I actually think Josh Schwartz & Jamie Moss got ripped on this one.

POSSIBLE SPOILERS: I'm talking about diff drafts of the script.

I read Sheldon Turner's 2nd draft of XMEN ORIGINS: MAGNETO back in late 2006. I'm a fan of his, but this script was pretty bad. At least for Xmen fans. It was much more similar to HANNIBAL RISING than any Xmen script. It was largely a Holocaust survival script, that had to do with Nazis experimenting on people and wanting to do experiments on Magneto. The only mutants in this script were Xavier and Magneto. From the look of the trailer for First Class, Prof. X setting up the school is the only thing they kept from Turner's script. And for the most part, that would be part of ANY Xmen script that was set during this time period.

Schwartz & Moss wrote a script called XMEN: YEAR ONE which is the script that got Vaughn and Co
onboard this project. This script had a ton more mutants and was much more like an Xmen script. I also enjoyed it much more than Turner's take.

But Vaughn works with Goldman, so it's no surprise he had her come in and do her own version. They were working from the Schwartz/Moss draft. The draft Vaughn's team is much more similar to the Schwartz & Moss draft and doesn't even have the ghost of Turner's left in it.

Turner is most likely getting credit simply because the guild heavily favors the first writer on a project.

Not knocking any writers here. The reason for the confusion is general Hollywood practice. It just gets super muddy when it is an adaptation that has had multiple very distinct and different scripts.

Looking forward to more on the subject of "credits" in the future.

I had situation where I believe (belief is all I have in this matter) I helped develop entire story with the writer during days worth of one on one verbal conversations, and then spent months reviewing each page of each draft and giving notes in person (story editor credit?) When i asked for a credit, ANY credit, the answer was "no", though I was paid. I even was chastised for publicly mentioning the work I did on the script. Not being a WGA member I never thought I had any recourse.

This specific issue is long ago/far away now, but am interested for myself and other non-WGA how to handle such matters in future.