Spino attempts to break Rexes neck and tears its teeth out
Its not stupid; it won't try something it has no chance of achieving.
If Spinosaurus and Tyrannosaurus fought, I would have to give it to Trex. There is no way an animal built to eat fish can successfully kill a creature that has the strongest bite force in history. Yes, its like putting a Greyhound VS a Bull Dog.
I mean, its SPINE is right there. Oh ya, it allows them to intimidate other predators. But it also means they can die really, really easily. Plus the jaw size, shape, bite strength...
It may be longer, but that doesn't mean anything when you include its other flaws. Spino would probably run if it faced a Trex. Also, they aren't monsters: Animals will try to intimidate others before resorting to violence. So its not like a fight like that WOULD happen if they came face to face...