The question isn't how plausible it is for them to attack each other, or whether it could have happened in real life. One could argue the same about superheroes duking it out. I'm freely disregarding the specifics of what happened in JPIII, just because it's obvious the specific way it happened in the movie is unrealistic. (And not the only inconsistency--the raptors' legs dislocated when they walked, the Pteranodons had teeth...)
I find it annoying just because it seems like most people who prefer Spinosaurus, do so because Jack Horner says it would happen that way. I mean c'mon... really? But the number of other paleontologists who disagree with him apparently means nothing. I've yet to see him address many of the points brought up in Tyrannosaurus's favor, which frankly smacks of personal bias.
* One of Horner's favorite arguments seems to be that T. rex was just a wimpy scavenger, and so couldn't possibly win. Hunting behavior
does not matter in this kind of scenario to begin with, but for the sake of the argument let's think about this for a moment. It almost seems like Horner has made the mistake of assuming that an inclination to scavenge == devoted scavenging behavior == weakness. Even animals that
are normally considered scavengers tend to be opportunists... even turkey vultures will snatch squirrels out of trees. The really ironic part is that Spinosaurus is probably *more* suited to scavenging than T. rex. That long, low snout with the nostrils set back on the face would be excellent for digging around in dead dinos' guts, as some paleontologists (eg. Darren Naish) have suggested in the past.
* Ecology and basic logistics support this. T. rex was the only really big carnivore in its environment, so for all intents and purposes it
should be the apex predator. The German Shepherd-sized raptors that lived alongside it certainly weren't taking on elephant-sized hadrosaurs and ceratopsians on a regular basis, but something had to be keeping their numbers in check. T. rex was the only animal that really fit the bill.
* In contrast, Spinosaurus lived alongside other large predators, eg. Carcharodontosaurus and various abelisaurs. How do you fit multiple very large predators in the same environment? You stick them in different niches so they won't compete for the same resources. So which seems like the more likely all-around apex predator out of this bunch? It must be either Carcharodontosaurus, which is a dead ringer for South America's own apex predator Giganotosaurus, and uses the same body plan as the stegosaur-slaying Allosaurus. Or it would be Spinosaurus, with the crazy long snout and other peculiarly specific adaptations, whose best-known relative was found with half-digested fish scales and juvenile dino bones in its gut.
* Then there's just basic morphology. Spinosaurus's neck--if it was anything like its close relative Baryonyx--was long, slim, and not very flexible. Its jaws had inferior leverage given their length and thinness, and its jaw muscles just wouldn't have had the same amount of crushing power. T. rex's feet were about twice as big as those of same-sized Acrocanthosaurus (another relative of Carchar). It was the bulldog of theropods. Its smaller dimensions only mean that it had a lower center of gravity, which would actually have been an advantage. Its worst weakness would probably be its tiny arms. Maybe a Spinosaurus could win
if it managed to pin the T. rex down. That WAS one advantage Spinosaurus had, was that it had pretty hefty forelimbs. It certainly couldn't have used them the way it did in JPIII, but it would have been able to pick itself up off the ground if it tripped, or break a fall. Tyrannosaurus, not so much. But of course, Spinosaurus would have to be able to get Tyrannosaurus on the ground in the first place.
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