Keith,

That is incorrect. Shell actually predated case, although it was originally restricted to lower velocity smoothbore types with lower chamber pressures such as howitzers and mortars. Shell was no more restricted to being a cylindrical or rifled round than solid shot was.

Shell was used in ACW smoothbores and in fact made up a considerable portion of the ammunition chest of some types. About the only common ACW field smoothbore that didn't fire shell as a standard load was the U.S. 6 pounder gun (the 12 pounder gun--not Napoleon--was rare.) The CSA had trouble producing case (shrapnel) for its smoothbores and substituted shell, often with a polygonal cavity to produce even fragmentation. So for the CSA who used 6 pounders far longer, their most common 6 pounder explosive round appears to have been shell rather than case.

The 1864 US Field Artillery Tactics lists the following in each ammunition chest:
6 pounder gun: 25 shot, 20 case, 5 canister.
12 pounder Napoleon: 12 shot, 12 case, 4 shell, 4 canister
12 pounder howitzer: 15 shell, 20 case, 4 canister
(Note the lack of solid shot for howitzers.)

Shell with its larger powder charge was more dangerous for ground bursts. In the heavy naval smoothbores it could displace a lot more earth as well.