The Proto-Tethys Ocean was an ancient ocean that existed from the latest Ediacaran to the Carboniferous (550–330 Ma). It was an ocean predecessor of the later Paleo-Tethys Ocean. The ocean formed when Pannotia disintegrated, Proto-Laurasia (Laurentia, Baltica, and Siberia) rifted away from a supercontinent that would become Gondwana. Proto-Tethys formed between these two supercontinents. The ocean was bordered by Panthalassic Ocean to the north, separating it from Panthalassa by island arcs and Kazakhstania. The Proto-Tethys expanded during the Cambrian. The ocean was at its widest during the Late Ordovician to Middle Silurian. The ocean was situated between the Siberia to the west, and Gondwana to the east. The ocean began to shrink during the Late Silurian, when North China, and South China moved away from Gondwana and headed north. In the late Devonian, the microcontinent of Kazakhstania collided with Siberia, shrinking the ocean even more. The ocean closed when the North China craton collided with Siberia-Kazakstania continent in the Carboniferous, while the Paleo-Tethys Ocean expanded.
See also 
- Proto-tethys ocean – at global history
- Paleo-Tethys and Proto-Tethys – at global history
- Early Carboniferous – at PaleoMap Project (North China microcontinent begins to collide with Siberia-Kazakstania continent closing the Proto-Tethys)
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