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Topic Last Updated on 15-07-2024
Post on topic: Traces of the Ancient Seas.
We Live at the Bottom of the Ocean
So, how exactly did the sea leave traces over almost all of the Earth’s landmass? Many areas previously occupied by the sea were eventually raised by powerful tectonic movements. Where this uplift occurred smoothly, plateaus and highlands were formed, and where there was a crushing of rocks into folds at the same time as the uplift, mountains grew. For example, many mountain systems in Eurasia — the Alps, the Caucasus, and the Himalayas — consist mostly of the former bottom of the extinct Tethys Ocean, the remnants of which are the Mediterranean, Black, and Caspian seas.
In addition, during warmer geological periods, when there were no glaciers on our planet, all the water that is now concentrated in the frozen masses was located in the World Ocean, resulting in an almost 230 ft increase in sea level. This means that many lowlands were flooded at that time.
So, almost any point of modern land was once home to a sea. Of course, there are places where there are no marine deposits — for example, virtually all of Finland. But this does not mean that the sea has never been there — it’s just that this country is near the center of a relatively recent glaciation, so the glacier has scraped off all traces of it.
PERMIAN, 225 MILLION YEARS AGO
TRIASSIC, 200 MILLION YEARS AGO
CRETACEOUS, 65 MILLION YEARS AGO
ANTHROPOGENIC, MODERN TIME
Fossil Pie
Moreover, the order of these sets of fossils is the same in different places. When Smith decided to classify rock layers based on these sets of fossils (which is why they are called index fossils), it turned out that the vertical sequence of the layers is the same everywhere. He then assumed that layers with the same index fossils were formed at the same time, and he combined layers from several different outcrops in one diagram.
The Fossil Puzzle
Of course, the rock does not preserve the entire animal, only its solid parts, like the skeleton and teeth of vertebrates, shells of shellfish, and shells of crustaceans. In addition, these solid parts undergo mineralization, in which their substance passes into a more stable form or is even replaced by another mineral. However, the shape is often very well preserved, and you can fairly accurately judge what kind of animal it was. If a fossil animal is related to any modern species, then even a small detail can be used to reconstruct its entire appearance. For example, the appearance of the Megalodon shark was reconstructed only from teeth and a few vertebrae. However, as the species has no analogs in modern times, paleontologists can only guess. For example, scientists have been arguing for more than 100 years about where the ancient fish of the Helicoprion genus had so-called “tooth whorls.”
Layers of Time: Sedimentary Rocks Reveal Earth’s History
The sea has left sedimentary rock that can be found almost anywhere on the Earth’s landmass. It usually consists of several layers, which lie on top of each other like layers of a cake. Back in the 17th century, the Danish naturalist Niels Stensen formulated a law that states that the higher the layers of rocks lie, the younger they are.
That is, if you have a geological outcrop (for example, a cliff washed away by a river or sea), and it displays three layers with the remains of marine animals, you can conclude that, once upon a time, there lived animals from the lower layer, then they were replaced by those whose remains we can find in the middle layer, with the remains of the upper layer belong to the closest to us in time. If there is no outcrop, you can drill a well to see how the depth of one rock is replaced by another. This way, you can not only find out which marine animals once lived here but also understand which of them lived here before and which came later.
Stone Chronicles of the Earth
If at some spot on the surface of the Earth’s modern land, the sea existed from the time of the formation of our planet until recently, and all its deposits were preserved to the present moment, we could study the geological outcrop or drill a well at that point to learn how marine organisms gradually replaced each other over time. Unfortunately, there is no such piece of land anywhere. First of all, in the geological past, the sea flooded one part of the land, then another — there is no place where it existed at all times. Secondly, once on land, the sedimentary rocks formed in the sea are often destroyed: they are washed away by rivers, blown away by the wind, or plowed over by glaciers.
Fossil Detectives: Matching Layers Across Continents
Fortunately, scientists have come up with a way around this obstacle and have begun to compare the deposits left by ancient seas around the world. In one place, they found one set of layers, and in another a different set, but when compared, it often turned out that some layers, including those found in different parts of the world, contain the same set of fossils. In the mid-19th century, the English zoologist and paleontologist Thomas Henry Huxley, suggested that layers that contain the same set of fossils are of the same age.
Based on this assumption, paleontologists have identified a dozen or more layers that contain more or less the same set of fossils and decided that each of these layers was formed during a certain period. They are called geological periods. Later, each period was divided into smaller parts. The result was a fairly detailed picture of changes in marine life over time. However, it covers only the last 540 million years — until that time, marine animals had no solid body parts that could be preserved in sediments. Therefore, we know very little about the life that inhabited the older seas, although the sediments of those times have also been found.
ABSOLUTE DATING
For quite a long time, it was completely unknown how long all these periods lasted and how long ago they took place. For example, just over a hundred years ago, the length of the entire geological history of the Earth was estimated at 20–40 million years. It was only by the mid-20th century that radioisotope dating produced more or less accurate absolute dates, and it became clear that the age of our planet is approximately 4.5 billion years.
What Was the Ancient Sea like?
The sediments left by the ancient seas do not just tell us about their inhabitants. They can also tell us a lot about the sea itself. How? First, the very species of fossil animals often suggest what kind of natural conditions were present. For example, if there are many remnants of ancient corals in the rock, the sea was likely warm because corals do not grow at low temperatures. Alternatively, if there are remnants of sea urchins, it means that the sea was quite salty — sea urchins do not tolerate low salinity (for example, they don’t live in the Black Sea, where the salinity is twice as low as the average in the World Ocean).
