Leedsichthys

Leedsichthys was a giant pachycormiform actinopterygian fish that lived during Middle Jurassic period, and is known from the Callovian Oxford Clay Formation.

The generic 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 specific epithet problematicus, because the remains were so fragmented that they were extremely hard to recognize and interpret.

The remains of Leedsichthys have been found in the Callovian of England, northern Germany and France, the Oxfordian of Chile, and the Kimmeridgian of France.

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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 9 and 10 metres. Recent work on growth ring structures within the remains of Leedsichthys have 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 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.