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23.01.2016 00:59 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part10 Amphibians ch.1Temnospondyls
Автор: valentint Категория: Забавление   
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Amphibians (Amphibia)
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Amphibians were the first group of vertebrates to develop limbs and to be able to leave the water to conquer the land. Even if they are seen as simple and primitive animals by most people, amphibians show a wide diversity of survival strategies which have allowed them to occupy most terrestrial and fresh-water habitats.
Amphibians differ from other vertebrates in three main ways: first, newborn hatchlings live underwater and breathe via gills, which then disappear as the juvenile undergoes a "metamorphosis" into its adult, air-breathing form. Juveniles and adults can look very different, as in the case of baby tadpoles and full-grown frogs. Second, adult amphibians lay their eggs in water, which significantly limits their mobility when colonizing the land. And third, the skin of modern amphibians tends to be "slimy" rather than reptile-scaly, which allows for the additional transport of oxygen for respiration.
The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish.These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills.
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Probably the creature known as Tiktaalik is the closest animal to the mid-point between the osteolepiformes and the amphibians. The first recorded amphibians were labyrinthodonts meaning that their teeth had layers of dentin and enamel forming a structure similar to a maze.
There were four main groups of primitive amphibians, each characterized by: a group that includes the first animals that were able to get out of water, a second group which contains the ancestors of the amniotes (reptiles, birds and mammals) and two more groups, both candidates to be the ancestors of modern amphibians.
Ichthyostegalians were the first tetrapods to be able to leave the water. They appeared at the late Devonian period and they were big animals with large wide heads, short legs and an aquatic or semi aquatic lifestyle (they probably were pretty clumsy on land). They moved around using mainly their muscular tail with rays similar to that of fish.
It"s only in the late Carboniferous period, from about 310 to 300 million years ago, that we can comfortably refer to the first true amphibians. By this time, some genera had attained relatively monstrous sizes--a good example being Eogyrinus ("dawn tadpole"), a slender, crocodile-like creature that measured 15 feet from head to tail. Interestingly, the skin of Eogyrinus was scaly rather than moist, evidence that the earliest amphibians needed to protect themselves from dehydration. Another late Carboniferous/early Permian genus, Eryops, was much shorter than Eogyrinus but more sturdily built, with massive, tooth-studded jaws and strong legs.
As a general rule, the amphibians of the Carboniferous and Permian periods can be divided into two camps: small and weird-looking (the lepospondyls), and big and reptile-like (the temnospondyls). The lepospondyls were mostly aquatic or semi-aquatic, and more likely to have the slimy skin characteristic of modern amphibians. Some of these creatures (such as Ophiderpeton and Phlegethontia) resembled small snakes; others, like Microbrachis, were reminiscent of salamanders, and some were simply unclassifiable. A good example of the last is Diplocaulus: this three-foot-long lepospondyl had a huge, boomerang-shaped skull, which might have functioned as an undersea rudder.
Dinosaur enthusiasts should find the temnospondyls easier to swallow. These amphibians anticipated the classic reptilian body plan of the Mesozoic Era: long trunks, stubby legs, big heads, and in some cases scaly skin, and many of them (like Metoposaurus and Prionosuchus) resembled large crocodiles. Probably the most infamous of the temnospondyl amphibians was the impressively named Mastodonsaurus, the name means "nipple-toothed lizard" and has nothing to do with the elephant ancestor, which had an almost comically oversized head that accounted for nearly a third of its 20-foot-long body.
For a good portion of the Permian period, the temnospondyl amphibians were the top predators of the earth"s land masses. That all changed with the evolution of the therapsids, "mammal-like reptiles", toward the end of the Permian period; these large, nimble carnivores chased the temnospondyls back into the swamps, where most of them slowly died out by the beginning of the Triassic period. There were a few scattered survivors, though: for example, the 15-foot-long Koolasuchus thrived in Australia in the middle Cretaceous period, about a hundred million years after its temnospondyl cousins of the northern hemisphere had gone extinct.

Temnospondyls
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Eryops 2. Zygosaurus 3. Archegosaurus 4. Nigerpeton 5. Acheloma
The Temnospondyls were an extremely diverse and succesful group of "labyrinthodont" tetrapods. They represent one of the three primary lines of labyrinthodont evolution, the other two being the reptilomorphs and lepospondyls. Although some temnospondyls were completely aquatic and even had external gills as adults, others probably became almost as terrestrial as reptiles, returning to water only to lay their eggs.The Temnospondyli evolved in the Early Carboniferous. Most Paleozoic forms became extinct before or during the Lopingian (Late Permian). However two aquatic Gondwanan clades, the dvinosaurs and stereospondyls, survived and even prospered in the early Mesozoic. The youngest known temnospondyl is Koolasuchus from the Middle Cretaceous (Barremian) of Australia.
Perhaps the single most successful group of non-amniote tetrapods, the temnospondyls have a long and diverse history.
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Temnospondyls first appeared in the Early Carboniferous around 330 million years ago (Mya). During the Carboniferous, temnospondyls included basal medium-sized forms such as Dendrerpeton or large semiaquatic forms such as Cochleosaurus. Other, more derived temnospondyls, such as the amphibamids, were smaller and more terrestrial. They resembled salamanders, and some taxa, such as the genus Branchiosaurus, even retained external gills like the modern-day axolotl. During the latest Carboniferous and Early Permian around 300 Mya, several groups, such as the dissorophids and trematopids evolved strong, robust limbs and vertebrae and became adapted to life on land while others such as the eryopids, developed into large semiaquatic predators. The dvinosaurs, a group of small aquatic temnospondyls, evolved from terrestrial ancestors in the Late Carboniferous.
During the Late Permian, increasing aridity and the diversification of reptiles contributed into a decline in terrestrial temnospondyls, but semiaquatic and fully aquatic temnospondyls continued to flourish, including the large Melosaurus of Eastern Europe. Other temnospondyls, such as archegosaurids, developed long snouts and a close similarity to crocodiles, although they lacked the armor characteristic of the latter group. These temnospondyls included the largest known batrachomorph, the 9-m-long Prionosuchus of Brazil.
As temnospondyls continued to flourish and diversify in the Late Permian (260.4 - 251.0 Mya), a major group called Stereospondyli became more dependent on life in the water. The vertebrae became weak, the limbs small, and the skull large and flat, with the eyes facing upwards. During the Triassic period, these animals dominated the freshwater ecosystems, evolving in a range of both small and large forms. During the Early Triassic (251.0 - 245.0 Mya) one group of successful long-snouted fish-eaters, the trematosauroids, even adapted to a life in the sea, the only known batrachomorphs to do so with the exception of the modern crab-eating frog. Another group, the capitosauroids, included medium- and large-sized animals 2.3 to 4 m (7.5 to 13.1 ft) in length, with large and flat skulls that could be over a meter long in the largest forms such as Mastodonsaurus. These animals spent most or all their lives in water as aquatic predators, catching their prey by a sudden opening of the upper jaw and sucking in fish or other small animals.
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1. Mastodonsaurus 2. Capitosaurus 3. Promastodonsaurus 4. Sclerothorax 5. Wetlugasaurus
In the Carnian stage of the Late Triassic (228.0 - 216.5 Mya), capitosauroids were joined by the superficially very similar Metoposauridae. Metoposaurids are distinguished from capitosauroids by the positioning of their eye sockets near the front of their skulls. Another group of stereospondyls, the plagiosaurs, had wide heads and gills, and adapted to life at the bottom of lakes and rivers. By this time, temnospondyls had become a common and widespread component of semiaquatic ecosystems. Some temnospondyls, such as Cryobatrachus and Kryostega, even inhabited Antarctica, which was covered in temperate forests at the time.
Triassic temnospondyls were often the dominant semiaquatic animals in their environments. Large assemblages of metoposaurs with hundreds of individuals preserved together have been found in the southwestern United States. They have often been interpreted as mass death events caused by droughts in floodplain environments. Recent studies show these dense assemblages were instead probably the result of currents accumulating dead individuals in certain areas. These environments seem to have had little diversity, as they were inhabited almost exclusively by metoposaurs.
The Triassic-Jurassic extinction event around 199.6 Mya led to the extinction of most Mesozoic temnospondyls. The brachyopoids survived, as well as a few capitosauroids and trematosauroids. While the latter two groups soon became extinct, brachyopoids persisted and grew to large sizes during the Jurassic. Among brachyopoids, the brachyopids flourished in China and the chigutisaurids became common in Gondwana. The most recent known temnospondyl was the giant chigutisaurid Koolasuchus, known from the Early Cretaceous of Australia. It survived in rift valleys that were too cold in the winter for pseudosuchians that normally would have competed with them. Koolasuchus was one of the largest of the brachyopoids, with an estimated weight of 500 kg (1,100 lb)


The largest known amphibian of all time was the 30 ft long temnospondyli Prionosuchus.
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Prionosuchus is an extinct genus of large temnospondyl. A single species, P. plummeri, is recognized from the middle Permian period (270 million years ago). Its fossils have been found in what is now northeastern Brazil.
The fragmentary remains of this animal have been found in the Pedra do Fogo Formation in the Parnaiba Basin of Northeastern Brazil, and it was described by L.I. Price in 1948.The incomplete skull of the holotype specimen has been estimated to be 50 centimetres (20 in) long.Several more fragmentary specimens have been found. One very fragmentary but very large specimen (BMNH R12005) appears to have come from an individual nearly three times the size of most other specimens, and may have had a skull that measured up to 1.6 metres (5.2 ft) long.Based on related species, the total body length of this specimen has been estimated at about 9 metres (30 ft), making it the largest known species of temnospondyl.
With an elongated and tapered snout, numerous sharp teeth, long body, short legs, and a tail adapted for swimming, its general appearance was very similar to a modern crocodile, particularly to the gharial, and it probably had a similar lifestyle as an ambush aquatic predator feeding on fish and other aquatic animals.
Prionosuchus has been classified as an archegosaurian by Carroll.The genus is monotypic with P. plummeri being the only species described. The archegosaurs were a group of temnospondyli that occupied the ecological niche of crocodiles and alligators during the Permian, and of which the European genus Archegosaurus is typical. The group went extinct at the end of the Permian and the niche was subsequently filled by other, new temnospondyls, later joined by reptiles such as the phytosaurs in the Triassic period.
Cox and Hutchinson re-evaluated Prionosuchus in 1991 and synonymized it with the genus Platyoposaurus from Russia. On the basis of this study, the Pedra do Fogo Formation was reevaluated to be of Middle to Late Permian age.However, studies based on plants and pollens indicate that this formation is actually early Permian in age, making Prionosuchus not contemporary with Platyoposaurus.
Prionosuchus lived in a humid and tropical environment as indicated by the petrified forest of the Pedra do Fogo formation in which the fossil has been found. The strata composed of siltstones, shales and limestones were deposited in lagoonal and fluvial environments.Other animals discovered in the same rocks include fish (primitive sharks, palaeoniscids, and lungfishes) and amphibians.
Most temnospondyl amphibians are perceived to be hunters of aquatic organisms such as fish and other amphibians, and there certainly is no evidence to refute this for Prionosuchus. With the additional possibility of individuals growing to exceptional sizes, then Prionosuchus may well be one of the key apex predators of the Permian. This would mean that larger Prionosuchus would be able to attack and kill almost anything else in the water, even smaller members of their own species. It is also not impossible that they may have attacked land dwelling animals that came to the water to drink in a similar manner as modern day crocodiles, though it should be pointed out that there is no evidence for this.


Another huge temnospondyli was Mastodonsaurus giganteus
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The name Mastodonsaurus means ‘breast tooth lizard’, and this came about from the observation of G. F. Jaegar who was describing a broken tooth. Later, when other teeth of Mastodonsaurus were found they were found to be no different from the teeth of most other temnospondyls.
The signature features of Mastodonsaurus are the two teeth in the front of the lower jaw that have enlarged to the point of becoming tusks. These are so large that there are two openings in the upper jaw which these tusks fit through when the mouth is closed. Without these openings the mouth simply would not be able to close fully due to the size of the tusks. These tusks may have been for prey capture, allowing a Mastodonsaurus to get a grip upon prey. However, other temnospondyls seem to have managed just fine without these specialisations, so they may have also served a display purpose that allowed Mastodonsaurus to differentiate between themselves and similar temnospondyls.
Mastodonsaurus seems to have been a genus that spent most if not indeed all of its time in the water. The evidence for this is quite compelling, but we’ll begin with noted observations of the body. The head of Mastodonsaurus was so large that it took up almost a quarter of the total body length, something that would have been very cumbersome if on land. The eyes are situated midway on top of the skull, which meant that they were best placed for looking up at whatever may have been swimming above them. Sensory sulci that formed a lateral line of sensory organs were also present, and this would have allowed a Mastodonsaurus to pick up upon changes in water pressure caused by the movement of other aquatic animals. Finally for the body, the limbs were greatly reduced in size, and would have been incapable of lifting the body clear off the ground if out of the water. The joints of these limbs are also particularly weak, in turn suggesting limited muscles.
Further support for an entirely aquatic lifestyle comes from the discoveries of several Mastodonsaurus that seem to have died together after the body of water that they were living in dried out. Had they been able to walk about well on land, they should have at least wandered around for a bit searching for another water source. Coprolites associated with Mastodonsaurus are also mostly composed of fish remains, a prey source that would only be abundant within water.
With all of these factors combined, it seems very likely that Mastodonsaurus were restricted to a lifetime in the water. The wide distribution of the genus across Europe might be explained by individuals using ancient river systems to get around, and perhaps spreading into new areas when these rivers flooded, granting temporary access to new locations, as well as possibly creating traps as new bodies of water were isolated when the flood waters receded, but were not replenished by subsequent floods before they dried out. While the limbs were weak, they were likely plenty strong enough for aquatic life where the weight of the body would have been supported by the water. So it is more likely that the limbs were used for locomotion along the bottom, pushing through dense aquatic plants, to even steering by gentle paddling. Also, while fish seem to have been the main prey source for Mastodonsaurus, there are bones from other temnospondyl amphibians that bear tooth marks seemingly created by the teeth of creatures that had teeth similar to Mastodonsaurus. With this in mind it is possible that Mastodonsaurus may have occasionally hunted other temnospondyl amphibians, or they were quite aggressive in defending their territories against competition from other species.
There have been a great many species assigned to the Mastodonsaurus genus over the years, though at the time of writing only three are considered to be valid. Some of the other former species are now considered to be synonymous to these, while other species have been assigned to different genera. As such, some fossils formerly assigned to Mastodonsaurus have now been moved to Capitosaurus, Cyclotosaurus, Eupelor, Heptasaurus, Parotosuchus, Plagiosternum and Stenotosaurus. Further species such as M. andriani, M. indicus, M. laniarius, M. lavisi, M. meyeri, M. pachygnathus and M. silesiacus are considered to be based upon remains that are so indeterminate that their placement within the genus is highly questionable.



Cyclotosaurus

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1. Eryops megacephalus 2. Mastodonsaurus jaegeri 3. Aphaneramma kannemeyeri 4. Cyclotosaurus robustus
Cyclotosaurus is an extinct genus of temnospondyl within the family Mastodonsauridae. It was of great size for an amphibian, reaching 3–4.3 m (9.8–14.1 ft) in length with an elongated 70 cm (28 in) skull. By the end of the Triassic, the temnospondyl amphibians had already been greatly reduced in number to what they were in the earlier Permian and Carboniferous. They were not all gone however, and some like Cyclotosaurus were without doubt flourishing. So far fossils of Cyclotosaurus have been found all the way from Greenland, across much of Europe, and even as far as Thailand in south East Asia. On top of this the largest individuals of Cyclotosaurus grew to over four meters long, meaning that large Cyclotosaurus were even capable of taking down early dinosaurs. The dentition of Cyclotosaurus however indicates a more piscivorous (fish hunting lifestyle), though Cyclotosaurus could have still snatched smaller animals from the water’s edge.

Koolasuchus at 16 ft long, but only 1 ft high.

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Koolasuchus is an extinct genus of brachyopoid temnospondyl in the family Chigutisauridae. Fossils have been found from Victoria, Australia and date back 120 Ma to the Aptian stage of the Early Cretaceous. Koolasuchus is the latest known temnospondyl. Koolasuchus is known from several fragments of the skull and other bones such as vertebrae, ribs, and pectoral elements. The type species K. cleelandi was named in 1997.
Koolasuchus was an aquatic temnospondyl estimated to have been around 4 to 5 metres (13 to 16 ft) in length.Its mass has been estimated to be up to 500 kilograms (1,100 lb). Although represented by incomplete material, the skull was likely 65 centimetres (26 in) long.Like other chigutisaurids, it had a wide, rounded head and tabular horns projecting from the backside of the skull.
Koolasuchus was named in 1997 from the Aptian Strzelecki Group of the Wonthaggi Formation in Victoria.It is known from four fragments of the lower jaw and several postcranial bones, including ribs, vertebrae, a fibula, and parts of the pectoral girdle. A jawbone was found in 1978 in a fossil site known as the Punch Bowl near the town of San Remo. Later specimens were found in 1989 on the nearby Rowell"s Beach. A partial skull is also known but has not been fully prepared. Koolasuchus was named for the palaeontologistLesley Kool. The name is also a pun on the word "cool" in reference to the cold climate of its environment.The type species K. cleelandi is named after geologistMike Cleeland.
Koolasuchus inhabited rift valleys in southern Australia during the Early Cretaceous. During this time the area was below the Antarctic Circle, and temperatures were relatively cool for the Mesozoic. Based on the coarse-grained rocks in which remains were found, Koolasuchus likely lived in fast-moving streams. As a large aquatic predator, it had a similar lifestyle to crocodilians. Although crocodilians were common during the Early Cretaceous, they were absent from southern Australia 120 million years ago, possibly because of the cold climate. By 110 Ma, represented by rocks in the Dinosaur Cove fossil locality, temperatures had warmed and crocodilians had returned to the area. These crocodilians likely displaced Koolasuchus, leading to its disappearance in younger rocks.

However, an unnamed Late Triassic or Early Jurassic brachiopoid from Lesotho in southern Africa is estimated to have been far larger. At an estimated 7 metres (23 ft), the brachiopoid from Lesotho is one of the largest amphibians sensu lato ever known.This estimate is based on a single jaw fragment found in 1970 by a French expedition near Alwynskop in Quthing.
Because of its size, the fragment was initially considered to be from a mastodonsaur.However, the specimen was redescribed as a brachyopoid in 2005. Several features of the specimen indicate that it is from a brachyopoid. There is a large tusk protruding from the ectopterygoid, a bone of the palate, and the dental morphology is similar to that of other brachyopoids. When viewed from the side, the upper margin of the jaw appears concave.


 






 

 




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Автор: valentint
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