Size and Weight:
The "last" (i.e., species) name of Leedsichthys is "problematicus," which should give some clue about the controversy occasioned by this prehistoric fish. The problem is that, although Leedsichthys is known by dozens of fossil remains from around the world, they don't consistently add up to a convincing snapshot, leading to grossly divergent size estimates: more conservative paleontologists venture guesses of about 30 feet long and 5-10 tons, while others say superannuated Leedsichthys adults could attain lengths of over 70 feet and weights of over 50 tons. (This latter estimate would make Leedsichthys the largest fish that ever lived, bigger even than the giant shark Megalodon.)
We're on firmer ground when it comes to Leedsichthys' feeding habits. This Jurassic fish was equipped with a whopping 40,000 teeth, which it used not to prey on the larger fish and marine reptiles of its day, but to filter-feed plankton. By opening its mouth extra-wide, Leedsichthys could gulp in hundreds of gallons of water every second, more than enough to cover its outsized dietary needs.
Tantalizingly, an analysis of one Leedsichthys fossil hints that the individual may have been attacked by the giant marine reptile Liopleurodon--and lived to see another day!