Ashley Miller & Zack Stentz along with Jane Goldman & director of the film Matthew Vaughn are receiving a shared screenplay writing credit. The first writers on the project, Josh Schwartz and Jamie Moss, will receive no credit for their services rendered.
How does someone who didn't write on the movie get a story credit, yet the two people who created the story don't? It seems that this has more to do with how Marvel Studios is cultivating and farming out its writers than any rules set forth by the Writers Guild of America. Sheldon Turner was originally hired to write a film entitled X-Men Origins: Magneto, which was supposed to be a companion piece to X-Men Origins: Wolverine.
Josh Schwartz, at around the same time, was also hired to write an X-Men prequel, which would later become X-Men: First Class. When X-Men Origins: Magneto was put on the backburner, that character was worked into a new draft of X-Men: First Class as director Matthew Vaughn came aboard. So, certain elements from Sheldon Turner's X-Men Origins: Magneto were retained, hence the story credit.
Josh Schwartz and Jamie Moss, on the other hand, though they served in writing the first draft of X-Men: First Class, are not being credited because they borrowed liberally from the 2006-2007 X-Men: First Class Marvel comic book miniseries, which incorporated may of the characters from Bryan Singer's original X-Men seen as teenagers. That of course changed when Vaughn joined the project.
Josh Schwartz is still seeking a credit, even though his work is based on the work of other Marvel writers, so this fight may not be officially over.
For a more in-depth analysis of how Marvel and the WGA chose who should be credited for screenwriting on X-Men: First Class,
X-Men: First Class was released June 3rd, 2011 and stars James McAvoy, Laurence Belcher, Michael Fassbender, Bill Milner, Kevin Bacon, Rose Byrne, Jennifer Lawrence, Beth Goddard. The film is directed by Matthew Vaughn.