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Big thanks to Concaventor Wrangler, DragonSaber73, Michael Benucci, and Therapod King for helping create this list.

10th - Helicoprion
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Helicoprion is most known for its extremely bizarre "tooth whorl." While named in 1899, paleontologists initially had no way of knowing what the whorl was used for or even its position on the body. However, recently it has been determined that this whorl was actually the animal's teeth. Being a cartilaginous fish, very little of Helicoprion has been preserved, so not much is known about it. 
9th - Basilosaurus
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Basilosaurus was a large predatory whale from the Eocene. Due to its "serpentine" physique, it was originally though to have been a Mosasaur. The brain of Basilosaurus was not as large as that of modern whales. It also lacked the "melon" which is used for communication and echolocation. It possessed small hind limbs, which are not visible in modern whales, although the bones still exist in some species. Basilosaurus was an apex predator, probably feeding on smaller whales such as Dorudon

8th - Kronosaurus
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Kronosaurus was a genus of Pliosaur, which is a family of Plesiosaurs known for proportionally shorter necks and more robust bodies. It that lived in shallow seas across the Pacific Ocean during the Mid Cretaceous. It was an apex predator, and is known to have fed on smaller Plesiosaurs. It was also extremely large, growing to nearly 11 meters in length. Some consider it to be the largest of all Plesiosaurs. 
7th - Dunkleosteus
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Dunkleosteus was a genus of Placoderm, an entire class of fish that has been extinct since the end-Devonian, around 360 million years ago. These ancient fish had osteoderms, plates of exposed bone that could have served as protection. Dunkleosteus was one of the largest, and probably the most powerful. It's bite force has been estimated at 750 kg. Because of this, it was probably a hypercarnivore, feeding on tough prey like ammonites or other placoderms. 
6th - Pliosaurus
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While the aforementioned Kronosaurus is considered by some to be the largest Pliosaur, Pliosaurus is the more common selection. The largest species, P. funkei, which has received the somewhat ominous popular name "Predator X," could have reached anywhere from 10 to 13 meters in length. It probably fed primarily on marine reptiles like smaller Plesiosaurs. 
5th - Livyatan
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While Livyatan was named fairly recently, in 2010, it has already become quite popular. It was a genus of Physeteroid whale, the same group that contains modern sperm whales. However, it would have been more similar to raptorial sperm whales such as Zygophyseter.  It was an apex predator. Although it had to coexist with the large shark C. megalodon, some paleontologists believe Livyatan was the behaviorally dominant of the two. Current estimates suggest large Livyatan could reach 18 meters in length. It's teeth are also the largest teeth any known animal has ever used for eating, although, elephant tusks are larger. 
4th - Anomalocaris
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Anomalocaris is by far the oldest genus on this list. It lived during the Early-Mid Cambrian, which is the oldest of the Paleozoic periods. It is possibly one of the first apex predators. Some have suggested it fed on ancient soft-bodied organisms. The eyes of Anomalocaris are also extremely sophisticated. 
3rd - Elasmosaurus
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Plesiosaurs are some of the most iconic of all prehistoric aquatic predators, and Elasmosaurus is one of the most iconic of the Plesiosaurs. It is the type genus of Elasmosauridae, a family known for relatively long necks and comparatively short tails. They also possessed eyes facing upward, meaning that their depth perception in this direction would have been excellent. This suggests that they typically stayed lower in the water column than their prey. While Elasmosaurus was a very large predator, reaching lengths in excess of 10 meters, it also seems to have been attacked occasionally by the prehistoric shark Cretoxyrhina
2nd - Mosasaurus
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Large Mosasaurs like Mosasaurus were the aquatic apex predators of the Late Cretaceous. Perhaps exceeding 13 meters in length, Mosasaurus was one of the largest, and possibly the largest, of all the Mosasaurs. It probably fed primarily on smaller marine reptiles and large fish such a Xiphactinus . Mosasaurus has also become famous in popular media, due in part to its slightly exaggerated reconstruction in the film Jurassic World
1st - Megalodon
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The top entry on the list is a little unusual because C. megalodon is a single species as opposed to a genus. However, it easily stands out more than any other prehistoric aquatic predator. It is certainly the most famous prehistoric aquatic predator, and among the most famous of all prehistoric animals, being featured frequently in games and popular media. It is also well known for possibly being the largest fish ever to exist.

In life, C. megalodon was an apex predator, feeding primarily on whales. Its bite force has been estimated to have been at least 10 tonnes. It also seems to have rammed prey from below, perhaps to stun them, as the fossilized bones of whales occasionally show signs of serious impact wounds.