But, given this is a PM tip post, a few pragmatic thoughts for PMs in this sort of situation..

1. Your feature team relationships are the most important you have.  When issues like this one come up it is often a symptom of a disconnect or mistrust across the feature team.  It should be a single to you to invest more in these relationships.  What is really going on? 

2. Keep the conversation focused on what needs to be done.  Don't fall into the trap of each person on the team just defining some "traditional" work.  Help guide the team to the point that everything that being done actually needs to be done by this team at this time.  For example, do you need exhaustive test cases while you are still incubating?  Do you need an API level spec while basic design tenants are still being established? 

3. Be the most flexible member of the team.   When you do get into dividing up what needs to be done, be the most open minded and flexible person.  For example,  if dev wants to own defining the APIs for the feature, fine, you pick another area to add value on and figure out a way to influence the design though customer feedback, app building, etc.