Потребителски вход

Запомни ме | Регистрация
Постинг
16.01.2016 23:18 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part10 Amphibians ch.1 Temnospondyls
Автор: valentint Категория: Забавление   
Прочетен: 3376 Коментари: 0 Гласове:
0

Последна промяна: 12.12.2018 21:34


Amphibians (Amphibia)
image


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.
image
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.

imageAs 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.

The largest known amphibian of all time was the 30 ft long temnospondyli Prionosuchus.
image

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
image
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
image
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.


Melosaurus
image
Melosaurus is an extinct genus of temnospondyl amphibian. Fossils of it have been found in Russia. Fossil remains of Melosaurus uralensis were discovered at Belebei in Russia. Its fossils dated of the Asselian-Olenekian stages (299 - 245 million years ago). Fossils of Melosaurus sp. were also unearthed at the site. Fossils of Melosaurus uralensis were found at the Sterlitamak site in Bashkortostan in Russia. The span of the strata is listed as Wordian (268 - 265.8 million years ago).
Melosaurus was 2.5–3 m (8.2–9.8 ft) in length. It lived around ponds, lakes, rivers, and marshes. The diet of Melosaurus consisted of fish and smaller tetrapods. Melosaurus was a powerful predator. It very likely preyed upon Discosauriscus.


Koskinonodon size: About 3 meters long. Skull about 65 centimeters long.
image
Koskinonodon
was originally named as Buettneria back in 1922, however it was later realised that Buettneria had already been used to name a genus of katydid (bush cricket). Therefore in 1929 this temnospondyl amphibian was renamed Koskinonodon. In addition to this a species of Metoposaurus, M. bakeri was moved to create a new species of Koskinonodon, K. bakeri, in 1931. Additionally another species of Metoposaurus, M. maleriensis is now included with Koskinonodon.
Koskinonodon is regarded as a metoposaurid temnospondyl amphibian, which means that it is closest to genera such as Metoposaurus and Apachesaurus. These temnopondyls are noted for having eyes that were situated in a more forward position on their skulls than the eyes of other similar temnospondyl groups. Also like its relative genera, Koskinonodon seems to have been more at home in the water where it probably hunted for fish and possibly other amphibians. The limbs are generally not that well supported for terrestrial locomotion, and the presence of a lateral line formed by sensory sulci would have detected changes in water pressure, allowing them to pick up upon the movements of nearby swimming animals.
Further evidence for a mostly aquatic lifestyle can be inferred from collections of Koskinonodon which can be interpreted as mass graves where a body of water dried out, leaving many Koskinonodon exposed to the dry air. The remains of these Koskinonodon are found so close together that they seem to have clustered together in the last remnants of water before death. This may have been the result of a body of water not being replenished by seasonal rain or flood water.
Koskinonodon is best known from the United States, particularly the state of Arizona, where Koskinonodon fossils are known from many members of the Chinle Formation. Remains from India however indicate that Koskinonodon had a much wider distribution than previously thought. Koskinonodon lived in the latter portion of the Triassic, and by the time the Jurassic period started, most of the temnospondyls including Koskinonodon had disappeared. Only a rare few exceptions such as the genera Siderops and Koolasuchus are known to have survived well beyond this point.

 

Metoposaurus
image
Metoposaurus pronounced Me-top-o-sore-us) meaning "front lizard" is an extinct genus of Stereospondyli temnospondyl amphibian, known from the Late Triassic of Germany, Italy, Poland, and Portugal.This mostly aquatic animal possessed small, weak limbs, sharp teeth, and a large, flat head.This highly flattened creature mainly fed on fish, which it captured with its wide jaws lined with needle-like teeth. Metoposaurus was up to 3 m (10 feet) long and weighed 454 kg (1,000 pounds),Many Metoposaurus mass graves have been found, probably from creatures that grouped together in drying pools during drought.
The genus Metoposaurus was originally named as Metopias by Christian von Meyer in 1842, but the name was replaced and changed to Metoposaurus by Richard Lydekker in 1890. Metoposaurus is the type species of the Metoposauridae, a group of temnospondyl amphibians that are particularly noted for having eyes that are placed further forward on the head. Relatives of Metoposaurus include Apachesaurus, Bogdania, Dutuitosaurus and Koskinonodon.
Like relative genera, Metoposaurus lived during the closing stages of the Triassic, and shows developments that best support a primarily if not an entirely aquatic lifestyle. Most obvious is the limb reduction, with the legs seemingly not large of strong enough to lift the body clear off the ground if an individual Metoposaurus found itself out of the water. The shape of the skull is also quite flat on top which means that a Metoposaurus near the surface of the water would not expose itself that much to anything that was on the surface. Grooves in the skull also hint at the presence of a sensory system similar to the lateral line of a fish which would have detected changes in water pressure caused by the movement of nearby animals. The latter development is not unique to the metoposaurid however since similar adaptations can also be seen on other genera such as Mastodonsaurus, though again this genus is widely believed to have been entirely aquatic. Another piece of evidence that supports an entirely aquatic lifestyle is the discovery of several Metoposaurus clustered together in what is thought to have been a dried out body of water, an occurrence which has also been seen in other related genera.











 

 




Гласувай:
0
0



Няма коментари
Търсене

За този блог
Автор: valentint
Категория: Политика
Прочетен: 222786
Постинги: 161
Коментари: 25
Гласове: 13
Архив
Календар
«  Март, 2019  
ПВСЧПСН
123
45678910
11121314151617
18192021222324
25262728293031