Leedsichthys is an extinct genus of pachycormid fish that lived in the oceans of the Middle to Late Jurassic. It was the largest ray-finned fish, and amongst the largest fish known to have ever existed. 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. In it he named the species Leedsichthys problematicus. The generic name Leedsichthys means "Leeds' fish", from Greek ἰχθύς, ichthys, "fish". 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.

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, Glasgow. the second time Leedsichthys remains were mistaken for stegosaurian bones; Leeds himself was able to disabuse von Huene the same year. 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.

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". However, Liston later concluded that the presumed distinguishing traits of this species, depressions on the gill rakers, were artefacts caused by erosion;

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. These occurrences span a temporal range of at least five million years.

Description

left|thumb|350x350px|Modern speculative [[life reconstruction of Leedsichthys aided with pachycormid relatives. While much of the skeleton is known, it's too poorly preserved and difficult to interpret, so its exact appareance cannot be determined with certainty ]]

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 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. an order of magnitude less than Leedsichthys. The giant oarfish (Regalecus glesne) might rival Leedsichthys in length, but has a much thinner body width. The lack of a preserved vertebral column has made it difficult to estimate the exact length of Leedsichthys. 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 compared the bones of Leedsichthys to a pachycormid that he had recently discovered, Asthenocormus. The unusual proportions of that specimen gave a wide range of possible sizes. Subsequently, a length of thirty metres (hundred feet) was often mentioned in popular science publications, sometimes one as high as .

Liston in his studies concluded to much lower estimates. Documentation of historical finds and the excavation of "Ariston", the most complete specimen ever from the Star Pit near Whittlesey, Peterborough, support Woodward's figures of between . With "Ariston" the pectoral fins are apart, indicating a narrow body of no excessive size, even though it was initially thought to have been long. In 2007 Liston stated that most specimens indicated lengths between . A linear extrapolation from the gill basket would be flawed because the gills grow disproportionally in size, having to increase their surface allometrically to ensure the oxygen supply of a body increasing in volume to the third power. The growth ring structures within the remains of Leedsichthys have indicated that it would have taken 21 to 25 years to reach these lengths, and isolated elements from other specimens showed that a maximum size of just over is not unreasonable.

In 2013, Liston and colleagues estimated that the age of the five specimens (PETMG F174, NHMUK PV P10000, GLAHM V3363, NHMUK PV P6921 and NHMUK PV P10156) would have ranged between 19 and 40 years old. The largest specimen, NHMUK PV P10156, on the basis of its gill basket with a preserved width of and height of , would have been 38 years old (2 years younger than the holotype NHMUK PV P6921) and measured long. In 2018, Ferron and colleagues estimated that this specimen would have weighed .

Phylogeny

Woodward initially assigned Leedsichthys to the Acipenseroidea, considering it related to the sturgeon, having the large gill rakers and branching finrays in common. In 1905, he changed this to the Pachycormidae. The Pachycormidae have a somewhat uncertain position. Often they are considered very basal Teleostei—if so, Leedsichthys would be the largest known teleost—others see them as members of a Pachycormiformes forming the sister group of the Teleostei, and sometimes they are seen as even more basal Amiiformes. In the latter case the extant bowfin, Amia calva, would be the closest living relative of Leedsichthys.

Within the Pachycormidae, a cladistic analysis found Leedsichthys to be the sister species of Asthenocormus, their clade being the sister group of Martillichthys.

Paleobiology

Like the largest fish today, the whale sharks and basking sharks, Leedsichthys problematicus derived its nutrition as a suspension feeder, using an array of specialised gill rakers lining its gill basket to extract zooplankton, small animals, from the water passing through its mouth and across its gills. It is less clear whether also phytoplankton, algae, were part of the diet. Leedsichthys could have been a ram feeder, making the water pass through its gills by swimming, but could also have actively pumped the water through the gill basket. In 2010, Liston suggested that fossilised furrows discovered in ancient sea floors in Switzerland and attributed to the activity of plesiosaurs, had in fact been made by Leedsichthys spouting water through its mouth to disturb and eat the benthos, the animals dwelling in the sea floor mud. Also problematic is how Leedsichthys could increase its size quickly during the first year of its life. Teleostei typically lay relatively small eggs and this has been seen as an obstacle for them attaining giant sizes.

In 1986, Martill reported the presence of a tooth of the marine crocodile Metriorhynchus in a bone of Leedsichthys. The bone would have healed, a sign that the about Metriorhynchus was actively hunting the much larger fish. However, in 2007 Liston concluded the bone tissue had not in fact healed and that this was probably a case of scavenging. A 2.5 m-long specimen FBS 2012.4.67.80, assigned to Metriorhynchus cf. superciliosus, was found with the gill apparatus of Leedsichthys and remains of invertebrates inside its stomach. Such content indicates that the diet of metriorhynchids was varied, and this individual most likely ate already dead fish. An apex predator of the Oxford Clay seas large enough to attack Leedsichthys was the pliosaurid Liopleurodon.

In 1999 Martill suggested that a climate change at the end of the Callovian led to the extinction of Leedsichthys in the northern seas, the southern Ocean offering a last refuge during the Oxfordian.

Recent studies have uncovered some estimations regarding metabolic rate and speed for Leedsichthys. Using data from living teleost fish as a comparison, scientists discovered that Leedsichthys could have cruised along at potential speeds of while still maintaining oxygenation of its body tissues.

Footnotes

  • For more on the Star Pit dig of 2002-2003 look here and also here where the dig was featured in the BBC-Open University series Fossil Detectives.
  • For a more accurate reconstruction of Leedsichthys visit Paleocreations. The artist went on to produce the most accurate reconstruction of Leedsichthys to date, which was used on 'Fossil Detectives' (see above).
  • Article on Leedsichthys by Darren Naish.
  • Profile of Leedsichthys problematicus from the BBC series Sea Monsters
  • "Biggest Fish Ever Found" Unearthed in U.K. National Geographic News. Published October 1, 2003.
  • Article on the latest discovery of a Leedsichthys problematicus fossil, estimated 22 meters = long.
  • Picture of Leedsichthys problematicus
  • Article on Leedsichthys from BBC news