Leedsichthys

''Leedsichthys'' is an extinct genus of pachycormid fish that lived in the oceans of the Middle to Late Jurassic.Liston, JJ (2004). An overview of the pachycormiform ''Leedsichthys''. In: Arratia G and Tintori A (eds) Mesozoic Fishes 3 - Systematics, Paleoenvironments and Biodiversity. Verlag Dr. Friedrich Pfeil, München, pp 379–390. It is the largest ray-finned fish, and amongst the largest fish known to have ever existed.

The first remains of ''Leedsichthys'' were identified in the nineteenth century. Especially important were the finds by the British collector Alfred Nicholson Leeds, after whom the genus was named "Leeds' fish" in 1889. The type species is ''Leedsichthys problematicus''. ''Leedsichthys'' fossils have been found in England, France, Germany and Chile. In 1999, based on the Chilean discoveries, a second species was named ''Leedsichthys notocetes'', but this was later shown to be indistinguishable from ''L. problematicus''.

''Leedsichthys'' fossils have been difficult to interpret because the skeletons were not completely made of bone. Large parts consisted of cartilage that did not fossilize. On several occasions the enigmatic large partial remains have been mistaken for stegosaurian dinosaur bones. As the vertebrae are among the parts that have not been preserved, it is hard to determine the total body length. Estimates have varied significantly. At the beginning of the twentieth century, a length of was seen as plausible, but by its end ''Leedsichthys'' was sometimes claimed to have been over long. Recent research has lowered this to about for the largest individuals. Skull bones have been found indicating that ''Leedsichthys'' had a large head with bosses on the skull roof. Fossilised bony fin rays show large elongated pectoral fins and a tall vertical tail fin. The gill arches were lined by gill rakers, equipped by a unique system of delicate bone plates, that filtered plankton from the sea water, the main food source.

Along with its close pachycormid relatives '' Bonnerichthys'' and '' Rhinconichthys'', ''Leedsichthys'' is part of a lineage of large-sized filter-feeders that swam the Mesozoic seas for over 100 million years, from the middle Jurassic until the end of the Cretaceous period. Pachycormids might represent an early branch of Teleostei, the group most modern bony fishes belong to; in that case ''Leedsichthys'' is the largest known teleost fish.

Discovery and naming

During the 1880s, the gentleman farmer Alfred Nicholson Leeds collected large fish fossils from loam pits near Peterborough, England. In May 1886 these were inspected by John Whitaker Hulke, who in 1887 partially reported them as the back plates of the stegosaurian ''Omosaurus''. On 22 August 1888, the American dinosaur expert Professor Othniel Charles Marsh visited Leeds' farm at Eyebury and quickly concluded that the presumed dinosaurian armour in fact represented the skull bones of a giant fish. Within two weeks British fish expert Arthur Smith Woodward examined the specimens and began to prepare a formal description published in 1889.Woodward, Smith, A (1889). Preliminary notes on some new and little-known British Jurassic fishes. Geological Magazine Decade 3 Volume 6: 448–455. In it he named the species ''Leedsichthys problematicus''. The generic name ''Leedsichthys'' means "Leeds' fish", from Greek ἰχθύς, ''ichthys'', "fish". 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. After a second publication in 1889, objections were raised against the perceived "barbaric" nature of the generic name, which simply attached a non-Latinised British family name to a Classical Greek word. Woodward therefore in 1890 changed the genus name to ''Leedsia'', resulting in a ''Leedsia problematica''. However, by modern standards this is a non-valid junior synonym.

The holotype specimen, BMNH P.6921, had been found in a layer of the Oxford Clay Formation dating from the Callovian, about 165 million years old. It consists of 1133 disarticulated elements of the skeleton, mostly fin ray fragments, probably of a single individual. Another specimen, BMNH P.6922, contains additional probable fragmentary remains of ''Leedsichthys''. Woodward also identified a specimen previously acquired from the French collector Tesson, who had in 1857 found them in the Falaises des Vaches Noires of Normandy, BMNH 32581, as the gill rakers of ''Leedsichthys''. Another specimen bought in 1875 from the collection of William Cunnington, BMNH 46355, he failed to recognise.Liston, J.J., 2010, "The occurrence of the Middle Jurassic pachycormid fish ''Leedsichthys''", ''Oryctos'' 9: 1-36

Leeds continued to collect ''Leedsichthys'' fossils that subsequently were acquired by British musea. In March 1898, Leeds reported to have discovered a tail which he on 17 March 1899 sold for £25 to the British Museum of Natural History, which exhibited it as specimen BMNH P.10000; a new inventory number range was begun for the occasion. Already in July 1898, the front of probably the same animal had been bought, BMNH P.11823. On 22 July 1905 specimen BMNH P.10156 was acquired, a gill basket. In January 1915 Leeds sold specimens GLAHM V3362, a pectoral fin, and GLAHM V3363, the remainder of the same skeleton with 904 elements, to the Hunterian Museum of Glasgow.

Leeds had a rival, the collector Henry Keeping, who in 1899 tricked pit workers into selling dorsal fin rays by misinforming them that Leeds had lost interest in such finds. Keeping again sold these to the University of Cambridge where they were catalogued as specimen CAMSM J.46873. In September 1901, they were examined by the German palaeontologist Friedrich von Huene, who identified them as tail spikes, ''Schwanzstacheln'', of ''Omosaurus'', the second time ''Leedsichthys'' remains were mistaken for stegosaurian bones; Leeds himself was able to disabuse von Huene the same year.

In 2001, students at the Dogsthorpe Star Pit discovered a major new British specimen that they nicknamed "Ariston" after a 1991 commercial for the Indesit Ariston washing machine that claimed it went "on and on and on" — likewise the bones of ''Leedsichthys'' seemed to endlessly continue into the face of the loam pit. From 2002 until 2004 "Ariston" or specimen PETMG F174 was excavated by a team headed by Jeff Liston; to uncover the remains it was necessary to remove ten thousand tonnes of loam forming an overburden of thickness. The find generated considerable media attention, inspiring an episode of the BBC
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