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15.01.2016 00:32 - Largest prehistoric animals Vol.1 Vertebrates part8 Ichthyosaurs,compliance of whales and dolphins among reptiles
Автор: valentint Категория: Забавление   
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Последна промяна: 29.11.2016 18:00


Ichthyosaurs (Ichthyosauria)


The largest
ichthyosaur was Shastasaurus sikanniensis at 21 metres (69 ft) in length.

Shastasaurus ("Mt. Shasta lizard") is an extinct genus of ichthyosaur from the middle and late Triassic, and is the largest marine reptile that has yet been found.Specimens have been reported from the United States, Canada, and China.

Shastasaurus lived during the late Triassic period. The type species Shastasaurus pacificus is known from California. A second possible species of Shastasaurus, S. sikanniensis, is known from the Pardonet Formation British Columbia, dating to the middle Norian age (about 210 million years ago). If S. sikkanniensis belongs to Shastasaurus, it would be the largest species, measuring up to 21 metres (69 ft).

Shastasaurus was highly specialized, and differed considerably from other ichthyosaurs. It was very slender in profile. The largest specimens had a ribcage slightly less than 2 metres (6.6 ft) deep despite a distance of over 7 metres (23 ft) between its flippers.Due to its unusually short, toothless snout (compared to the long, toothed, dolphin-like snouts of most ichthyosaurs) it was proposed that Shastasaurus was a suction feeder, feeding primarily on soft-bodied cephalopods although current research indicates ichthyosaur jaws do not fit the suction-feeding profile.

In S. liangae, the only species with several well preserved skulls, the skull measures only 8.3% of the total body length (9.3% in a juvenile specimen). Unlike the related Shonisaurus, even juvenile Shastasaurus completely lacked teeth. The snout was highly compressed via a unique arrangement of skull bones. Unlike almost all other reptiles, the nasal bone, which usually forms the mid part of the skull, extended to the very tip of the snout, and all bones of the snout tapered to abrupt points.

Shastasaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in any species. The upper fluke of the tail was probably also much less developed than the shark-like tails found in later species.

The type species of Shastasaurus is S. pacificus, from the late Carnian of northern California. It is known only from fragmentary remains, which have led to the assumption that it was a 'normal' ichthyosaur in terms of proportions, especially skull proportions. Several species of long-snouted ichthyosaur were referred to Shastasaurus based on this misinterpretation, but are now placed in other genera (including Callawayia and Guizhouichthyosaurus).

Shastasaurus may include a second species, Shastasaurus liangae. It is known from several good specimens, and was originally placed in the separate genus Guanlingsaurus. Complete skulls show that it had an unusual short and toothless snout. S. pacificus probably also had a short snout, although its skull is incompletely known. The largest specimen of S. liangae (YIGMR SPCV03109) measures 8.3 metres (27 ft) long. A juvenile specimen (YIGMR SPCV03108) has also been found, measuring 3.74 metres (12.3 ft) in length.

S. sikkanniensis was originally described in 2004 as a large species of Shonisaurus. However, this classification was not based on any phylogenetic analysis, and the authors also noted similarities with Shastasaurus. The first study testing its relationships, in 2011, supported the hypothesis that it was indeed more closely related to Shastasaurus than to Shonisaurus, and it was reclassified as Shastasaurus sikkanniensis.However, a 2013 analysis supported the original classification, finding it more closely related to Shonisaurus than to Shastasaurus. Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia, dating to the middle Norian age (about 210 million years ago).

In 2009, Shang & Li reclassified the species Guizhouichthyosaurus tangae as Shastasaurus tangae. However, later analysis showed that Guizhouichthyosaurus was in fact closer to more advanced ichthyosaurs, and so cannot be considered a species of Shastasaurus.

Dubious species that were referred to this genus include S. carinthiacus (Huene, 1925) from the Austrian Alps and S. neubigi (Sander, 1997) from the German Muschelkalk. S. neubigi, however, has recently been re-described and reassigned to its own genus, Phantomosaurus


Shonisaurus



Shonisaurus is a genus of ichthyosaur. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period, about 215 million years ago.

Shonisaurus lived during the Norian stage of the late Triassic period. S. popularis measured around 15 metres (49 ft) long. A second species from British Columbia was named Shastasaurus sikanniensis in 2004. S. sikkanniensis was one of the largest marine reptiles of all time, measuring 21 metres (69 ft). However, phylogenetic studies later showed S. sikanniensis to be a species of Shastasaurus rather than Shonisaurus.

Shonisaurus had a long snout, and its flippers were much longer and narrower than in other ichthyosaurs. While Shonisaurus was initially reported to have had socketed teeth (rather than teeth set in a groove as in more advanced forms), these were present only at the jaw tips, and only in the very smallest, juvenile specimens. All of these features suggest that Shonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line. It was historically depicted with a rather rotund body, but studies of its body shape since the early 1990s have shown that the body was much more slender than traditionally thought. S. popularis had a relatively deep body compared with related marine reptiles.

Shonisaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in Shonisaurus. The upper fluke of the tail was probably also much less developed than flukes found in later species.

Fossils of Shonisaurus were first found in a large deposit in Nevada in 1920. Thirty years later, they were excavated, uncovering the remains of 37 very large ichthyosaurs. These were named Shonisaurus, which means "lizard from the Shoshone Mountains", after the formation where the fossils were found.

S. popularis, was adopted as the state fossil of Nevada in 1984. Excavations, begun in 1954 under the direction of Charles Camp and Samuel Welles of the University of California, Berkeley, were continued by Camp throughout the 1960s. It was named by Charles Camp in 1976.

The Nevada fossil sites can currently be viewed at the Berlin-Ichthyosaur State Park.

The Nevada bonebed represents a large assemblage of Shonisaurus which died at varying times and became preserved on the sea floor in a curiously regular arrangement of bones. The lack of fossil invertebrates encrusting the remains indicates that the carcasses sank in relatively deep water poor in oxygen.

 

Himalayasaurus


Himalayasaurus is an extinct genus of ichthyosaur from the Late Triassic of Tibet. The type species Himalayasaurus tibetensis was described in 1972 on the basis of fragmentary remains, including teeth, limb bones, and vertebrae. The entire body length of Himalayasaurus is estimated to have been over 15 metres (49 ft) in length. Himalayasaurus has since been considered a nomen dubium or "dubious name" because of the lack of features that set it apart from other ichthyosaurs although the presence of distinct cutting edges on its teeth have more recently been proposed as a unique feature of the genus (cutting edges have also been found in the recently described ichthyosaur Thalattoarchon from the western United States). Himalayasaurus belongs to the family Shastasauridae, which includes other large-bodied Triassic ichthyosaurs like Shonisaurus.

Temnodontosaurus

Temnodontosaurus is one of the oldest and largest ichthyosaurs known to science, and while there are a few species associated with the genus, the most obvious differences between these are the size and proportions of the jaws. Temnodontosaurus fossils have been so well preserved that they reveal stomach contents of ammonites and cephalopods like squid. Juvenile specimens have also been reported inside Temnodontosaurus remains, but in a position that suggests they were waiting to be born rather than being digested. Live birth in ichthyosaurs like Temnodontosaurus has long been suspected as their fusiform bodies that are so much like fish would make it impossible for them to climb onto land.Study of other genera such as Ophthalmosaurus amongst others have helped to confirm the theory of live birth in ichthyosaurs. Like most large ichthyosaurs, Temnodontosaurus is thought to have been a marine reptile of deep water, mostly rising to the surface just to breathe in fresh air.
Temnodontosaurus had large eyes that were roughly twenty centimetres across. These large eyes would have enabled Temnodontosaurus to see better when in deep water where most of the above sunlight could not penetrate. The large eyes providing a greater catch area for the little available light would allow Temnodontosaurus to distinguish ammonite forms against the dim oceanic backdrop. Aside from ammonites found inside Temnodontosaurus fossils, further support for this hunting behaviour comes from study of the teeth which reveals that they had robust roots so that they could withstand the stresses of cracking shells without breaking off.
Ammonites would have been a plentiful food supply back in the Jurassic oceans, and one that could not have required a great amount of energy expenditure to catch. This suited the large body of Temnodontosaurus as it would require large amounts of a stable food supply to keep going. It is hard to say how the large size came about however but a larger body can carry more oxygen for Temnodontosaurus to stay hunting down in the depths for longer. It’s possible that the earlier ancestors of Temnodontosaurus came across the plentiful supply of deep water cephalopods and grew larger to take better advantage of it. However this greater size also brought a greater reliance upon them, a specialisation that could bring the end of the genus with the disappearance of the prey.

Thalattoarchon saurophagis

Thalattoarchon is an extinct genus of ichthyosaur from the Middle Triassic of the western United States. The type species Thalattoarchon saurophagis (meaning "lizard-eating sovereign of the sea" in Greek) was discovered in Nevada in 2010 and formally described in 2013. It is known from a single skeleton consisting of a partial skull, vertebral column, hip bones, and parts of the hind fins. The total length of Thalattoarchon is estimated to have been at least 8.6 metres (28 ft). Thalattoarchon is thought to have been one of the first marine macropredators capable of eating prey that was similar in size to itself, an ecological role that can be compared to that of modern
Orcas. Thalattoarchon lived four million years after the first appearance of ichthyosaurs in the Early Triassic and is therefore the oldest known marine reptile to have been an apex predator. It lived eight million years after the Permian-Triassic extinction event, predating the first post-extinction appearance of apex predators on land in the Carnian stage of the Late Triassic.Therefore, the existence of Thalattoarchon suggests that marine ecosystems recovered faster than terrestrial ecosystems after the mass extinction.
Thalattoarchon is a large-bodied ichthyosaur that is estimated to have grown at least 8.6 metres (28 ft) in length. The only known skeleton of Thalattoarchon is incomplete, but it can be inferred on the basis of other early ichtyosaurs to have had an elongated body and a weakly developed caudal fin. Thalattoarchon is diagnosed by a single distinguishing feature that is unique among ichthyosaurs: large, thin teeth that bear two cutting edges and that have smooth tooth crowns. Most later ichtyosaurs have much smaller cone-shaped teeth. The Late Triassic ichthyosaur
Himalayasaurus tibetensis also has large teeth with cutting edges, but can be distinguished from Thalattoarchon saurophagis by the presence of grooves across the surfaces of its tooth crowns. Thalattoarchon is very similar in appearance to Cymbospondylus, another large-bodied Middle Triassic ichthyosaur, but differs in having a head that is about twice as large as that of Cymbospondylus relative to its body.







 



Тагове:   сова,   prehistoric animals,   giant animals,   sea monsters,


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Автор: valentint
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