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Hey Keith,

water based clays crack to 97% if they loose to much water.
The WED Clay EM 217 from Laguna is one of the only water based clays, which hold its shape very long and its cracking very late in the drying process.
Pottery- clays are done for drying complete without cracking.
The clay itself begins to shrink and if there is a hard core in the sculpture, which dant shrink as the clay does. the clay will crack. 

Did you use a hard core under your sculpt?

Kai is correct.  Water based clay will pretty much always crack on you as it dries out.  As the moisture evaporates out of the clay it begins to shrink and crack.  If it shrinks around a hard core, it will crack.  If it shrinks faster in some areas than others, it will crack. 

If you want the sculpture to last a bit longer (if you are still working on it) you can place damp paper towels on the surface and cover it with a plastic garbage bag to keep the moisture in.  You then need to check on it every few days, adding new moist paper towels.  You are always working agains the clock, as you can't keep that process going forever.  I've known sculptors who have kept a water based clay sculpture (usually WED clay which is a bit more stable) alive for months, but you have to care for it like a living thing.

Water based clays are not intended to last forever.  Some can be fired in a kiln to create pottery, others would have the be molded so you can cast replicas of the original sculpt.  

Sometimes a dried water clay sculpt can be saved by lightly misting water onto it, then storing it with the damp paper towels and garbage bag mentioned above.  This gives the moisture time to return to the clay, and would need to be repeated slowly until the moisture can penetrate deep into the clay.  That process will take a few days, possibly over a week.  If you add moisture too quickly the outer layers will turn to soup and all you will have is a mess.  You don't need a lot of moisture, just a lot of time.

If you just want a sculpture to keep around, you can work in an oil based or polymer clay.

Sorry you've run into these problems.  It's never fun to lose a sculpture (though there is a possibility this one can be saved), at least you gained experience!  

Here is a video from by friend Gordon Tarpley that covers rehydrating a dried out WED clay sculpture.
https://www.youtube.com/watch?v=jlIy50M43tw

Hope that helps!

/Chris