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20.01.2016 01:27 - Largest prehistoric animals Vol.1 Vertebrates part12 Fish Pachycormidae - Largest fishes of all time
Автор: valentint Категория: Забавление   
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Последна промяна: 03.12.2016 02:48


Pachycormidae is an extinct family of ray-finned fish known from Mesozoic deposits from Eurasia and the Americas. They were characterized by having serrated pectoral fins, reduced pelvic fins and a bony rostrum. Certain members of the group were large filter-feeding planktivores. Their relations with other fish are unclear.

The largest bony fish of all time was the pachycormid Leedsichthys problematicus at around 27 metres (89 ft) long.

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Claims of larger individuals persist. Leedsichthys is a giant member of the Pachycormidae, an extinct group of Mesozoic bony fish, that lived in the oceans of the Middle Jurassic period.

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 fossilise. 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 wildly. At the beginning of the twentieth century a length of nine metres was seen as plausible, but by its end Leedsichthys was sometimes claimed to have been over thirty metres long. Recent research has lowered this to about sixteen meters for the largest individuals. Skull bones have been found indicating that Leedsichthys had a large head with bosses on the skull roof. Fossilised bony finrays 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 who 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.

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.

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.

The gill arches are equipped with rows of parallel three to twelve centimetres 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 two millimetres 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.

Leedsichthys is the largest known member of the Osteichthyes or bony fishes with the exception of the largest descendants of land-dwelling Tetrapodomorpha (e.g. giant sauropods and whales), not commonly thought of as "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 King of Herrings might rival Leedsichthys in length but is a much more elongated animal. 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 274 centimetres, 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 nine metres 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, AsthenocormusThe unusual proportions of that specimen gave a wide range of possible sizes. Some were as low as 13.5 metres, but extrapolating from the gill basket resulted in an estimated length of 27.6 metres for Leedsichthys specimen NHM P.10156 (the earlier BMNH P.10156). Martill considered the higher estimate as a plausible size of the largest individuals. Subsequently, a length of thirty metres was often mentioned in popular science publications, sometimes one as high as thirty-five metres.

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 nine and ten meters. With "Ariston" the pectoral fins are 1005 millimetres apart, indicating a narrow body of no excessive size, even though it was initially thought to have been twenty-two metres long. In 2007 Liston stated that most specimens indicated lengths between seven and twelve metres. 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 sixteen metres (53 feet) is not unreasonable. In 2013 a new study by Liston found most specimens to be between seventeen and thirty-one years old. Specimen GLAHM V3363, "Big Meg", could have been between 11.4 and 14.9 metres long. The exception was NHM P.10156. Its gill basket, with a preserved width of 114 centimetres and height of 1545 millimetres, indicated a body length of 16.5 metres and an age of forty-five years.

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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.

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.

Much is still uncertain about the life cycle of Leedsichthys. Liston"s 2013 study suggested a slow, nearly linear, growth. A French study in 1993 of its bone structure concluded however, that the metabolism was rather high.Problematic is also 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 the about three metres long 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. 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. However, in 2010 Liston pointed out that Leedsichthys during the later Kimmeridgian was still present in the north, as testified by Normandian finds.Liston did nevertheless consider in 2007 that the lack of any vertebrate suspension feeders as large as 0.5 metres prior to the Callovian stage of the Mesozoicum might indicate that the Callovian had seen a marked change in productivity as regarded zooplankton populations. Indeed, further studies supported this, viewing Leedsichthys as the beginning of a long line of large (>2 metres in length) pachycormid suspension feeders that continued to flourish well into the Late Cretaceous, such as Bonnerichthys and Rhinconichthys and emphasising the convergent evolutionary paths taken by pachycormids and baleen whales.


Bonnerichthys gladius
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Bonnerichthys is a genus of fossil fishes within the family Pachycormidae that lived during the Late Cretaceous Period Fossil remains of this taxon are known from the Smoky Hill Member of the Niobrara Chalk Formation of Kansas (Late Coniacian-Early Campanian, about 87-81 million years ago). It grew to around 30 feet in length, though not quite as large as the related Leedsichthys from the Jurassic of Europe which likely grew up to 60-80 feet.
One of the most significant features of Bonnerichthys is the recognition that it was a filter feeder, living on plankton. This recognition that many large-bodied fish from the Mesozoic in the Pachycormidae were filter feeders shows that this niche was filled for at least 100 million years before previously known. The modern niche is filled by several species of sharks and the baleen whales.

Rhinconichthys is an extinct species of bony fish which existed during the upper Cretaceous period.
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Along with its close cousins the great-white-shark-sized or larger Bonnerichthys and the immense Leedsichthys, Rhinconichthys forms a line of giant filter-feeding bony pachycormid fish that swam the Jurassic and Cretaceous seas for over 100 million years.
Rhinconichthys was a medium sized fish. R.uyenoi grew to around 3.5-4.5 meters.Rhinconichthys was first named in 2010, from fossils that had been discovered in England. Then in 2016 a description of two new species, R. uyenoi from Japan and R. purgatoirensis from the USA were described, revealing to us that this fish genus potentially had a global distribution. Rhinconichthys is noted for the large hyomandibulae bones which would have allowed this to have had an incredibly wide gape, in turn allowing Rhinconichthys to filter large amounts of plankton from the sea water.

Protosphyraena
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Protosphyraena is a fossil genus of swordfish-like marine fish, that throve worldwide during the Upper Cretaceous Period (Coniacian-Maastrichtian). Though fossil remains of this taxon have been found in both Europe and Asia, it is perhaps best known from the Smoky Hill Member of the Niobrara Chalk Formation of Kansas (Late Coniacian-Early Campanian). Protosphyraena was a large fish, averaging 2–3 metres in length. Protosphyraena shared the Cretaceous oceans with aquatic reptiles, such as mosasaurs and plesiosaurs, as well as with many other species of extinct predatory fish. The name Protosphyraena is a combination of the Greek word protos ("early") plus Sphyraena, the genus name for barracuda, as paleontologists initially mistook Protosphyraena for an ancestral barracuda. Recent research shows that the genus Protosphyraena is not at all related to the true swordfish-family Xiphiidae, but belongs to the extinct family Pachycormidae.

In its general body plan, Protosphyraena resembled a modern sailfish, though it was smaller with a shorter rostrum, was somewhat less hydrodynamic, and adults possessed large blade-like teeth (adults of living swordfish species are toothless). Complete skeletons of Protosphyraena are relatively rare, but in parts of the Niobrara Chalk, the Mooreville Chalk Formation of Alabama, and other geological formations, fragmentary specimens are quite common and most often include isolated teeth, the distinctive rostrum, and fragments of the long saw-edged pectoral fin first described by Mantell. Usually, portions of the skull and postcranial skeleton are found separately. This preservational bias can be explained by the fact that the skeleton of Protosphyraena was less ossified than that of most bony fishes and tended to be torn apart by scavengers or decay before burial and fossilization (Everhart, 2005; p. 93). Like most of the Cretaceous marine fauna, Protosphyraena became extinct at the end of the Mesozoic; the resemblance to living swordfish apparently results from convergent evolution.













 

 

 



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