In answer to the first question, yes, it was a huge ocean.
Regarding the rest of the questions, much of higher vertebrate life evolved along the equatorial regions of Pangea. Because it was a supercontinent, much of the surface had continental climate. During the last 300 million years the Earth did not change much in size. The Pangea supercontinent had lots of plants, but much of the southern half of it was covered by ice, a huge ice age until around 270 million years ago. For some detail about basic paleontology and a nice chart showing Earth climate over the last 500 million years visit our page on Robert Carroll and scroll down to the Science section.
Also check out the Wikipedia page on Pangea.
At our school we have been studying Pangaea and so far we have found that Pangaea was a large 'supercontinent'-- that is, all of the continents from today put together. Pangaea was first studied by Mr. A. Wegener, a German scientist. While studying a map he found that the ridges of South America and Africa looked the same. He studied these similarities for years before he came to the conclusion that the continents, which looked kind of like a puzzle, could be put together. With his notes, he went to many other scientists, who disagreed with his theories. Wegener continued to study his 'Pangaea' and after several more years, once again showed the scientists. He had many more notes to prove his theory and after explaining more thouroughly, the scientists agreed. Today we also have many more examples to prove this. There are Ice Caps that could have once been connected, mountain ranges that had somehow been built together, and even dinosaur remains showed that they had thrived together as the ground moved (of course, the dinosaurs were not alive at this time, but came later when some of the continents were still connected). Wegener is not commonly famous, but is still heard of as a famous scientist. All over the world different scientists are trying to piece together his notes and have been so far successful. If you happen to look at a world map or globe any time soon, note how close the Eastern Ridge of South America and the Western Ridge of Africa look.