hello grade 11 welcome in this video let's take a closer look at the geologic time scale and describe the relationships between events that have occurred throughout the earth's history our discussion will focus on the learning competency describe how earth's history can be interpreted from the geologic time scale previously you learned about the methods used by geologists in determining the age of rocks and fossils which led to the creation of the geologic time scale the geologic time scale is a timeline that illustrates earth's history based on geologic events and life forms this is a system that chronologically relates geological strata to time [Music] the table of geologic periods presented here agree with the nomenclature dates and standard color codes set forth by the international commission on stratigraphy there are four major divisions of the geologic time scale we have the eon the era the period and the epoch this geologic time units is based on stratigraphy which is the correlation and classification of rock strata the eon is the largest time span of the geologic time scale the geologic time scale is divided into two eons we have the pre-cambrian eon which refers to the earliest part of earth's history and the phanerozoic eon which is the current geologic eon in the geologic time scale the pre-cambrian eon is divided into three eons the heidi antion the archaean eon and the proto-rhizobic eon hadayan eon occurred 4.6 billion to 4 billion years ago during this time the solar system was forming within a cloud of gas and dust known as the solar nebula which eventually spawned planets moon and etc during this time the earth started cooling and the outer edge of the planet solidified from a molten lava to a solid crust water rained from the atmosphere and created oceans [Music] the arcadian eon started between 4 billion and 2.5 billion years ago the first form of life on our planet was created in the oceans exactly when or how it happened is unknown but microfossils of this time indicate that life began in the oceans about 3.5 billion to 2.8 billion by the end of the archaean the ocean floor was covered in a living mat of bacterial life proterozoic ion also called the age of hidden life started 2.5 billion years ago in this time enough shield rock had formed to start recognizable geologic processes such as plate tectonics proterozoic eon contained many definite traces of primitive life forms the fossil remains of bacteria and blue-green algae as well as the first oxygen-dependent animals the next eon is the phanerozoic eon the time in which abundant animal and plant life has existed it covers 541 million years up to the present the phanerozoic ion is subdivided into three eras we have the paleozoic era the mesozoic era and the cenozoic era the paleozoic era ran from about 542 million years ago to 251 million years ago paleozoic era began with the breakup of one super continent in the formation of another because of this plants became widespread [Music] this era is divided into six periods of time we have the cambrian period the ordovician period silurian period devonian period carboniferous period and permian period paleozoic era began with the cambrian period 53 million years ago this era is known for ushering in an explosion of life on earth this cambrian explosion included the evolution of anthropods the ancestors of today's insects and crustaceans and coordinates animals with rudimentary spinal cords next is the ordovician period which occurred 45 million years ago this period marked in the fossil record by an abundance of marine invertebrates perhaps the most famous of this invertebrates was the trilobite an armored anthropod that scuttled around the seafloor for about 270 million years before going extinct next is the silurian period which occurred 443 million to 416 million years ago this period saw the spread of jawless fish throughout the seas mollusks and corals thrived in the oceans and land the first undisputed evidence of terrestrial life this was the time when plants evolved though they most likely did not yet have leaves or vascular tissue to sip water and nutrients next is the carboniferous period 359 million to 299 million years ago long before birds evolved tetrapods began laying eggs on land for the first time during this period allowing them to break away from an amphibious lifestyle tetrapods were becoming more specialized and two new groups of animals evolved the first were marine reptiles including lizards and snakes the second were the archosaurs which would give rise to crocodiles dinosaurs and birds most creepily this era is sometimes referred to as the age of the cockroaches because rocha's ancient ancestors was found all across the globe during the carboniferous period the last of the paleozoic era is the permian period which occurred 299 million to 259 million years ago this period would end with the largest mass extinction ever the permian extinction before the permian mass extinction though the warm seas teamed with life coral reefs flourished providing shelter for fish and shelled creatures such as nautiloids and ammonites modern conifers and ginkgo trees evolved on land terrestrial vertebrates evolved to become herbivores taking advantage of the new plant life that had colonized the land the next era is the mesozoic era which is divided into three periods the triassic jurassic and cretaceous periods this period spans from about 252 million years ago to about 66 million years ago was known as the age of reptiles or the age of dinosaurs because of its famous inhabitants this era is also known as the middle life era life diversified rapidly in giant reptiles dinosaurs and other monstrous beasts roamed the earth first we have the triassic period which occurred 252 million to 200 million years ago this period saw the rise of reptiles and the first dinosaurs next is the jurassic period which occurred 200 million to 145 million years ago this period marked the presence of dinosaurs and the first appearance of birds and we have the cretaceous period 145 million to 66 million years ago this period is also known for its iconic dinosaurs and we also have the xenozoic era which is divided into three periods the paleogene neochain and quaternary periods this era began about 65 million years ago and continues into the present this is the third documented era in the history of earth the current locations of the continents and their modern day inhabitants including humans can be traced to this period the cenozoic era is also known as the age of mammals because the extinction of many groups of giant mammals allows smaller species to thrive and diversify because their predators no longer existed first of the cenozoic era is the paleogene period which existed 65 million to 23 million years ago this period consists of the palyucine aeocene and oligocene epox the beginning of the polyujin period was a time for the mammals that survived from the cretaceous period later in this period rodents and small horses are common and rhinoceroses and elephants appear as the period ends dogs cats and peaks become commonplace next is the neogene period which happened 23 million to 2.6 million years ago this period includes the myosin and lyocene epox the neogene period gives rise to early primates including early humans bovides including couple sheep goats antelopes and gazelle flourished during this period last is the quaternary period 2.6 million up to the present year this period consists of flies to sin and holocene efforts in this period cave lions saber-toothed cats cave bears giant deer woolly rhinoceroses and woolly mammoths were prevailing species of the quaternary period and that ends our lesson congratulations i hope you learned something today thank you for watching [Music] you