We got a thread debating how well Rexy goes up against the Indominus, so I figured, since the other Tyrannosaurs from TLW and JP3 would only do very comparable than the Queen of Nublar against the hybrid; why not call in the other super predator on the block? It's the Untameable Terror to Nublar vs. the Pharaoh of Sorna!
Now let's assume the fight is set up exactly the same way as in JW's climax, only instead of Rexy we get Spiny charging out of Paddock 9 and crashing through a Tyrannosaurus skeleton
because we're in an AU that JP3 was considered a classic because of plot convenience playhouse. And then, Spiny confronts and charges the Indominus.
The conditions beforehand are identical, as is passage of time. The Indominus is already injured from the fights with other dinosaurs, Owen, ACU, and the InGen mercs; but still in seemingly good condition. We're also assuming it won't be using its smarts or camouflage ability any more than it did in canon. Spiny is in peak form, but not exactly the same way we saw it fourteen years ago. It would be older than we saw it in JP3, thus a tid bit larger and aged, but not as past its prime as Rexy might have been. We'll say its 48 feet long, 18 feet at the head, and 9 tons. So size wise it's pretty much around the same size as the I.rex and thus a bit larger than Rexy; however not by any significant degree.
There are some changes however that will occur due to changing one combatant from Rexy to Spiny. For one, the Spinosaurus fights a bit differently. It wouldn't be using its jaws quite as heavily as Rexy did for obvious reasons. Now this is both a favorable and unfavorable change for our sail-back. It means Spiny wouldn't be able to land those big chomps that Rexy was able to dish out, as its jaws aren't as powerful. It could still headbutt, grasp, and grapple with its mouth; but not bite as hard. On the plus side, this means the I.rex's biggest advantage against Rexy, her claws, wouldn't be as lopsided as the Spinosaurus has just as strong a set it isn't shy about using. It also means the Indominus wouldn't be able to negate Spiny's attacks quite as well as she did Rexy's via restraining Rexy's neck or jaws. If she did that here, it would leave her open for Spiny to slash at her.
Spiny weighing a bit more is also a bit of a trade-off change. On one hand, it means that colliding with the Indominus means the force be in its favor more so than it was Rexy's. It also means tossing Spiny through the storefront would be a bit more difficult as well as any shoves Spiny gives her would have more momentum behind them. However, it could also mean Spiny would be a bit less agile in the fight.
Now let's have them fight. For the outcome I'll put five outcomes in the poll based on how well the Spinosaurus does.
Outcome
#1 - The Spinosaurus beats down the Indominus alone
This is the most optimal outcome for Spiny. Here he/she/it manages to not only go head to head against the hybrid, but overpowers it without need of a second wind. This could end either like the film, with the Spinosaurus lasting long and damaging the Indominus enough to shove her into the Mosasaur pool, or would the Spinosaurus mauling the Indominus to death either via slashing or breaking its neck like it did the JP3 Tyrannosaurus.
Outcome
#2 - The Spinosaurus beats the Indominus with help from Blue
Basically the movie outcome. Spiny loses in the first round but gets a reprieve thanks to Blue long enough to catch its second wind and fight again. Like
#1, this can either end way it did in the movie with the Mosasaur or Blue and Spiny kill the I.rex personally.
Outcome
#3 - The Spinosaurus can at least hold it off; but still loses solo
Bad ending. The Spinosaurus just can't deal or tank the same damage Rexy could and gets killed by the Indominus as it fights it alone, before Blue can show up to help.
Outcome
#4 - The Spinosaurus loses even with Blue's help
Worst ending. Even with Blue's help, Spiny just can't go the distance and loses to the Indominus soundly.
And before anyone says so, no I really don't think it matters that the Tyrannosaurus rex the Spinosaurus fought in the third film was a couple of feet smaller than the ones from TLW and JP/JW. That fight was pretty much a curbstomp in Spiny's favor given as nothing the Tyrannosaurus threw at it did much more than stall it. I've restrained from saying this on JPL to avoid sparking a flame-war, but I think it needs be said. Just because the Spinosaurus soundly trounced a "sub-adult" (it really wasn't, subadult would be more like 25-32 feet, not 37), doesn't mean it now auto-loses to a slightly larger adult. It's like saying you could lift a 100lb weight easily, but suddenly a 115lb weight would break your spine. IMNO, it's probably most accurate and easiest just to say the Jurassic Park universe Spinosaurus and Tyrannosaurus are just very comparable at adult age and either one of them could kill the other.