Sheppard/Mastiff???

This good girl Nova was a rescue from Saskatchewan Canada. They said she was a Sheppard Mastiff. I'm thinking there has to be some Collie/ Boxer/Lab maybe.. she can normally be seen lounging in her favorite spot, the loveseat.. I know she has to be Sheppard because she always lies on her back with her little feeties in the air.. what do you guys think of Nova?