How many years ago was the paleozoic era

This precambrian era couldn’t have been different from the explosion of life that followed. When Gondwana broke apart from Pannotia, Pannotia ceased to be a supercontinent. Gondwana – the miniature supercontinent (550-150 mya) Gondwana was a supercontinent that existed from about 550 to 180 million years ago.

precambrian paleozoic mesozoic cenozoic. How long ago was this era. 4.6 billion years-543 million years ago. Life began on earth because. Chemicals in the ocean reacted with radiation from the sun. Prokaryotes. Single celled organism after 3 billion years. Cyanobacteria. What prokaryotes evolved to.Periods of the Paleozoic Era. The Paleozoic Era begins after the Pre-Cambrian about 297 million years ago and ends with the start of the Mesozoic period about 250 million years ago. Each major era on the Geologic Time Scale has been further broken down into periods that are defined by the type of life that evolved during that span of time.Jan 8, 2020 · The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale. At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. Mar 17, 2021 · The major divisions of the Paleozoic Era, from oldest to youngest, are the Cambrian (541 million to 485.4 million years ago), Ordovician (485.4 million to 443.8 million years ago), Silurian (443.8 million to 419.2 million years ago), Devonian (419.2 million to 358.9 million years ago), Carboniferous (358.9 million to …. Paleozoic (541-252 million years ago) means 'ancient life.' The oldest animals on Earth appeared just before the start of this era in the Ediacaran Period, but scientists had not yet discovered them when the geologic timescale was made. Life was primitive during the Paleozoic and included many invertebrates (animals without backbones) and the earliest fish and amphibians.The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure \(\PageIndex{1}\)). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time.Perhaps the most famous creatures to emerge during the Cambrian were the trilobites. Relatives of insects, crabs, and spiders, there were over 20,000 trilobite species that lived between the Cambrian and the end of the Paleozoic Era when they went extinct, some 252 million years ago. The Devonian (/ d ɪ ˈ v oʊ n i. ən, d ɛ-/ də-VOH-nee-ən, deh-) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, 419.2 million years ago (), to the beginning of the Carboniferous, 358.9 Ma. It is named after Devon, England, where rocks from this period were first studied.. The first significant …

The Paleozoic Era (543 to 251 million years ago). This era, which lasted some ... The many fossil shrimp dating back to the Jurassic Period all possessed exactly ...Meet the Fish of the Paleozoic, Mesozoic and Cenozoic Eras. Wikimedia Commons. The first vertebrates on the planet, prehistoric fish lay at the root of hundreds of millions of years of animal evolution. On the following slides, you'll find pictures and detailed profiles of over 30 different fossil fish, ranging from Acanthodes to Xiphactinus. 02.The Cambrian* Period begins the Phanerozoic Eon, the last 542 million years during which fossils with hard parts have existed. It is the first division of the Paleozoic Era (542Ma -251Ma). Marine animals with mineralized skeletons make their first appearance in the shallow seas of the Cambrian, though only "small shelly fossils" (tiny …The Paleozoic Era, which ran from 541 million to 251.9 million years ago, was a time of great change on Earth. The era began with the breakup of one supercontinent and the formation of...The Paleozoic Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago , it succeeds the Neoproterozoic and ends 251.9 Ma at the start …Oct 24, 2022 · Paleozoic Era Timeline. The Paleozoic Era began about 541 million years ago and lasted till 251.9 million years ago. It was the first era of the Phanerozoic Eon and is otherwise known as the “Age of Ancient Life.”. The other two eras that follow this are the Mesozoic (age of middle life) and the Cenozoic (age of recent life). Perhaps the most famous creatures to emerge during the Cambrian were the trilobites. Relatives of insects, crabs, and spiders, there were over 20,000 trilobite species that lived between the Cambrian and the end of the Paleozoic Era when they went extinct, some 252 million years ago. The current view of science is that Earth is around 4.6 billion years old, and the first 4 billion years of its development are known as the Precambrian ...

End Ordovician: 440 million years ago, 86% of all species lost, including graptolites. Late Devonian: 375 million years ago, 75% of species lost, including most trilobites. End Permian, The Great Dying: 251 million years ago, 96% of species lost, including tabulate corals, and most trees and synapsids. Paleozoic Era. The Silurian Period. The Silurian (443.7 to 416.0 million years ago)* was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of ...Figure 26.1 B. 1: Gymnosperms of the taiga: This boreal forest (taiga) has low-lying plants and conifer trees, as these plants are better suited to the colder, dryer conditions. Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period about 390 million years ago.The Paleozoic Era started 542 million years ago with the emergence of complex life forms and ended 251 million years ago with the largest mass extinction the world has ever experienced. It is the ...Figure 31. The Paleogene/Neogene world, 2 to 65 million years ago. The 10-mile-wide comet impact at Chicxulub 65 million years ago caused climate changes thought to have killed the dinosaurs and many other forms of life. Modified from Scotese (2003); used with permission. The Cretaceous Period Figure 29. The Cretaceous world, 65 to 140 million ...The Paleozoic Era is literally the era of “old life.”. It lasted from 544 to 245 million years ago and is divided into six periods. Major events in each period of the Paleozoic Era are described in Figure below. The era began with a spectacular burst of new life. This is called the Cambrian explosion.

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The current view of science is that Earth is around 4.6 billion years old, and the first 4 billion years of its development are known as the Precambrian ...By the end of the Paleozoic, cycads, glossopterids, primitive conifers, and ferns were spreading across the landscape. The Permian extinction, 251.4 million years ago, devastated the marine biota: tabulate and rugose corals, blastoid echinoderms, graptolites, the trilobites, and most crinoids died out. The current era on the geologic time scale is the Cenozoic Era. The era began after the K-T extinction resulted in the end of the Mesozoic Era around 65 million years ago. The extinction of the dinosaurs gave mammals the chance to prolifera...About how many million years ago did the paleozoic Era begin? 540. What animals were common in the Cambrian Period? brachiopods. The final period of the paleozoic Era ...MIT geologists have now reconstructed a timeline of the Earth’s temperature during the early Paleozoic era, between 510 and 440 million years ago — a pivotal period when animals became abundant in a previously microbe-dominated world. ... Both sites are known for their exposed rocks that date back to the early Paleozoic era. In 2016 and ...The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 538.8 to 251.9 Ma. During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals.

Oct 20, 2023 · The Cenozoic Era is generally divided into three periods: the Paleogene (66 million to 23 million years ago), the Neogene (23 million to 2.6 million years ago), and the Quaternary (2.6 million years ago to the present); however, the era has been traditionally divided into the Tertiary and Quaternary periods. By the end of the Paleozoic, cycads, glossopterids, primitive conifers, and ferns were spreading across the landscape. The Permian extinction, 251.4 million years ago, devastated the marine biota: tabulate and rugose corals, blastoid echinoderms, graptolites, the trilobites, and most crinoids died out. Eons are divided into eras, which are further divided into periods, epochs, and ages. Geologic dating is extremely imprecise. For example, although the date listed for the beginning of the Ordovician period is 485 million years ago, it is actually 485.4 with an uncertainty (plus or minus) of 1.9 million years.The time before the Cambrian period is known as the Ediacaran Period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era ().Ediacaran fossils were first found in the Ediacaran hills of Southern Australia. There are no living representatives of these species, which have left …520 million years ago. Study with Quizlet and memorize flashcards containing terms like How long ago did oceans begin to form?, What does the geologic time scale confirm about the Cretaceous period?, Scientists discovered that the Eocene time period existed in Earth's history around 34 MYA. What do they need to do to determine what time period ...The Paleozoic Era begins after the Pre-Cambrian about 297 million years ago and ends with the start of the Mesozoic period about 250 million years ago. Each major era on the Geologic Time Scale has been further broken down into periods that are defined by the type of life that evolved during that span of time.Jan 23, 2017 · Massive Loss of Life. The Permian, however, represented the last gasp for much early prehistoric life. The period, and the Paleozoic era, came to a calamitous close 251 million years ago,... Learn more about the time period that took place 488 to 443 million years ago. During the Ordovician period, part of the Paleozoic era, a rich variety of marine life flourished in the vast seas ...End Ordovician: 440 million years ago, 86% of all species lost, including graptolites. Late Devonian: 375 million years ago, 75% of species lost, including most trilobites. End Permian, The Great Dying: 251 million years ago, 96% of species lost, including tabulate corals, and most trees and synapsids.The Cenozoic (65.5 million years ago to present) is divided into three periods: the Paleogene (65.5 to 23.03 million years ago), Neogene (23.03 to 2.6 million years ago) and the Quaternary (2.6 million years ago to present). Paleogene and Neogene are relatively new terms that now replace the deprecated term, Tertiary.Introduction. During the Paleozoic Era (541 to 251.9 million years ago), fish diversified and marine organisms were very abundant. In North America, the Paleozoic is characterized by multiple advances and retreats of shallow seas and repeated continental collisions that formed the Appalachian Mountains.Seed plants first appeared during the _____. Paleozoic. The first prokaryotic cells appeared during the _____. Precambrian. Animals first appeared during the _____. Precambrian. The Cenozoic era began approximately _____ million years ago. 65. _____ were the dominant vertebrate life form during the Mesozoic era.

Paleozoic Era, or Palaeozoic Era , Major interval of geologic time, c. 542–251 million years ago.

About how many million years ago did the paleozoic Era begin? 540. What animals were common in the Cambrian Period? brachiopods. The final period of the paleozoic Era ... Paleozoic Era. During the Paleozoic Era (541 to 252 million years ago) Fish diversified and marine organisms were very abundant during the Paleozoic. Common Paleozoic fossils include trilobites and …Jan 8, 2020 · The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale. At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. The Phanerozoic eon began 541 million years ago (or, 0.541 billion years ago). Thus, the Phanerozoic eon represents a paltry 12% of Earth's history! Instead, most of Earth's history is represented by the three Precambrian eons. ... The Paleozoic ("old life") era is characterized by trilobites, the first four-limbed vertebrates, and the origin ...Eons are divided into eras, which are further divided into periods, epochs, and ages. Geologic dating is extremely imprecise. For example, although the date listed for the beginning of the Ordovician period is 485 million years ago, it is actually 485.4 with an uncertainty (plus or minus) of 1.9 million years.Feb 13, 2017 · 252 million years ago, Earth was rocked by the Permian–Triassic Extinction Event. It was, and still is, the world’s most devastating extinction event. It is known as ‘The Great Dying’. The Permian–Triassic Extinction Event marked the end of the Paleozoic Era and the start of the Mesozoic Era. The Permian period lasted from 299 to 251 million years ago* and was the last period of the Paleozoic Era. The distinction between the Paleozoic and the Mesozoic is made at the end of the Permian in recognition of the largest mass extinction recorded in the history of life on Earth. It affected many groups of organisms in many different ...

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The Paleozoic Era begins with the Cambrian Period (541 million years ago) and ends with the Permian Period (252 million years ago). The oldest fossil teeth that can be attributed with certainty to Chondrichthyes were discovered in the Lower Devonian of Spain and have been dated to approximately 418 million years ago.Introduction During the Paleozoic Era (541 to 251.9 million years ago), fish diversified and marine organisms were very abundant. In North America, the Paleozoic …Trilobites are a diverse group of prehistoric marine invertebrate animals that first appeared around 521 mya (million years ago), during the Cambrian Period of the Paleozoic Era. Trilobites are among the most successful early multicellular animals and were present on Earth for over 250 million years (that’s many millions of years longer …In geologic time , the Paleozoic Era, the first era in the Phanerozoic Eon , covers the time between roughly 544 million years ago (mya) and until 245 mya. The Paleozoic Era spans six geologic time periods including the Cambrian Period (544 to 500 mya); Ordovician Period (500 mya to 440 mya); Silurian (440 mya to 410 mya); Devonian (410 mya to ...The Devonian period, from 437 to 408 million years ago, was named for the English county where it was first identified. It has sometimes been called the Age of Fishes. ... In geologic time, the Devonian Period, the fourth period of the Paleozoic Era, covers the time roughly 410 million years ago (mya) until 360 mya.The Paleozoic Era The Paleozoic Era is the oldest of the three Eras and dates from 540 Million to 248 Million Years Ago. During the Paleozoic Era, multicelled living things acquired hard body parts, bones, vertebral columns, mandibles, and teeth. Common in the Paleozoic Era were trilobites, crinoids, brachiopods, fish, insects, amphibians, and ...Figure 26.1 B. 1: Gymnosperms of the taiga: This boreal forest (taiga) has low-lying plants and conifer trees, as these plants are better suited to the colder, dryer conditions. Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period about 390 million years ago.The Paleozoic Era lasted from about 540 million years ago to about 248 million years ago. During this time period of about 292 million years, shallow seas came inland several times. Sharks and other fish, along with many other kinds of animals, lived in the water. The water completely covered the area that we now call North Dakota and at times ...Here's a geological time scale of the three eras of the Phanerozoic. The three eras within the Phanerozoic eon are the Paleozoic era (541 million to 252 million years ago) the Mesozoic era (252 million to 66 million years ago) the Cenozoic era (66 million years ago to the present) The Phanerozoic eon is the eon of abundant complex life.Eons are divided into eras, which are further divided into periods, epochs, and ages. Geologic dating is extremely imprecise. For example, although the date listed for the beginning of the Ordovician period is 485 million years ago, it is actually 485.4 with an uncertainty (plus or minus) of 1.9 million years. ….

The Paleozoic Era. 543 to 248 Million Years Ago. The 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 …Paleozoic Era, or Palaeozoic Era , Major interval of geologic time, c. 542–251 million years ago.The start of the Paleozoic era, between roughly 542 mya and 530 mya, is a time when a large number of body plans appears for the first time in the fossil record. ... The Mesozoic (meaning "middle animals") era extends from about 251 million years ago mya to 65 mya and is separated into three geologic periods: Triassic, Jurassic, and Cretaceous.Meet the Fish of the Paleozoic, Mesozoic and Cenozoic Eras. Wikimedia Commons. The first vertebrates on the planet, prehistoric fish lay at the root of hundreds of millions of years of animal evolution. On the following slides, you'll find pictures and detailed profiles of over 30 different fossil fish, ranging from Acanthodes to Xiphactinus. 02.The Paleozoic Era lasted from about 540 million years ago to about 248 million years ago. During this time period of about 292 million years, shallow seas came inland several times. Sharks and other fish, along with many other kinds of animals, lived in the water. The water completely covered the area that we now call North Dakota and at times ... Study with Quizlet and memorize flashcards containing terms like Paleozoic Era (542 - 251 million years ago), Cambrian Period (542 - 488 million years ago), ...8.6: Paleozoic. Figure 8.6.1 8.6. 1: The trilobites had a hard exoskeleton and were an early arthropod, the same group that includes modern insects, crustaceans, and arachnids. The Phanerozoic eon is the most recent eon and represents time in which fossils are common, 541 million years ago to today.The Phanerozoic Eon is a period of geological history that spans 542 million years and is typically subdivided into three eras. These eras are: Paleozoic Era: 542 to 251 million years ago. Mesozoic Era: 251 to 65 million years ago. Cenozoic Era: 65 million years ago to present. Mesozoic. The dinosaurs and the mammals appeared during the ...Oct 20, 2023 · The Cenozoic Era is generally divided into three periods: the Paleogene (66 million to 23 million years ago), the Neogene (23 million to 2.6 million years ago), and the Quaternary (2.6 million years ago to the present); however, the era has been traditionally divided into the Tertiary and Quaternary periods. How many years ago was the paleozoic era, The Neoproterozoic is the terminal era of the Proterozoic eon (or informally, the Precambrian) extending from about 1,000 million years ago (mya) at the end of the Mesoproterozoic to 542 mya at the beginning of the Cambrian of the Paleozoic era. It is one of three major divisions (eras) of the Proterozoic in the geologic time scale ., Pre-Cambrian Animal Life. The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure \(\PageIndex{1}\)). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time., Paleozoic Era, or Palaeozoic Era, Major interval of geologic time, c. 542–251 million years ago. From the Greek for “ancient life,” it is the first era of the Phanerozoic Eon and is followed by the Mesozoic Era.It is divided into six periods: (from oldest to youngest) the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. ..., Trilobites are a diverse group of prehistoric marine invertebrate animals that first appeared around 521 mya (million years ago), during the Cambrian Period of the Paleozoic Era. Trilobites are among the most successful early multicellular animals and were present on Earth for over 250 million years (that’s many millions of years longer …, 30 de out. de 2013 ... The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon, spanning from roughly 541 to 252.2 million ..., Mar 17, 2017 · Meet the Fish of the Paleozoic, Mesozoic and Cenozoic Eras. Wikimedia Commons. The first vertebrates on the planet, prehistoric fish lay at the root of hundreds of millions of years of animal evolution. On the following slides, you'll find pictures and detailed profiles of over 30 different fossil fish, ranging from Acanthodes to Xiphactinus. 02. , Paleozoic Era, or Palaeozoic Era , Major interval of geologic time, c. 542–251 million years ago., Fossilized fecal material, better known as coprolites, have been found at many archeological sites across the globe (Appelt et al., 2016) and date back as far as the Paleozoic era (270 million years ago) (Dentzien-Dias et al., 2013). Fecal samples have also been analyzed after removal from the intestinal tracts of mummies. , In many ways, bone, both external and internal, was the key to vertebrate evolution. Acanthodii: early jawed fishes. The next class of fishes to appear was the Acanthodii, containing the earliest known jawed vertebrates, which arose in the Late Silurian, more than 416 million years ago. The acanthodians declined after the Devonian but lasted ..., The start of the Paleozoic era, between roughly 542 mya and 530 mya, is a time when a large number of body plans appears for the first time in the fossil record. ... The Mesozoic (meaning "middle animals") era extends from about 251 million years ago mya to 65 mya and is separated into three geologic periods: Triassic, Jurassic, and Cretaceous., The Ordovician* lasted about 45 million years and saw the transition from very primitive to relatively modern life-forms in the seas. The “Ordovician radiation” which followed the late Cambrian extinctions, lead to a tripling of marine diversity, the greatest increase in the history of life, and giving the highest levels of diversity seen during the …, Answer to: How many years ago was the Paleozoic era? By signing up, you'll get thousands of step-by-step solutions to your homework questions. You... , 13 de out. de 2020 ... Crinoids thrived during the Paleozoic Era (490 through 250 million years ago) and reached their apex during the Mississippian Period (360 ..., According to old mining tradition, the Continental Permian of Central Europe is subdivided into Rotliegend and Zechstein. With a span of some 45 million years, ..., The Paleozoic took up over half — approximately 300 million years (542 mya to 251 mya)* — of the Phanerozoic. During the Paleozoic there were six major continental land masses; each of these consisted of different …, The Paleozoic Era: The History of the Geologic and Evolutionary Changes that Began Over 500 Million Years Ago ... Select Format. Kindle – $4.99., , The Cambrian is the first period of the Paleozoic Era and ran from 539 million to 485 million years ago. The Cambrian sparked a rapid expansion in the diversity of animals, in an event known as the Cambrian explosion , during which the greatest number of animal body plans evolved in a single period in the history of Earth. , The Paleozoic Era is a time period in Earth’s history that lasted from 541 to 252 million years ago. It is divided into six periods: the Cambrian, Ordovician, Silurian, …, The Paleozoic Era lasted between 542 and 251 million years ago. The Paleozoic Era is further subdivided into 7 smaller units of time called periods., Terms in this set (23) What are the 4 major divisions of geologic history. Precambrian time, Paleozoic era, Mesozoic era, and Cenozoic era. About when did the Precambrian time begin. 4.6 billion years ago. What are 2 differences between early earth and the present earth. Early earth had an atmosphere made of gases and was a place of great termoil., Here's a geological time scale of the three eras of the Phanerozoic. The three eras within the Phanerozoic eon are the Paleozoic era (541 million to 252 million years ago) the Mesozoic era (252 million to 66 million years ago) the Cenozoic era (66 million years ago to the present) The Phanerozoic eon is the eon of abundant complex life., For instructions, click here. Scientists have recorded five significant ice ages throughout the Earth’s history: the Huronian (2.4-2.1 billion years ago), Cryogenian (850-635 million years ago ..., The Carboniferous Period is famous for its vast swamp forests, such as the one depicted here. Such swamps produced the coal from which the term Carboniferous, or "carbon-bearing," is derived. The Carboniferous Period lasted from about 359.2 to 299 million years ago* during the late Paleozoic Era. The term "Carboniferous" comes from England, in ..., The Palaeozoic (or Paleozoic) era is the earliest of the three eras of the Phanerozoic.Its name means early life.It lasted from about 541 to 252 million years ago (mya), and …, During the Paleozoic Era, several hundred million years ago, ... Between about 425 million and 380 million years ago, Avalonia collided with Laurentia and forced up a mountain range along the suture. This collision was an early part of the Acadian Orogeny, the second major mountain-building phase of the Appalachian Mountains. ..., Eons are divided into eras, which are further divided into periods, epochs, and ages. Geologic dating is extremely imprecise. For example, although the date listed for the beginning of the Ordovician period is 485 million years ago, it is actually 485.4 with an uncertainty (plus or minus) of 1.9 million years., Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous Period), after living on Earth for about 165 million years. If all of Earth time from the very beginning of the dinosaurs to today were compressed into 365 days (one calendar year), the dinosaurs appeared January 1 and became extinct the third week of September. , By the end of the Paleozoic, cycads, glossopterids, primitive conifers, and ferns were spreading across the landscape. The Permian extinction, 251.4 million years ago, devastated the marine biota: tabulate and rugose corals, blastoid echinoderms, graptolites, the trilobites, and most crinoids died out., Terms in this set (23) What are the 4 major divisions of geologic history. Precambrian time, Paleozoic era, Mesozoic era, and Cenozoic era. About when did the Precambrian time begin. 4.6 billion years ago. What are 2 differences between early earth and the present earth. Early earth had an atmosphere made of gases and was a place of great termoil., Here's a geological time scale of the three eras of the Phanerozoic. The three eras within the Phanerozoic eon are the Paleozoic era (541 million to 252 million years ago) the Mesozoic era (252 million to 66 million years ago) the Cenozoic era (66 million years ago to the present) The Phanerozoic eon is the eon of abundant complex life., The Ordovician ( / ɔːrdəˈvɪʃi.ən, - doʊ -, - ˈvɪʃən / or-də-VISH-ee-ən, -⁠doh-, -⁠VISH-ən) [9] is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya ..., The Neoproterozoic is the terminal era of the Proterozoic eon (or informally, the Precambrian) extending from about 1,000 million years ago (mya) at the end of the Mesoproterozoic to 542 mya at the beginning of the Cambrian of the Paleozoic era. It is one of three major divisions (eras) of the Proterozoic in the geologic time scale .