Above is a link to a scale representation of prehistoric sea creatures. For the looks of it Leedsichthys seems to be the biggest sea creature. Megalodon was estimated to be around 33 feet in length. The Liopluredon was an estimated 25 feet. The Sarcosuchus was estimated to be 37-40 feet. Thus leaving the Leedsichthys, Leedsichthys Problematicus was a giant pachycormid (an extinct group of bony fish) that lived in the oceans of the late Jurassic period, 165-155 million years ago. Only partial skeletons have been discovered, so its exact size is uncertain. Estimates range from 15 to 30 meters, but the best estimates believe it to have grown to around 20-22m (= 66 to 72 feet) long, making it the largest fish ever to exist. Hopefully this helps. By no means is this answer absolute I am not educated in Prehistoric Sea Creatures, it's just data gathered from searching the internet, as well as various National Georgraphic sites I have come across. There is a program that is hosted by Nigel Marvin called Prehistoric Sea creatures that touches upon all creatures listed above.
---The Megalodon has actually been recorded to be 55-60 feet in length, I also found this out from a Nat Geo site. I am also not a "Prehistoric Sea Creature" specialist.. but I've looked everywhere on the internet to find the definite world's largest. Just to put in "Predator X" or "Liopleurodon" measured up to 50 feet in length.
Leedsichthys problematicus is the largest prehistoric fish. Some scientists estimate it to be 20 meters (65 feet) long.
The Basilosaurus was certainly among the largest being 59 ft long.
megalodon
Basilosaurus
The largest prehistoric animal ever was the Basilosaurus which was a 59 foot whale. The largest crocodilian was the 39 foot long Sarcosuchus Imperator.
sinead of course
It is the Leedsichthys. It is over 90 ft.
No. Even the prehistoric Megaldon was smaller than the Blue Whales found in the ocean today. The blue whale is the largest animal on the planet.
t.rex
Titanoboa
Megaladon
yes it is an prehistoric animal
It can tell about prehistoric diets that the animal had.
The largest prehistoric mammal ever was the Indricotherium a prehistoric rhino like mammal the height of a modern giraffe and weighing 12 tons.
MegalodonOtodusPtychodusIsurus hastalisEdestus