,
Land rising and falling, resulting from collisions between continents, and rises and falls in sea levels linked to the polar ice caps were significant events that shaped the world during this period. Many fossils of marine and terrestrial life forms date back to the Carboniferous Period
The Permian Period (290 to 248 million years ago)
At the end of the Permian Period, another mass disappearance took place that represented the final end of the Paleozoic Era. The fossil record shows that during this huge disappearance, 90%-95% of living species became extinct. Nonetheless, some Permian life forms have survived right down to the present day. Fossil specimens from the Permian such as dragonflies and spiders prove that evolution never took place at any time in the past.
The Mesozoic Era (248 to 65 million years ago)
The Mesozoic Era is divided into three separate periods: the Triassic, Jurassic and Cretaceous. It was during this era that dinosaurs lived and became extinct.
The Triassic Period (248 to 206 million years ago)
The Mesozoic Era began with the Triassic Period.
The Jurassic Period (206 to 144 million years ago)
This part of the Mesozoic saw large numbers and varieties of dinosaurs. At the end of the Jurassic, some ammonites, sea sponges, oyster and mussel species had become extinct.
. One of the earliest known fossil crocodiles, for instance, is around 200 million years old. There are also examples of fossilized Tuatara lizards that are more than 200 million years old. The many fossil shrimp dating back to the Jurassic Period all possessed exactly the same perfect systems and complex structures as they do today.
The Cretaceous Period (146 to 65 million years ago)
This, final stage of the Mesozoic, is known as the age in which the dinosaurs became extinct, as did a large number of terrestrial reptiles and plant species.
The Cenozoic Era (65 million years ago to the present day)
The Cenozoic Era, in which we are still living, began with the end of the Cretaceous Period. Until recently, geologists and paleontologists divided the Cenozoic into two separate periods of unequal length: the Tertiary and the Quaternary. The Tertiarycomprised a time frame from 65 million to 1.8 million years ago, and the Quaternary encapsulated the last 1.8 million years. Recently, however, the Cenozoic Era has been divided into three separate periods. Under this new system, its three components are the Paleogene, the Neogene and the Quaternary.
Fossils are widely dispersed just about everywhere on Earth. Almost no fossils are encountered in some types of rock, but large numbers are found in others. Geologists have divided rock types into three main groups:
The igneous category includes granite or basalt-type rocks formed by the cooling of magma present in the depths of the Earth, or else emitted by volcanoes in the form of molten lava. Sedimentary rocks form when sand, silt, mud, and other small particles or substances carried in water are deposited on top of one another. Metamorphic rocks are igneous or sedimentary ones that have undergone structural changes due to high temperature and pressure deep in the Earth.
Few fossils are generally encountered in igneous seams. The rare examples discovered are fossils that have resulted when a plant or animal gets trapped inside molten lava. Very few fossils can survive the high temperatures and pressures that transform sedimentary strata into metamorphic rock. Almost all fossils are found in sedimentary seams or deposits.
Nearly all sedimentary rocks are formed by substances carried by wind or water or else from the erosion of still other rocks. Some forms, such as coal, are made of plant or animal remains. Clastic is the name given to sedimentary rock formed by minute particles or grains. Sandstone and schist are examples of such rocks. If there has been dissolution in the substances transported, then due either to chemical solution or vaporization, "organic" sedimentary beds form. Examples of such rocks are limestone and dolomite. In general, sedimentary rock seams are a mixture of clastic and organic seams. Fossils are usually seen in shales, schists, sandstone and limestone formed from calcium carbonate.