In the swamps of prehistoric Egypt, there was a dead giant. A Paralatitan, one of the largest sauropods to ever live, had been wounded by a pack of Rugops and now their work had payed off. The small abelisaurs walked towards the giant mountain of flesh, but they were nervous, for the smell of flesh was drawing deadlier carnivores. The pack leader walked forward nervously, but she couldn't see any large predators. She would have to be wary of Sarcosuchus, though. But her caution failed to pay off. Downwind of her, she was being watched by a predator, the largest of it's kind the world had ever seen.
The Rugops would never know what hit her. A Spinosaurus crashed out of the trees and sliced her into ribbons with one slash of it's titanic claws. The rest of the pack fled, unwilling to share their alpha's fate. The Spinosaurus turned to eat, but instead, it faced something unexpected. While the fight was progressing, another carnivore had come and was dragging off a massive chunk of meat. The Spinosaurus put it's foot down, quite literally, on the chunk of meat and leaned in towards this carnivore, ready to teach it that nothing and nobody messes with a Spinosaurus. The carnivore raised it's head and snarled defiantly. It was a Carcharodontosaurus, the only thing on the planet that the Spinosaurus was afraid of besides another member of it's species.
This particular Carcharodntosaurus was a large female, who was very angry, very hungry and not in the least bit afraid of the pharoah of prehistoric Egypt. She had, in the space of a month, lost her entire pack during a fight with another Spinosaurus, had her tail crushed by a sauropod, starved to the brink of death and lost an arm to the same pack of Rugops that the Spinosaurus had scared away. She would die if she didn't get this meat, and if she chose to fight the Spinosaurus, she was probably dead too. She had nothing to lose. The most dangerous animal is one that is injured and hungry, and the most dangerous foe is one who has nothing to lose. That Spinosaurus should have been afraid, very afraid. But it underestimated it's opponent, and that was it's fatal error.
With a roar that could be heard for miles, the Carcharodontosaurus lunged forward and grabbed the Spinosaurus by the throat. The surprised Spinosaurus whacked the Carcharodontosaurus, leaving deep gashes in her skull. They would take some time to heal. This broke her concentration long enough for the badly wounded Spinosaurus to pull away. The Spinosaurus make the next move, grabbing at the female's neck with her jaws and trying to bash her on the back of the neck. The Carcharodontosaurus pulled away and whirled around, trying to grab at the Spinosaurus's leg, but it shoved and threw her off balance. It reared up, ready to deliver the killing blow, but the female roared, lesning defiantly towards the Spinosaurus's skull and clamoing onto it. She dragged it towards the ground and it attempted to get up. She shoved it back down with her foot and it landed on it's sail, breaking it's spine. The Spinosaurus was paralyzed and the female dispatched it easily.
The scent of blood drew more Rugops, but they were content to wait in the shadows until an opportunity presented itsekf. The female gorged on both the Paralatitan and the Spinosaurus. Mosquitos in the swamp had been disturbed by the fight and they too were attracted to the blood. The Carcharodontosaurus shook her head to dislodge the little insects and one flew right into a tree and had the bad luck to get lodged in resin dripping down the trunk…
Unlike the other contestants, Carcharodontosaurus (I just DARE you to say that ten times fast!) has unique qualifications for replacing Spinosaurus in JPIV or JPV - it was Spinosaurus' mortal enemy in real life! They would have clashed over things like territory, food, water and getting too close to the eggs for comfort. Could they have won at one of these fights? Let us take a look at the fossil record.
Except for the now-destroyed holotype, all Spinosaurus remains are fragments, mostly skull fragments. Due to the fragmentary nature of Spinosaurus remains, we still haven't found evidence of injury by Carcharodontosaurus. Could it be that Carcharodontosaurus was responsible for smashing Spinosaurus jaws and creating the fragments in the first place? Without more intact specimens, we have no way of knowing if the two giants fought, but to believe two giant predators could comfortably live alongside each other is preposterous at best.
The thing about Spinosaurus is that it's strength was massively exaggerated in Jurassic Park III. The T. rex probably could have defeated it in real life, unless it was a juvenile as some have theorized. In reality, size would have given Spinosaurus some advantage over the smaller, faster, stronger Carcharodontosaurus, and those arms could have given a Carch one heck of a punch. I say they were evenly matched.
Go to the link at the top of the page to vote for top carnivore.
UPDATE: As it turns out, there is possible evidence for fighting between Carcharodontosaurus and Spinosaurus! In 2008, scientists found part of a Spinosaurus' sail had been bitten off by an unknown predator. The consensus is that the injuries were induced by a Carcharodontosaurus. Whether or not they proved fatal is unknown.