Makes sense. It could simply have been removed, or the second version could have been coded from scratch for all we know.
This all makes sense from what I know. From what I know, what you have here is a continuation of the original work by Stance Nixon. I do not know what additional, additional work was done by Thomas. As is often the case when you leave a project unfinished, your credit is removed. (As a side note, there is a actually a movement in the game industry to ensure people who started projects, had works derived from their work or a sequel created from their work, are given appropriate credit - http://www.igda.org/credit-standards.)