Charles River - The Jurassic Period: The History and Legacy of the Geologic Era Most Associated with Dinosaurs

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Gerhard Boeggemanns depiction of the landscape during the Jurassic

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Dmitry Bogdanovs illustration depicting Muraenosaurus

The early history of our planet covers such vast stretches of time that years, centuries and even millennia become virtually meaningless. Instead paleontologists and scientists who study geochronology divide time into periods and eras.

The current view of science is that planet Earth is around 4.6 billion years old. The first four billion years of its development are known as the Precambrian period. For the first billion years or so, there was no life in Earth. Then the first single-celled life-forms, early bacteria and algae, began to emerge. We dont know where they came from or even if they originated on this planet at all. This gradual development continued until around four billion years ago when suddenly (in geological terms!) more complex forms of life began to emerge.

Scientists call this time of an explosion of new forms of life the Paleozoic Era and it stretched from around 541 to 250 million years ago (Mya). First of all, in the oceans and then on land, new creatures and plants began to appear in bewildering variety. By the end of this period, life on Earth had exploded into a myriad of complex forms that filled virtually every habitat and niche available in the seas and on the planets only continent, Pangea.

Then a mysterious event that became known to early paleontologists as The Great Dying wiped out more than 95% of all life on Earth. No-one is entirely certain what caused this, but the effect of this cataclysm was as if someone had pressed a great, cosmic reset button and it took thirty million years for the development of life on Earth to start again.

The next period of Earths history is known as the Mesozoic Era, from about 252 to 66 Mya. This era is further divided into three periods, the Triassic, Jurassic and Cretaceous. During this era, one type of life came to dominate the planet more completely and for a longer period than had been seen before or since; this was the Age of Reptiles.

Beginning in the Triassic but especially in the Jurassic period, reptiles came to dominate the oceans, the land and even the skies. There has never been anything else quite like this period in terms of the success of a particular type of creature. For almost two hundred million years, reptiles were the only significant creatures on Earth. They were so successful and so diverse that they evolved to take advantage of every available habitat and no other type of large creature had a chance to develop.

To put the two hundred million years of reptile dominance in perspective, the entire span of recorded human history, the time since people advanced from tribes of primitive, nomadic hunter-gatherers into recognizable societies, covers less than six thousand years. To put this in context, if the entire history of the planet were to be laid out on the length of a football field, the period of dominance of the age of reptiles would not begin until the five-yard line and would stretch for twelve feet. All of human history would occupy a tiny strip at the end of the field, less than the width of a human hair.

It was during the Jurassic period that reptiles began rule the Earth and some of the best-known prehistoric creatures first emerged. The Jurassic Period: The History and Legacy of the Geologic Era Most Associated with Dinosaurs looks at the development of the era, the extinction events that preceded it, and how life began to evolve during it. Along with pictures depicting important people, places, and events, you will learn about the Jurassic Period like never before.

The modern understanding of the creation and development of planet Earth really began to emerge in the early 19th century when geologists first began to analyze rock strata and to recognize that different layers contained different kinds of fossil remains. It was quickly realized that these provided a snapshot of life at various periods in the ancient past and it didnt take long before scientists began to use agreed terms to describe the vast stretches of time that comprise the history of our planet.

One of the first was a self-taught English geologist named John Phillips and in the 1830s he published a book that would change our understanding of the history of the planet and finally standardize the terminology used to describe these ancient periods. He did this by ordering rock strata according to the different types of fossils found within them and used this to define different periods of the development of life on Earth.

Phillips

These classifications describe the history of life after the Precambrian period in terms of three eras, with each being further subdivided into several periods. These eras are:

The Paleozoic Era (meaning the era of ancient life) was the oldest and a period of dramatic upheaval and change. It covers a period from five hundred and fifty million to two hundred and fifty million years ago. This era began with the emergence of the first multi-celled life and by its end, the first large reptiles, creatures such as Dimetrodon and Edaphosaurus, had appeared on the single continent, Pangea. This era is further subdivided into six geologic periods; Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.

The Paleozoic Era was generally a period of unbroken evolution of increasingly complex forms of life. However, it also included at least three of what have become known as mass extinction events. For reasons that are not fully understood but are most likely associated with some form of climate change, there were three occasions during this era when many species completely died and were replaced by other forms of life.

The first of these events occurred between the Ordovician and Silurian periods. The Earth grew colder, glaciers appeared and sea levels and sea temperatures dropped dramatically. Around 25% of all species on Earth were killed. In the oceans the effect was even more dramatic and around 60% of all marine species vanished forever.

At the end of the Devonian period there was a second and even more catastrophic extinction event that led to the death of around 70% of all species on Earth. No-one is quite certain what caused this event though current theories include excessive sedimentation of the oceans, a period of rapid global warming (or cooling), the impact of a comet or large meteorite or even habitat changes caused by massive nutrient runoff from the land.