"So, it isn't possible for a cat to have a similar nasal cavity of a fox?"
I have no idea. But that is not the issue.
You would never look at just one isolated point like just sinus cavities, but a whole bunch of key points put together (plus shape of gross structures like hips, joints, front "paws," eye location (font side, distance from each other, distance from nose, depth of socket, etc.) to make a conclusion like that.
The "one point" was simply a question as to whether it is possible that "details" makes the species (especially considering that such details vary so widly between races of the same species.)