Panthalassa (Wikipedia)

In the Early Jurassic the Pacific Plate opened originating from a triple junction between the Panthalassic Farallon, Phoenix, and Izanagi plates.
Panthalassa was a hemisphere-sized ocean, much larger than the modern Pacific.
Farallon Plate (Wikipedia) The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart and after the formation of the Pacific Plate at the centre of the triple junction during the Early Jurassic. It is named for the Farallon Islands, which are located just west of San Francisco, California.

became

Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek πᾶν “all” and θάλασσα “sea”), was the superocean that surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During the Paleozoic–Mesozoic transition c. 250 Ma it occupied almost 70% of Earth’s surface. Its ocean floor has completely disappeared because of the continuous subduction along the continental margins on its circumference. Panthalassa is also referred to as the Paleo-Pacific (“old Pacific”) or Proto-Pacific because the Pacific Ocean is a direct continuation of Panthalassa.