Indiana Code Title 36. Local Government § 36-3-1-5

Sec. 5. (a) When a first class city becomes a consolidated city, the officers who become the executive and legislative body of the consolidated city under section 4(c) of this chapter also become the executive and legislative body of the county.

(b) The members of the board of commissioners of the county are entitled to remain in office until their terms expire, although the board is no longer the executive of the county.  As their terms expire or their positions become vacant, they shall be replaced by the following officers in the following order:

These three (3) officers then serve ex officio as commissioners under IC 36-3-3-10.