This is why I am always ammused by people spouting "scientific fact" about dinosaurs on threads like these. Like how "a spinosaurus could never beat a t-rex in a fight because the spinosaurus' skeleton was too frail to allow it to beat the t-rex". Really? You know this how exactly? Doc Brown loan you his Delorian? You hit 88 miles per hour and go back in time to research living dinosaurs first hand? No you didn't. You learned everything from books, books written by people who guess at what dinosaurs were like and wrote down their opinions.
I think any paleontologist reading this paragraph would be insulted to see their work trivialized to such a degree.
Yes, much of what we know is theory, but a theory is HARDLY a "guess". It's a stance based on, and supported by countless tests, studies, and analysis. For instance, we KNOW that a Spinosaur could not survive the bite to the neck shown in JP3 because countless scientists have analyzed Rex skulls to see how and where muscles joined, analyzed the mechanics of the skull, jaws, and teeth, this coupled with all other knowledge born from studying the Rex anatomy and basic biology and physics gives scientists a damn good idea as to the strength of a t Rex bite: 30,000-60,000 newtons. They're also able to do similar analysis with the rest of the body, and studies have only validated the findings: the t Rex had the most powerful bite of any land animal, and it's teeth, skull, neck...it's entire body was designed to take advantage of that fact: without any hyperbole, the Rex was built to literally destroy whatever it bit. Again, that is hardly a guess. And as technology improves and more information is learned, these findings have only been validated more, or INCREASED. We also know that the rex had binocular vision while the spinosaur did not, which is obviously a huge advantage when you're in a face to face match.This knowdledge, coupled with the same supported-by-evidence knowledge of Spinosaurus being a slender boned (comparied to rex any way) less active dinosaur with a much less damaging bite and a thinner, weaker neck lends a heck of a lot of credence to the Rex v. Spino debate.
Could a spinosaur defeat a Rex? Anythings possible I suppose, but pound for pound, point by point, one would be a fool to ever bet against the tyrannosaur in that scenario.
Yes, there are plenty of biological and situational variables that we don't know or can't consider, but based on the proven knowledge we have and comparing it the fight we saw in the film, when the Rex got its jaws around the spinosaur's neck, we should have seen a nearly headless Spino drop to the ground. At the VERY least, it would have countless blood vessels punctured and, muscles destroyed, and crushed vertebrae and trachea...at the absolute minimum it would have immediately bled out and suffocated. That's not guesses, that's applying scientific knowledge (established by analysis and verifiable evidence) to the actions depicted in the movie.
It's just as likely that the t-rex was a scavenger as it is that the t-rex was a hunter. Point is until we master time travel and can observe them in person, we may never know the truth.
Yes, nothing beats the seeing the real thing, but I think that you misunderstand or at least underestimate the amount of knowledge we are able to discover from the fossil record...there's a difference between knowing a dinosaurs actual behavior, and knowing what they're capable of.
like I mentioned before, advancements in research technology has allowed us to gain an unprecedented amount of knowdledge about dinosaur biology. Sure, some things we are unable to know for certain without actually seeing the organism, like the predator/scavenger debate you mention, but we ARE able to know what each dinosaur is capable of to an ever increasing degree, which points us in the right direction in terms of their behavior. There is also supplemental evidence that allows us to get a clearer picture on behavior (ie: many Rex fossils show evidence of healed wounds and broken bones, and there's hadrosaur fossils found with broken rex teeth and massive wounds that had begun to heal before death...both scenarios suggest an active predatory life style in one degree or another.
In short, to use you're term "guess"...scientists may make EDUCATED guesses (supported by biological evidence) about dinosaur behavior since action is very rarely something that can be fossilized; but what an animal is physically capable of (speed, strength, ability, sensory abilities etc) can, and is actively studied and understood to an astonishing degree.