Did the Atlantic ocean exsit when all land masses were connected?

The answer is No, The Atlantic Oceans did not exist when all landmasses were connected, there used to be a supercontinent called Pangaea.

About 335 million years ago, all of the world's continents were joined together in a single landmass called Pangaea. This supercontinent surrounded a single ocean called the Panthalassa ocean.

As the Earth's crust moved over time, Pangaea began to break apart, forming the Atlantic Ocean and the other oceans we know today.