Your situation sounds very similar to a leading (and probably controlling) case, Effects Associates v. Cohen. You can read it at http://openjurist.org/908/f2d/555/effects-associates-inc-v-cohen -- and I suggest you do, as it's a fairly entertaining opinion. The general rule is that ownership of a copyright can't be transferred by an oral agreement, but the parties' conduct can create a nonexclusive license. Effects Associates holds that an artist who "created a work at defendant's...
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The answer is... it depends. But we can at least pin down the issue by name: This is a question often referred to as "substantial similarity," or "nonliteral copying." The classic case that copyright professors make copyright students read is Nichols v. Universal Pictures. We copyright geeks call it the Abie's Irish Rose case. It was written in 1930 by Learned Hand, who in addition to having a great name, has a well deserved reputation for having written opinions that human beings...
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