The author never actually tells us how this new discovery has affected general thinking. We don’t know if this new knowledge is widespread yet.

This is not stated in the argument. In fact, the author’s reasoning relies on assuming the opposite: that these fossils are from ocean creatures, thereby supporting the idea that the Antarctic ice melted into an ocean long ago.

This is a fair paraphrase of the conclusion. All the evidence is used to establish that the Antarctic ice melted three million years ago, which rebuts the previously-held belief that the ice hasn’t melted for 14 million years.

This is used to support the plausibility of Antarctica’s ice having melted three million years ago. Because it supports another claim, this can’t be the main conclusion.

There is a version of (E) that supports the plausibility of Antarctica’s ice having melted three million years ago, however, warming and volcanoes also aren’t given as the only options, but rather just as possible causes of melting. Even if (E) had gotten that right, it supports another claim, so it can’t be the main conclusion.