Also not a lawyer, but I remember reading on /r/gamedev of a geme developer who got screwed over by his artist saying that he could no longer use the assets, or something like that; I believe the same principle applies here. Copyright belongs to the creator of the work unless explicitly stated otherwise.
In the U.S., copyright is automatically granted to the creator of the work, unless the work is a "work made for hire", in which case the copyright goes to the employer. When someone's artist asserts ownership over something they were paid to create, they're essentially challenging its status as a "work made for hire".
The Copyright Office has a circular that they distribute to clarify and help people decide, in general, whether certain types of work qualify as "works for hire" or not. [0]
Ouch! Seems that a company does need a goodwill explicit contract between partners so that in the case of a split or disagreement, the company can use the assets already created, even if copyright is still owned by the parties themselves. Or else all that work can be killed by one spiteful party (or one aggrieved one). Yet... in this case, would that be a good thing? Conundrum.
