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 was 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, Argentina 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 with a unique system of delicate bone plates that filtered plankton from sea water as 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.Hulke, J.W., 1887, "Note on some Dinosaurian remains in the collection of A. Leeds Esq., of Eyebury, Northamptonshire", Quarterly Journal of the Geological Society of London, 43: 695-702 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,Woodward, A.S., 1889, "Notes on some new and little-known British Jurassic Fishes", Annals of the Magazine of natural History, series 6, 4: 405-407 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.Woodward, A.S., 1890, "Note on the gill-rakers of Leedsia problematica – a gigantic fish from the Oxford Clay", Geological Magazine December III(7): 292-293 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.Gunter, M., 2005, Conservation of P.10000: Leedsichthys problematicus. 35 pp.; London (RCA/V&A Conservation) 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, 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,Huene, F. von, 1901, "Notizen aus dem Woodwardian-Museum in Cambridge", Centralblatt für Mineralogie, Geologie und Palaeontologie 1901: 715-719 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.Douglas Palmer & Hermione Cockburn, 2012, The Fossil Detectives: Discovering Prehistoric Britain, Google eBook, p. 146 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.Dawn, A., 2004, "Leedsichthys problematicus", Geology Today 20: 53-55Dawn, A., 2004, "Leedsichthys problematicus", The Mercian Geologist, 16(1): 43-45 The find generated considerable media attention, inspiring an episode of the BBC Sea Monsters series, "The Second Most Deadly Sea", and a Channel Four documentary titled The Big Monster Dig, both containing computer-generated animated reconstructions of Leedsichthys. Liston subsequently dedicated a dissertation and a series of articles to Leedsichthys, providing the first extensive modern osteology of the animal.Liston, J.J., 2007, A Fish Fit For Ozymandias?: The Ecology, Growth and Osteology of Leedsichthys (Pachycormidae, Actinopterygii), Unpublished PhD Thesis. 464 pp. Faculty of BioMedical & LifeSciences, University of Glasgow, Scotland
Apart from the British discoveries, finds of a more fragmentary nature continued to be made in Normandy, France. In July 1982, Germany became an important source of Leedsichthys fossils when two groups of amateur palaeontologists, unaware of each other's activities, began to dig up the same skeleton at Wallücke. Remarkably, parts of it were again incorrectly identified as stegosaurian material, of Lexovisaurus.Michelis, I., Martin Sander, P., Metzdorf, R. & Breitkreuz, H., 1996, "Die Vertebratenfauna des Calloviums (Mittlerer Jura) aus dem Steinbruch Störmer (Wallücke, Wiehengebirge)", Geologie und Palaeontologie in Westfalen 44: 5-66 From 1973 onwards, fragmentary Leedsichthys fossils were uncovered in Chile. In March 1994, a more complete specimen was found, SMNK 2573 PAL. In 1999 the Chilean finds were named as a second species, Leedsichthys notocetes, the "Southern Sea Monster".Martill, D.M., Frey, E., Caceras, R.P. & Diaz, G.C., 1999, "The giant pachycormid Leedsichthys (Actinopterygii) in the southern hemisphere: further evidence for a Jurassic Atlanto-Pacific marine faunal province", Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1999: 243-256 However, Liston later concluded that the presumed distinguishing traits of this species, depressions on the gill rakers, were artefacts caused by erosion; Leedsichthys notocetes would be a junior synonym of Leedsichthys problematicus.Liston, J.J., 2013, "The plasticity of gill raker characteristics in suspension feeders: Implications for Pachycormiformes", In: G. Arratia, H.-P. Schultze & M. V. H. Wilson (eds.) Mesozoic Fishes 5 – Global Diversity and Evolution pp 121-143, Verlag Dr. Friedrich Pfeil, München
Fossil range The fossil remains of Leedsichthys have been found in the Callovian of England (Oxford Clay) and northern Germany, the Oxfordian of Chile (El Profeta Formation & Cerro Campamento Formation), and the Callovian and upper Kimmeridgian of France (Calcaire de Creully Formation).Liston, JJ (2008a). Leedsichthys des Vaches Noires ... au peigne fin (translation by M-C Buchy) L’Écho des Falaises (=Ech.des Fal.) No.12: 41–49, 2008 ISSN 1253-6946. These occurrences span a temporal range of at least five million years. A complete and isolated gill raker from the Vaca Muerta formation of Argentina (MOZ-Pv 1788), has been assigned to the genus and dates to the early Tithonian.
Description
Although the remains of over seventy individuals have been found, most of them are partial and fragmentary. The skeleton of Leedsichthys is thus only imperfectly known. This is largely caused by the fact that many skeletal elements, including the front of the skull and the vertebral centra, did not ossify but remained cartilage. Furthermore, those that did ossify were gradually hollowed out during the lifetime of the animal by resorption of the inner bone tissue. In the fossil phase, compression flattened and cracked these hollow structures, making it extraordinarily difficult to identify them or determine their original form.