Leedsichthys Problematicus

''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 ''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''. 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 and northern Germany, the Oxfordian of Chile, and the Callovian and upper Kimmeridgian of France.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.

The head was probably relatively large and wide but still elongated. The snout is completely unknown. Frontal bones are absent. The skull roof is rather robust with bosses on the parietals, continuing sideways over the dermopterotica, and the postparietals. The parietals have a notch on the front midline. A dermosphenoticum is present above the eye socket. The jaws are toothless. Behind the jaw joint a robust hyomandibula is present. The gill basket rests on paired hypohyalia. At least the first two gill arches have ossified hypobranchialia, the lower parts of the gill arch; a third hypobranchiale was likely present. The hypobranchials are attached at their lower ends at an angle of 21,5° via a functional joint that possibly served to increase the gape of the mouth, to about two feet. All five gill arches have ossified ceratobranchialia with a triangular cross-section, the middle sections of the arches. The hypobranchials are fused with their ceratobranchials. The fifth gill arch is fused with the front parts of the basket. Higher epibranchialia and pharyngobranchialia are present but poorly known. The fourth arches are supported by a midline fourth basibranchiale. An ossified operculum is present.Liston, J.J., 2008, "A review of the characters of the edentulous pachycormiforms ''Leedsichthys'', ''Asthenocormus'' and ''Martillichthys'' nov. gen.", In: ''Mesozoic Fishes 4 Homology and Phylogeny'', G. Arratia, H.-P. Schultze & M. V. H. Wilson (eds.): pp. 181–198, 10 figs., 1 tab. © 2008 by Verlag Dr. Friedrich Pfeil, München, Germany –

The gill arches are equipped with rows of parallel 3-to-12-centimetre-long (1.2-to-4.7-inch-long) gill rakers, in life probably attached to the ceratobranchials via soft tissue. On the top of each raker one or two rows of dozens of low "teeth" are present. When there are two rows, they are placed on the edges of the upper surface and separated by a deep trough, itself separated from an internal hollow space by a transverse septum. The teeth or "fimbriations" are obliquely directed towards the front and the top. They are grooved at their sides, the striations continuing over the sides of the raker. Detailed study of exquisitely preserved French specimens revealed to Liston that these teeth were, again via soft tissue, each attached to delicate 2-millimetre-long (0.08-inch-long) bony plates, structures that had never before been observed among living or extinct fishes. An earlier hypothesis that the striations would function as sockets for sharp "needle teeth", as with the basking shark, was hereby refuted. The rakers served to filter plankton, the main food supply of ''Leedsichthys'', from the sea water.

Large parts of the ''Leedsichthys'' fossils consist of bony finrays. ''Leedsichthys'' has two pectoral fins that probably were located rather low on the body. They are large, very elongated — about five times longer than wide — and scythe-like, with a sudden kink at the lower end, curving 10° to the rear. Also a dorsal fin is present, although its position is unknown. Pelvic fins at the belly are lacking; also a pelvic plate is absent. However, there are indications for a small triangular anal fin. The vertical tail fin is very large and symmetrical with paired upper and lower lobes; there is a smaller lobe in the middle protruding between them. The rays are unsegmented lepidotrichia, resulting in a rather stiff structure. They are bifurcated at up to three splitting points along their length, so a proximally single ray may have eight distal ends. A row of bony supraneuralia is present behind the head, at each side of the vertebral column. Uroneuralia at the tail are unknown. No bony scales are present.

Size

''Leedsichthys'' is the largest known member of the Osteichthyes or bony fishes. The largest extant non-tetrapodomorph bony fish is the ocean sunfish, ''Mola mola'', being with a weight of up to two tonnes an order of magnitude smaller than ''Leedsichthys''. The extant giant oarfish might rival ''Leedsichthys'' in length but is much thinner. The lack of a preserved vertebral column has made it difficult to estimate the exact length of ''Leedsichthys''. Arthur Smith Woodward, who described the type specimen in 1889, estimated specimen BMNH P.10000 to be of an around nine metre long individual, by comparing this tail of ''Leedsichthys'', having a preserved height of , with another pachycormid, '' Hypsocormus''. The length of ''Leedsichthys'' was not historically the subject of much attention, the only reference to it being made by Woodward himself when he in 1937 indicated it again as on the museum label of BMNH P.10000. However, in 1986, David Martill