Ichthyosaur

Life image of Shastasaurus, a representative huge ichthyosaurs of the Triassic

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The ichthyosaur ( Ichthyopterygia ) are a group of extinct reptiles from the Mesozoic era ( Mesozoic ). They were fully adapted to aquatic life and lived exclusively in the sea. A total of about eighty species have been described. They lived over a period of about 150 million years ago, died before 93 million years ago at the beginning of the Upper Cretaceous again, long before the extinction of dinosaurs. The ichthyosaur underwent in the late Triassic period and twice during the Jurassic larger radiations from which emerged the respective groups of Neoichthyosauria that Ophthalmosauridae and Platypterygiinae.

The first complete skeleton of an ichthyosaur was found in 1811 by the twelve year old Mary Anning in Lyme Regis / England when dinosaurs were still unknown. The fossils confused people, because the physique reminded in some respects to the land vertebrates. Nevertheless, they were kept for fish by some researchers initially. Others saw them amphibians or even marine mammals.

Etymology

Ichthyosauria comes from the Greek and means fish lizard ( ἰχθύς ichthys, fish ' and σαῦρος sauros, lizard '). The taxon was erected in 1835 by de Blainville. To integrate the basal forms, presented Owen 1840, the taxon Ichthyopterygia on ( " ichthyos " and " pteryx " = wing). Both are mentioned in the German Ichtyosaurs and also described here together. The Ichthyosauria be defined as a subgroup of Ichthyopterygia and include all the forms that are more closely related than Ichthyosaurus with Grippia.

Evolution and stratigraphy

Ichthyosaurs appeared in the Early Triassic. The first forms as Grippia, Utatsusaurus and Chaohusaurus still had a lizard- like, elongated body and moved probably offshore in shallow water by a Anguilliformes meandering of the whole body away. They had a large number of elongated vertebrae. Hupehsuchus, Mixosaurus, Parvinatator and nearly ten feet long expectant Cymbospondylus lived in the Middle Triassic. At the end of this time, all elongated and ichthyosaurs were disappeared in the Upper Triassic of the Shastasauriden, forms with a more spindle-like physique, detached. They are the sister group of all post- Triassic ichthyosaurs. Shonisaurus, Toretocnemus and Californosaurus lived in the early, Macgovania and Hudsonelpidia in late and Shastasaurus throughout the Upper Triassic. At the end of the latter three genera became extinct.

The greatest diversity of forms show the ichthyosaurs the Middle Triassic, when there was both purely piscivorous forms as well as those with heterodontem or cracking teeth ( Durophagie ). In Jurassic fossil record of species richness in the Aalenian and Kimmeridgian is highest in the Cretaceous, it reduces to the group of Platypterygiinae and the genus Malawania.

In the Lower Jurassic ichthyosaur which reached their greatest species richness. All Jurassic ichthyosaurs and later form a monophyletic taxon, the Neoichthyosauria. A significant archaeological site from this period is the Posidonia shale at the municipality of wood grubs in southern Germany. So far, the fossil remains of about 3,000 copies of the genera Eurhinosaurus, Stenopterygius and Temnodontosaurus were excavated. In the UK, many copies of the type genus Ichthyosaurus were found. More ichthyosaur from this period are Suevoleviatan, as well as the long-snouted Eurhinosaurier Leptonectes and Excalibosaurus. As known from the Middle Jurassic Chacaicosaurus, Mollesaurus and Ophthalmosaurus, can be in the Upper Jurassic brachypterygius and Caypullisaurus and continue to prove Ophthalmosaurus. All three genera died at the transition from the Jura from the Cretaceous period. During the Cretaceous can be both close with Ichthyosaurus related genus Malawania and various representatives of the Baracromia prove. From them the world's occurring Platypterygius survived to the end of the Cenomanian, as the last representative of the extinct ichthyosaurs with it ..

Features

Ichthyosaurs were reptiles and were therefore among the land vertebrates. They are secondarily transferred back to life in the water. Your shoulder belt is not fixed to the skull grow together like fish and their fins can be upper and lower arm bones, carpal bones and phalanges differ. Like other reptiles, they have diapside skull window, the top speed Ralf Rochester. The information presented in other Diapsiden lower pace Ralf Rochester were closed secondarily again.

How plesiosaurs and mosasaurs are the ichthyosaurs have been able to maintain their body temperature at a constant high, the same level warm 35-39 ° C ( endothermic ). The taphonomy of ichthyosaurs suggests that this group of animals - like most recent whale species also - have a higher specific gravity than seawater exhibited. Dark traces ( eumelanin ) show the fossil preserved soft tissue from Ichthyosaurierfossilien she dyed dark and some ichthyosaurs, similar to the deep-diving sperm whales, were uniformly dark and no counter- shading showed a bright side of the abdomen.

Ichthyosaurs were a ( Mixosaurus ) to over twenty meters ( Shonisaurus ) are long.

Extremities

All ichthyosaurs, even the earliest, had fin-like limbs. These are homologous to those of other vertebrates, but they have changed drastically in adaptation to the aquatic environment over time. The bones of the front limbs shortened and widened as a result of evolutionary changes. At the same time the finger bones ( Hyperphalangie ) increased. Then the first of the five fingers disappeared ( of the human thumb corresponded ). This was followed on both sides of the remaining fingers to an increase of the fingers ( polydactyly ) itself The forelimbs were probably used for control and to change direction, while the propulsion on primitive forms by the eel-like meandering of the body, with advanced forms by blows with the tail fin was created. Some paleontologists suggest also an advance by the front limbs, but the shoulder girdle of ichthyosaurs is not very strong, but weaker than in vertebrates, which move in this way.

Eye

The eyes of ichthyosaurs were surrounded in proportion to their body length very large and of a ring-shaped, bony reinforcement, the scleral ring, which occurs in many vertebrates. The scleral ring was probably used to hold the flat, not round eyeballs of ichthyosaurs in shape since her large eyes were exposed to different water pressure while swimming. The portion of the eyes which is closer to the nose was exposed to a greater pressure than the water further back.

The biggest eye found in an ichthyosaur is also the largest eye of all vertebrates. It had a diameter of 26.4 cm and was platyodon to Temnodontosaurus. In relation to body length, however Ophthalmosaurus had bigger eyes.

The eyes were very bright. Compared with the aperture settings of a photo lens of the eye Ichthyosaurus had an f-number of 1 /1, 1 to 1/1, 3, reached by Ophthalmosaurus 1/0, 8 to 1/ 1, 1 It is known that the eye nocturnal animals have a low f-numbers. When an owl she is 1 /1, 1, in a cat with 1/0, 9 The human eye has an f-number of 1 /2, 1

Nutrition

The ichthyosaurs were able to see in low light quantity. This does not mean that they were nocturnal. Rather, they sought their food in the ocean depths.

The diet of some advanced Ichtyosaurs is known for the remains of prey in the stomach region. Here, especially the fishing hooks on the tentacles of belemnites and fish scales have received. The rostra of belemnites may have been spewed out. Probably ate very large forms, such as the length of ten meters reach Temnodontosaurus other vertebrates. In a fossil from the Upper Cretaceous of Australia, many young sea turtles Protostegidae the family and a bird have been found from the group of Enantiornithes in the stomach area. Later forms such as Ophtalmosaurus, had short strong jaws, blunt teeth and probably ate hard-shelled animals like clams or ammonites. The Ammonites were chewed before swallowed and thrown out the case.

Reproduction

Ichthyosaurs could not crawl ashore and lay eggs there. They were viviparous and came probably heard of viviparous ancestor. There over 50 fossils have been found in which some, a maximum of ten to eleven pups of the same kind are in the mother's body within the rib cage. The boys were born with the tail first As with dolphins ( the basal Chaohusaurus but with the head first), so they could quickly to the water surface to take its first breath. Some of the fossils, the animals should have died at the moment of birth, the boy is still with head or snout in the womb. More likely, however, that in these cases an embryo was expelled after the death of the mother or through putrefaction gases.

Outer systematics

The ichthyosaurs have fully adapted to life in the water, while their original features ( plesiomorphies ) have changed. Basal forms, which still show these characteristics, fossil rarely survived. Therefore, it has long been controversial, such as the ichthyosaurs are related to other amniotes. Finally, the primitive Ichthyopteryge Utatsusaurus hataii was found in layers of Lower Triassic in Japan. Through him, the relationship of ichthyosaurs became clearer.

Phylogenetic analyzes show that the ichthyosaurs belong to the diapsiden reptiles, but not to the crown group that includes as the second major group of marine reptiles in the Mesozoic, the Sauropterygia today Squamata, the tuatara, crocodiles and birds. Probably the line of ichthyosaurs split off before the division of the Diapsida in Archosauromorpha and Lepidosauromorpha. So you are not closer to the other large marine reptiles of the Mesozoic, the plesiosaurs and mosasaurs the, related, both belong to the Lepidosauromorpha.

The position of the ichthyosaurs within the reptiles illustrated in the following cladogram:

• Reptilia Parareptilia
• Diapsida Neodiapsida † Younginiformes
• † Ichthyopterygia
• Untitled monophyletic Lepidosauromorpha ( Squamata, tuatara )
• Archosauromorpha ( crocodiles, dinosaurs, birds)

The internal classification of ichthyosaurs is shown in the following cladogram:

Inside systematics

By September 2013, 50 valid species have been described.

• Ichthyosauria Thaisauridae Thaisaurus

• Utatsusaurus

• Grippia
• Chaohusaurus
• Gulosaurus

• Quasianosteosaurus

• Parvinatator

• Omphalosaurus
• Tholodus
• Xinminosaurus

• Isfjordosaurus

• Mixosauria Wimaniidae Wimanius

• Mixosaurinae Mixosaurus

• Phalarodon
• Contectopalatus

• Barracudasauroides

• Toretocnemidae Toretocnemus
• Qianichthyosaurus

• Cymbospondylus
• Phantomosaurus

• Merriamosauridae Pessopteryx

• Besanosaurus

• Shastasaurus

• Shonisaurus
• Himalayasaurus
• Guizhouichthyosaurus

• Guanlingsaurus
• Mikadocephalus

• Californosaurus
• Callawayia

• Hudsonelpidiidae Hudsonelpidia

• Macgowania

• Temnodontosauridae Temnodontosaurus
• Unnamed genus ( " Ichthyosaurus " acutirostris )

• Leptonectes
• Excalibosaurus
• Eurhinosaurus

• Suevoleviathan

• Without family assignment Malawania

• Ichthyosaurus

• Stenopterygius
• Chacaicosaurus
• Hauffiopteryx

• Arthropterygius
• Nannopterygius
• Ophthalmosaurinae Ophthalmosaurus
• Acamptonectes
• Leninia

• Brachypterygius
• Maiaspondylus
• Aegirosaurus
• Sveltonectes
• Platypterygius
• Caypullisaurus
• Athabascasaurus
• Undorosaurus

Extinction

The ichthyosaurs died out 93 million years ago, at the end of the Cenomanian. The last genus, which can be clearly identified, is the internationally established Platypterygius. The reasons for the disappearance are unknown. Has been suggested previously that the advent of mosasaurs, giant marine reptiles from the group of Squamata, has to do with it. However, they only developed a lot of ways than the ichthyosaurs were already extinct.

Another possible reason is the oceanic anoxic event at the end of the Cenomanian. It led to the sub-surface layer of oxygen -depleted, causing the oceans extinction of marine invertebrates and cephalopods. According to this theory, the ichthyosaurs were ruined because their food source disappeared. Some scientists refer to the reproductive strategy of the ichthyosaurs. An ichthyosaur got only a few relatively large, well-developed young. For them, the new, large predatory fish in the Upper Cretaceous were dangerous as Xiphactinus from the order of Ichthyodectiformes.