What if Jurassic Park featured a Spinosaurus instead of a T-Rex?

Well I feel there is two ways to look at this...

1. In Michael Crichton's novel the primary Dinosaur antagonist is the T.Rex, so adapting the novel for the big screen and giving that role to another Dinosaur would have been a dishonour to the original narrative.

2. On the other hand the T. Rex has been used as the defacto Dinosaur in any movie featuring the prehistoric creatures; think Journey to the Centre of the Earth, The Lost World (not the JP one), even 1933's King Kong. While its understandable that the creature is popular, had Jurassic Park used a Spinosaurus, a Ceratosaurus or an Allosaurus instead it could have worked just as well because its how the creature was depicted and executed on film that drives the suspension and fear, not what the creature looks like.

At its core, when you really strip it down to basics, Jurassic Park is no different to a slasher movie, 1987's Predator, or even 1979's Alien, all of which are just rehashing countless B-movies from the 1950's; Hollywood's most overused narrative premise (just think how many movies the following applies to)...

"A small group within an inescapable environment being eliminated one-by-one by an unseen antagonist."

Narratively whichever Dinosaur was used wouldn't have affected the scenes in which the T. Rex featured. The T.Rex was chosen and used for the same reasons that Velociraptors, Triceratops and Brachiosaurs were used; marketing. They used the most popular, most well known Dinosaurs to attract the widest audience possible.

I had always wondered about this. Well, as Gavin said the T. rex was already a popular villain in Hollywood movies.

And like Gavin said, the novel featured the T. rex as one of the main dinosaur antagonists. When adapted to a movie, of course Spielberg probably didn't want to change too much from the book, even though me personally 95% of the time I actually ENJOY changes made from book to the movies.

And a change like this if it had been done right would be an excellent example- the movie and the book would be even more distinct experiences. Whenever a movie is exactly like its book, I actually feel sad since I didn't get to experience something new. A Spinosaurus would have been just as cool (again, if done right.)

Then my generation may have had Spinosaurus as their personal "king" if it had been this way... Interesting.

Then again the original Jurassic Park aimed at scientific accuracy (well for the most part...) and since little was known about Spinosaurus at the time, Crichton and the film makers probably didn't want to choose a dinosaur with little scientific information about it that could easily change within a few years to be the primary antagonist.

If Spinosaurus was more well known even at the time, it still wouldn't trump T. rex, Spinosaurus being a fish eater and all.

I feel it'd be fine. If the movie was good (which it was) and the people making it knew what they were doing, then yes, it'd be cool. Plus this was pre-big-fish-eater-JP/// T-Rex killer- era. I think it'd be just fine. If anything, I think it'd be more intimidating. 

JPC, it would depend on which Spino they use: JP3 or the 2014 reconstruction.

T-Rex wasn't the main antagonist; the Raptors were.  She was merely the anti-heroine.

Though i like Spinosaurs better than a T-rex (Spino was the first dino i knew) it wouldn't have had the same impact on the public as T-Rex was and is still the most pouplar dino in history.

 I feel that the sheer heaviness and integrity of the T-Rex's skeleton makes him a very imposing/powerful character in the film. Not many dinosaurs exhibit such toughness, including Spinosaurs.