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First phase of the Tethys Ocean's forming: the (first) Tethys Sea starts dividing Pangaea into two supercontinents, Laurasia and Gondwana.

The Tethys Ocean was a Mesozoic era ocean that existed between the continents of Gondwana and Laurasia before the opening of the Indian Ocean.

In 1893, using fossil records from the Alps and Africa, Eduard Suess proposed the theory that a shallow inland sea had once existed between Laurasia and Gondwana. He named it the 'Tethys Sea' after the Greek sea goddess Tethys. The theory of plate tectonics later disproved or overrode many parts of Suess's theory, even determining the existence of an earlier body of water called the Tethys Ocean. However, Suess's overall concept was still relatively accurate and remarkably imaginative for its day, so he generally is credited with the discovery of both the Tethys Sea and the Tethys Ocean.

About 250 million years ago, during the late Permian Era, a new ocean began forming in the southern end of the Paleo-Tethys Ocean. A rift formed along the northern continental shelf of Southern Pangaea (Gondwana). Over the next 60 million years, that piece of shelf, known as Cimmeria, traveled north, pushing the floor of the Paleo-Tethys Ocean under the eastern end of Northern Pangaea (Laurasia). The Tethys Ocean formed between Cimmeria and Gondwana, directly over where the Paleo-Tethys used to be.

During the Jurassic Period (150 mya), Cimmeria finally collided with Laurasia. There it stalled, the ocean floor behind it buckling under, forming the Tethyan Trench. Water levels rose and the western Tethys came to shallowly cover significant portions of Europe. Around the same time, Laurasia and Gondwana began drifting apart, leaving the Atlantic Ocean between them. Between the Jurassic and the Cretaceous (100 mya), even Gondwana began breaking up, pushing Africa and India north, across the Tethys and opening up the Indian Ocean. As these land masses pushed in on it from all sides, up until as recently as the Late Miocene (15 mya), the Tethys ocean continued to shrink, becoming the Tethys Seaway or (second) 'Tethys Sea'.

Today, India, Indonesia and the Indian Ocean cover the area once occupied by the Tethys Ocean, and Turkey, Iraq, and Tibet sit on Cimmeria. What was once the Tethys Sea has become the Mediterranean Sea. Other remnants are the Black, Caspian and Aral Seas (via a former inland branch known as the Paratethys). Most of the floor of the Tethys Ocean disappeared under Cimmeria and Laurasia. We only know the Tethys existed because geologists like Suess found fossils of ocean creatures in rocks in the Himalayas. So, we know those rocks were underwater, before the Indian continental shelf began pushing upward as it smashed into Cimmeria. We can see similar geologic evidence in the Alpine orogeny of Europe, where the movement of the African plate raised the Alps.

Paleontologists also find the Tethys Ocean particularly important because much of the world's sea shelves were found around its margins for such an extensive period of time. Marine, marsh-dwelling, and estuarian fossils from these shelves are of considerable paleontological interest.

Like every science, geology is a continuously evolving system of theories, and the terms used to describe various pre-historic formations have fluctuated as more accurate theories have emerged. For example, many internet sources use "Tethys Ocean" to refer to the "Tethys Sea" and vice versa. Some even appear to erroneously refer to the growing Atlantic Ocean during the Jurassic as the Tethys Sea.

The western end of the Tethys Ocean is called Tethys Sea, Western Tethys Ocean or Alpine Tethys Ocean. The Black, Caspian and Aral Seas are thought to be its crustal remains (though the Black Sea may in fact be a remnant of the older Paleo-Tethys Ocean). However, this "Western Tethys" was not simply a single open ocean. It covered many small plates, Cretaceous island arcs and microcontinents. Many small oceanic basins (Valais Ocean, Piemont-Liguria Ocean) were separated from each other by continental terranes on the Alboran, Iberian, and Apulian plates. The high sealevel in the Mesozoic age flooded most of these continental domains forming shallow seas. During the Cenozoic, large parts of central and eastern Europe were covered by a northern branch, the Paratethys or Pannonian Sea, which gradually disappeared from the end of the Miocene, becoming an isolated inland sea that finally dried up during the Pleistocene.

The eastern part of the Tethys Ocean is likewise sometimes referred to as Eastern Tethys.

As theories have improved, scientists have extended the "Tethys" name to refer to similar oceans that preceded it. The Paleo-Tethys Ocean, mentioned above, existed from the Silurian (440 mya) through the Jurassic eras, between the Hunic terranes and Gondwana (later the Cimmerian terranes). Before that, the Proto-Tethys Ocean existed from the Ediacaran (600 mya) into the Devonian (360 mya), and was situated between Baltica and Laurentia to the north and Gondwana to the south. Neither Tethys oceans should be confused with the Rheic Ocean, which existed to the west of them in the Silurian era.

• Paleo-Tethys Ocean
• Proto-Tethys Ocean
• Tethyan Trench

• Palaeos Earth: The Tethys Sea
• Tethys Ocean - at global history