The largest fish that ever lived is Leedsichthys problematicus. It is thought to have grown to lengths of 22 m (72 ft), about the length of a school bus, although this estimate cannot be certain because no complete spine has been found. Regardless, it is quite certain that Leedsichthys is the largest fish to have ever lived that scientists are currently aware of. It is more than twice the size of the whale shark, the largest fish in the seas today. The closest living relative of Leedsichthys is the bowfish.
Leedsichthys problematicus was discovered by fossil collector Alfred Leeds, and the species was named in his honor. Leedsichthys means “Leed’s fish,” while problematicus is a reference both to the difficulty in imagining a fish of this size, and the difficulty of classifying it once it was found. Leedsichthys was a pachycormid, an extinct group of bony fish. Shortly it was discovered, it was realized it was the largest fish that ever lived.
Fossils of the fish have found in England and Germany have been dated to 155 million years ago, during the late Jurassic era, when dinosaurs roamed the Earth. Like the whale shark and numerous whales, Leedsichthys was a planktivore, meaning it exclusively consumed plankton. It would have been preyed upon by marine carnivores of the time, such as the plesiosaur Liopleurodon.
One of the biggest Leedsichthys fossils was found in Peterborough, England, where it was discovered as a tangled mass of fractured bones. Unlike large dinosaurs, Leedsichthys had delicate bones, many of which were crushed by the weight of clay over millions of years. One of the largest specimens, called Big Meg, filled over 20 museum drawers with its bones. Its tail alone included over 10,000 fragments.
Investigations of Leedsichthys fossils have shown that 8 – 10% of its body mass was compromised of organic material it consumed, like algae and plankton.
Pachycormids like Leedsichthys were eventually replaced by a new group called teleosts, which comprise 95% of all living fish today. Teleosts produced larger numbers of eggs, focusing less care on each individual, and this evolutionary strategy has proved successful. However, no teleost can claim the title of largest fish ever.