About 250 Ma, during the late Permian era, a new ocean began forming in the southern end of what geologists call the Paleo-Tethys Ocean. A rift formed along the northern continental shelf of Southern Pangea (Gondwana). Over the next 60 million years, that piece of shelf, known as Cimmeria, traveled north, pushing the floor of the Paleo-Tethys under the eastern end of Northern Pangea (Laurasia). The Tethys ocean formed between Cimmeria and Gondwana, directly over where the Paleo-Tethys used to be.
During the Jurassic period (150 Ma), Cimmeria finally collided with Laurasia. There it stalled, the ocean floor behind it buckling under, forming the Tethyan Trench. As water levels rose, 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 Ma), even Gondwana began breaking up, pushing Africa and India north, across the Tethys. As these land masses pushed in on it from all sides, up until as recently as the Late Miocene (15 Ma), the Tethys ocean contiued to shrink, becoming the Tethys Seaway or Tethys Sea.
Today, India, Indonesia and the Indian Ocean cover the area once occupied by the Tethys Ocean. Turkey, Iraq, and Tibet sit on the land once known as Cimmeria. Most of the floor of the Tethys Ocean disappeared under Cimmeria and Laurasia. We only know the Tethys existed because geologists like Suess have 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 Europe, where the movement of Africa raised the Alps.
Paleontologists 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 interest to them.