Supervising Art Director and Art Director are both valid credits.
What happens with the Senior Art Directors?
Credit them or don't?

The answer is in the rules:

The table below gives each of the crew roles available within DVD Profiler. For each category, include only those people credited with the roles listed in the "Role" and "Credited As" columns. If someone is not credited with one of these roles (or direct translations of these roles), do not include them in the Crew section.

Not in the list, not contributable... Stupid, yes, but we are requested to follow rules without thinking... 

To this special case: I see no reason why - if a job is divided between more people - not any one of them should be credited. In fact there are six art directors and the studio decided to credit them in a three level hirarchy - could be based on pride, envy or other deadly sins 

Quoting AiAustria:

Quote:
Thinking is not forbidden in the rules. 

As long as there is a common sense, thiniking should be the base of anything. Stepping back and hiding behind the rules is only a way to cope with argueing and quarreling beond any sense and boundary...

I totally agree with that. That is why I often asked in the past to correct obvious spelling mistakes (such as Harisson Ford...), and, even more, never add spelling mistakes. Unfortunately, I was always told that rules are rules, and exactly means exactly. I'm the first to ask to use common sense, but that means everywhere, not only where people want for them (and of course not for others...)

About this problem, you could have a look at this old topic