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The Jurassic Coast’s Geological Timescale For Fossil Hunters

Updated: Feb 15, 2022

Unlike the timescales, we use every day to describe years, months, days, and hours a geological timescale spans billions of years. The geologic timescale is the “calendar” of events in the Earth's history known as eons, eras, periods, epochs, and ages. So, when was the Jurassic?

Fossils help to establish a geological timescale through their timing of emergence and disappearance from the fossil record. It is these intervals or stratigraphy that marks the beginnings and endings of ages, epochs, periods, and other intervals.

Before we understand when the Jurassic Period prevailed along the Jurassic Coast it is important to understand the origins of the science behind the geological timescale. Below is a simple chart of the geological timescale managed by the International Commission on Stratigraphy (ICS) the largest and oldest constituent scientific body in the International Union of Geological Sciences (IUGS).

The geological timescale is similar to the periodic table we all learned at school. Rather than showing the relative properties of elements it is an authoritative reference map showing the relative chronological order of rocks and fossils throughout the Earth's history.

The understanding of rocks and their relationship to one another dates back throughout history with those involved in mining and building. Since the 17th-century science has influenced the study of geology.

In 1669, Nicolas Steno a geologist and anatomist from Denmark first advanced the development of the science of geology with two basic geologic principles. He would later in life become a Catholic bishop.

The principle’s stated, that sedimentary rocks were laid down in a horizontal manner, and the second stated that younger rock was deposited on top of older rock. Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. These principles described a process of rock succession.

It would still be over 100 years later in 1796 that the English engineer William Smith (image below) observed that where rock (lithology) was found in the same order or superposition it could be characterised by its unique fossil content. Up and until this point it was only the physical characteristics of rock, such as colour, mineralogy, and grain size that defined the rock.

Smith’s observations of the fossil content would change how rocks would be identified. He is credited with the "principle of faunal succession" that states different fossil species always appear and disappear in the same order and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks.

This work was further developed by James Hutton and Sir Charles Lyell (image below) in 1830 who published the book, "Principles of Geology" and outlined the contemporary “principle of uniformitarianism”.

Lyell suggested that "the present was the key to the past". Both Lyell and Hutton agreed that gradually changing natural processes shaped the Earth’s surface geology and landscape (geomorphology) over long periods of time.

Research at the time showed that sea levels had risen and fallen in the past, that volcanoes were built on older rocks, and that valleys form slowly by the erosional power of water and ice.

This new thinking replaced the prevailing idea developed by French naturalist Baron Georges Cuvier (1769–1832) of "catastrophism" that suggested the features seen on the surface of Earth, such as mountains, were formed by large, abrupt changes or catastrophes.

The “principle of uniformitarianism” was also supported by Charles Darwin who is today regarded as the founder of evolutionary biology. As a naturalist, biologist, and geologist Darwin undertook a five-year voyage around the world on the HMS Beagle starting in 1831. During this period his own observations and theories supported Charles Lyell's uniformitarian ideas.

In 1838 Darwin would present his "theory of natural selection" and established that the evolution of life took a long time and that the Earth's geology had shown to be extremely old. Therefore, there had also been enough time for millions of species to emerge, and either evolve into new species or go extinct.

These early scientific concepts of studying the characteristics of geology and the biology of unique fossil assemblages or index fossils made it possible to distinguish one layer of rock from another in terms of relative dating. This placed geologic events in chronological order without requiring a specific numerical age.

It was not until the emergence of radiometric dating that enabled fossils to be numerically quantified by means of absolute dates.

Radiometric dating or the study of geochronology compares the presence of specific radioactive isotopes within a sample to its known abundance on Earth and half-life (rate of decay). There are a number of different types of dating methods depending upon the type of rock or fossil sample.

In 1902, physicists Ernest Rutherford and Frederick Soddy had discovered that radioactive elements broke down into other elements in a definite sequence or series, through the process of nuclear fission. Between 1904 - 1906, Rutherford was the first to suggest and demonstrated that by using radioactivity millions of years of geologic time could be measured by using the radioactive decay of uranium-bearing fossils and sediments.

In 1946, Willard Libby an American chemist proposed a method for dating organic materials by measuring their content of carbon-14. This long-lived radioactive carbon isotope had only recently been discovered in 1940 by American physicists Martin Kamen and Sam Ruben. In fossils, radiocarbon (14C) dating is possible because all living organisms take in carbon from their environment, which includes a small amount of the radioactive isotope 14C.

The science behind the geological timescale and the mapping of the fossil record in any geographic area across the world has helped us to make sense of the Earth's complex history.

It is with confidence that we can now understand that the Jurassic Coast represents almost the entire Mesozoic Era (252 million - 66 million years) in one coastline and hence its importance as a World Heritage Site. The Mesozoic Era of “middle life” in Greek was the second of three major geological eras alongside the Paleozoic (541 million - 252 million years), Cenozoic (66 million years ago to the present day) of the Phanerozoic when physical life first emerged. The three periods of the Mesozoic are the Triassic, Jurassic, and Cretaceous and during these periods the ancestors of major plant and animal groups that exist today first appeared but is best known as the time of the dinosaurs or "Age of the Reptile". The Mesozoic also has had three of the five largest mass extinctions in Earth history which also saw the extinction of the dinosaurs.

Portrait Images: Source Wikipedia


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