Leedsichthys was a giant pachycormid (An Extinct Group of Mesozoic Bony Fish) that lived in the oceans of the Middle Jurassic period. The closest living relative of the pachycormids is the bowfin, Amia calva, but this is only very distantly related. The name Leedsichthys means “Leeds’ fish”, after the fossil collector Alfred Nicholson Leeds, who discovered it before 1886 near Peterborough, England. The fossils found by Leeds gave the fish the species epithet problematicus, because the remains were so fragmented that they were extremely hard to recognise and interpret.
Unfortunately, although the remains of over seventy individuals have been found, these are usually partial and fragmentary. This has made it difficult to estimate its length. Arthur Smith Woodward, who described the specimen in 1889, estimated it to be 30 feet (around 9 metres) long by comparing the tail of Leedsichthys with another pachycormid, Hypsocormus. In 1986, Martill compared the bones of Leedsichthys to a pachycormid that he had recently discovered, but the unusual proportions of that specimen gave a wide range of possible sizes.
More recent estimates, from documentation of historical finds and the excavation of the most complete specimen ever from the Star Pit near Whittlesey, Peterborough, support Smith Woodward’s figures of between 30 and 33 feet (9 and 10 meters). Recent work on growth ring structures within the remains of Leedsichthys has also indicated that it would have taken 21–25 years to reach these lengths, and isolated elements from other specimens indicate that a maximum size of just over 53 feet (16 metres) is not unreasonable.
Like the largest fish today, the whale sharks and basking sharks, Leedsichthys problematicus derived its nutrition using an array of specialised gill rakers lining its gill basket to extract zooplankton from the water passing through its mouth and across its gills. There is little direct evidence for predation as opposed to scavenging on Leedsichthys remains, but specimen P.6924 in the Natural History Museum of London shows signs of bites from a Liopleurodon-sized pliosaur. These bites have then healed, indicating that Leedsichthys could even escape the top predator of the Oxford Clay seas, probably as a result of its powerful tail.
