Потребителски вход

Запомни ме | Регистрация
Постинг
16.01.2016 18:45 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part9 Reptiles ch.7 Freshwater turtles and tortoises
Автор: valentint Категория: Забавление   
Прочетен: 1822 Коментари: 0 Гласове:
0

Последна промяна: 22.03 22:22


The largest seems to be the freshwater turtle Stupendemys, with an estimated total carapace length of more than 3.3 m (11 ft) and weight of up to 1,814–2,268 kg (3,999–5,000 lb
image
Stupendemys is a prehistoric genus of freshwater turtle. Its fossils have been found in northern South America, in rocks dating from the late Miocene to the very start of the Pliocene, about 6 to 5 million years ago.
Stupendemys"s carapace measured over 1.80 m (5.9 ft) in length and was also very wide. With an estimated total carapace length of more than 3.3 m (11 ft), it was the largest turtle that ever existed, surpassing even Archelon.The largest freshwater turtle living today is the Arrau turtle (Podocnemis expansa), a pleurodire closely related to Stupendemys, but the Arrau turtle measures only 75 centimetres (30 in).
Two species have been described to date. Stupendemys geographicus was more robust; its remains have been found in the Urumaco Formation of Venezuela. Stupendemys souzai, marginally smaller and more slender, was recovered from the Solimхes Formation in Acre State, Brazil.
Its weight helped Stupendemys stay under water for extended periods of time, grazing on aquatic plants. On the other hand, it was probably a very weak swimmer, unable to move its bulk against a swift current, and thus probably avoided smaller streams.
Since S. souzai fossils have been found in sites which yield a rich fossil fauna, even though little is known with certainty, much can be inferred about the ecology of these animals. Among the aquatic animals that shared the habitat with S. souzai were fish, including catfish such as Phractocephalus and Callichthyidae, characids such as Acregoliath rancii and the tambaqui (Colossoma macropomum), the South American lungfish (Lepidosiren paradoxa), trahiras (e.g. Paleohoplias assisbrasiliensis) and freshwater rays and sharks. Crocodilians and other Crurotarsi were diverse and abundant, among them such taxa as Charactosuchus fisheri, Gryposuchus, Mourasuchus, Nettosuchidae and the giant Purussaurus brasiliensis. Other turtles and tortoises found in the same deposits are Chelus columbiana (a prehistoric relative of the mata mata) and Chelonoidis. Further aquatic vertebrates included river dolphins and the large darter "Anhinga" fraileyi.
Terrestrial mammals were plentiful, and the fauna included many megaherbivores, like the ground sloth Acremylodon campbelli, Toxodontidae (e.g. Gyrinodon and Trigodon), Proterotheriidae, and caviomorph rodents, some of them of immense size also (e.g. Kiyutherium, Neoepiblema, Phoberomys burmeisteri, Potamarchus murinus, Telicomys amazonensis and Tetrastylus). Smaller mammals living in that time and place were the ateline monkey Stirtonia and the bulldog bat Noctilio lacrimaelunaris. Altogether, this fauna is massively dominated by large herbivores and generally lacks terrestrial carnivores. It can thus be assumed that the habitat was mostly low-lying rainforest that was seasonally flooded, as well as floodplains and swampland. The rivers must have been wide and slow-moving, as the fossil-rich rocks are alluvial deposits and do not show evidence of fast-flowing riverbeds that would have dug into the sediment deeply while depositing little of their own.

Carbonemys cofrinii has a shell that measures about 1.72 m (5.6 ft) and was estimated to weigh 916 kg (2,019 lb).
image
Carbonemys is an extinct genus of pelomedusoid turtle known from the early Paleocene Cerrejуn Formation of Colombia. It contains a single species, Carbonemys cofrinii.
In 2005, a 60-million-year-old fossil specimen was discovered in a Colombian coal mine by a North Carolina State doctoral student named Edwin Cadena. It had a shell that measured about 1.72 metres (5 ft 8 in), making it one of the world"s largest turtles.
They lived 5 million years after the mass extinction event of many species of dinosaurs. Their jaws were massive and would be powerful enough to eat a crocodile.
The Carbonemys holotype remains were first discovered in a coal mine, and since coal is essentially fossilised carbon, the name Carbonemys was chosen for the genus. The form of the shell is very interesting as it indicates that Carbonemys would have been a member of the Pleurodira, a group better known as the ‘side-necked turtles’. Side-necked turtles tend to have proportionately longer necks than other turtles, which means that they are too long to retract inwards under the shell. Because of this problem, these turtles instead bend their necks to one side so that they lay against the body and under the ridge of the carapace (upper shell), hence the name ‘side-necked turtle’.
Of course, being of a physically large size means that not many predators would pose a threat to you. However, even though the dinosaurs seem to have gone extinct from South America just a few millions of years before the first known of appearance of Carbonemys, giant super predators had already appeared on the landscape. For example, the giant snake Titanoboa, which at the time of writing may be the largest snake so far discovered, is also known from the same formation as Carbonemys. The presence of a giant turtle and a giant snake at the same time as one another is a good indication as to the kinds of fauna that rose up to replace the ecological gaps that were left behind by the extinction of the dinosaurs, and would continue to be quite dominant until the combined rise of mammals and birds.
Skull remains of Carbonemys are also known, and the form of the skull indicates that Carbonemys had a very strong bite. One idea is that these jaws may have been used for crunching through the armoured bodies of crocodiles such as the genus Cerrejonisuchus, which as the name suggests is also from the Cerrejуn Formation. However it must be remembered that the mouth of Carbonemys could have been used to kill either animals or crop vegetation, or perhaps even both. We also can’t look to other modern turtles of the Pleurodira because some genera are carnivorous while others are herbivorous, which can only further confusion as to the speculated diet of Carbonemys. Only the stomach contents of multiple individuals of Carbonemys could give us a clearer idea as to the exact diet of this genus.



Tortoises
image

Tortoises are a group of terrestrial turtles globally distributed in habitats ranging from deserts to forests and include species such as the Greek and the Galapagos tortoise. Some species evolved large body sizes with a shell length exceeding 1 metre whereas others are no larger than 6-8 centimetres. Despite a particular interest from naturalists ever since the times of Darwin, the evolution of gigantism in tortoises remains enigmatic.
Tortoises are unique among vertebrates in that the pectoral and pelvic girdles are inside the ribcage rather than outside. Tortoises can vary in dimension from a few centimeters to two meters. They are usually diurnal animals with tendencies to be crepuscular depending on the ambient temperatures. They are generally reclusive animals. Tortoises are the longest living land animal in the world, although the longest living species of tortoise is a matter of debate. Galбpagos tortoises are noted to live over 150 years, but an Aldabra giant tortoise named Adwaita may have been the longest living at an estimated 255 years. In general, most tortoise species can live 80–150 years.
The fact that all living giant tortoises are insular may suggest that their evolution followed the so-called island rule: a trend toward dwarfism of large animals and gigantism of small animals on islands. An example of insular dwarfism is the Florida key deer, a dwarf version of the mainland white-tailed deer; its small size may be an adaptation to the limited resources found on the islands. Insular gigantism is best exemplified by the famous dodo, an extinct flightless pigeon from Mauritius, probably evolving large body size due to release from predatory pressure. Previous studies on extant tortoises were partly inconclusive: giant size has been linked to the absence of predatory mammals in islands but it has been also proposed that tortoises were already giants when they reached the remote archipelagos.
The fossils reveal a very different picture of the past compared to the present. Giant size evolved on multiple occasions independently in mainland Asia, Africa, Europe, North and South America at different times of Earth history. However, all of these species went extinct at latest during the Pleistocene ice age.


Megalochelys
image
Megalochelys atlas is the largest known member of Testudinidae, with a shell length of about 2.1 m (6.9 ft), an estimated total length of 2.5 to 2.7 m (8.2 to 8.9 ft), and an approximate total height of 1.8 m (5.9 ft). Popular weight estimates for this taxon have varied greatly with the highest estimates reaching up to 4,000 kg (8,800 lb) in some instances.However, weights based on volumetric displacement of the skeleton,or inferences based on two-dimensional skeletal drawings,indicate that M. atlas was probably closer to 1,000 to 2,000 kg (2,200 to 4,400 lb) in mass.
Megalochelys atlas is an extinct species of giant cryptodiran tortoise from the Miocene through to the Pleistocene periods. During the dry glacial periods it ranged from western India and Pakistan (possibly even as far west as southern and eastern Europe) to as far east as Sulawesi and Timor in Indonesia.
Remains of the shell of Megalochelys atlas indicate that this tortoise lived around much of southeast Asia. Analyses of preserved fossils indicate that the genusMegalochelys was quite successful, with at least 4 recognized species (possibly 7) present from the Late Pliocene to the Early Pleistocene (Rhodin et al. 2015), or a span of about 2 million years.
The original, gigantic size for M. atlas can be traced back to its original description by Falconer and Cautley (as detailed in Murchison 1868). The authors repeatedly compared the size of their specimen to that of an Indian rhinoceros (Rhinoceros unicornis), which can reach masses of 2300 kg (Macdonald 2001; Clauss et al. 2005). This was not just hyperbole, though, as the authors wrote:
A specimen comprising the upper part of the humerus was exhibited, exceeding in size the same portion of the corresponding bone of the Indian Rhinoceros…
image

So, based on general size comparisons with an Indian rhino, it would appear that M. atlas may have reached slightly over two tonnes in mass. Of course, this is operating under the assumption that the rhino they looked at was a large male, and that they accounted for the differences in anatomical proportions between rhino and tortoise humeri.
Falconer and Cautley never gave a weight estimate to their M. atlas fossils, but Falconer did provide an estimate of overall length, based on a comparison of bony elements between M. atlas and a Chelonoidis nigra specimen available to him from the College of Surgeons. That specimen had a carapace length of 107 cm (42 inches), and a curved carapace length (CCL) of 137 cm (54 inches). According to the Galбpagos Islands tourism site, that translates roughly to an animal of about 227 kg (500 lbs) in weight. In contrast, the estimated carapace length and CCL for M. atlas was estimated as: 373 cm (12 ft 3 inches) and 480 cm (15 ft 9 inches) respectively. That is 3.5 times longer than the modern-day Galбpagos tortoise. If we assume that the overall anatomy was similar between both species, and if we assume that M. atlas scaled isometrically to C. nigra, then we can use known scaling relationships such as the fact that volume (and hence, mass) scales to the third power, to calculate the estimated mass for Falconer and Cautley’s large M. atlas specimen.
3.53 = 43 times heavier.
227 kg * 43 = 9,761 kg (9.7 tonnes)
This would have made Megalochelys atlas insanely huge, putting it on par with a very large bull African elephant. Of course, this is an isometric scaling attempt, which works for members of the same species (think Shaq vs. Jet Li), but doesn’t hold true across species. Modern-day giant tortoises are a lot lighter than they would appear based off a cursory glimpse of their skeletons. This is due to the evolution of weight saving features on their bodies, such as a honeycombing of the shell microstructure (Scheyer and Sбnchez-Villagra 2007). Although there has yet to be any microstructural analyses of M. atlas shells, gross anatomical descriptions indicate that similar weight-saving features were likely present in this animal as well (see below). As such, the estimated weight based on isometry is likely going to be too much. Exactly how much is difficult to assess, but it is still unlikely to bring the weight down to the typical heavyweight estimate of 4 tonnes.
image

M. atlas size compared to contemporary giant turtles and a 1.8 m (6 ft) person. 1. M. atlas based on Falconer & Cautley’s original measurements. 2. M. atlas size based on more accurate shell measurements. 3. Leatherback sea turtle at 2.1 m (7 ft) total length. 4. Galбpagos tortoise at 1.2 m (4 ft) carapace length. M. atlas silhouettes based on Aldabra tortoise.
The original dimensions of M. atlas, as estimated by Falconer and Cautley, were huge. Falconer calculated that Megalochelys atlas had a 3.73 meter (12 ft 3 inch) carapace, with a 1.8 meter (6 ft) neck, and a 0.6 meter (2 ft) head and tail. So, the initial total length of the animal was estimated at 6.8 meters (22 ft 3 inches). At this colossal size, a mass of 5 or 6 tonnes would not be out of the question. However, it turns out that the original estimates given by Falconer and Cautley (1844) were mismeasurements taken from a composite specimen (composite specimens will compound any errors in the original measurements). Lydekker (1889) first called this into question and suggested that the actual length of the carapace in the  large M. atlas specimen found by Falconer and Cautley, was probably about 1.8 meters (6 ft). This result agreed well with a second, preserved shell discovered by Barnum Brown in 1931. Upon reconstruction, Brown found the curved carapace length to be 2.2 meters (7 ft 4 inches). According to Brown (1931), the specimen they recovered was approximately the same size as the one Falconer and Cautley had discovered nearly a century earlier. Brown’s reconstruction resulted in a tortoise that was 1.6 times longer than a modern-day Galбpagos tortoise, which translates to a 4x estimated higher mass, or approximately 929 kg (2,046 lbs). Interestingly, Brown attempted to estimate the mass of Megalochelys atlas using a 1:1 scale model that was immersed in water to determine its volume via displacement. His results suggested that their M. atlas specimen would have weighed 955 kg (2,100 lbs), which isn’t too far off from the isometry estimate.



Meiolania

image

 

Meiolania had an unusually shaped skull that sported many knob-like and horn-like protrusions. Two large horns faced sideways, giving the skull a total width of 60 centimetres (2.0 ft), and would have prevented the animal fully withdrawing its head into its shell. The tail was protected by armored "rings", and sported thorn-like spikes at the end.The body form of Meiolania may be viewed as having converged towards those of dinosaurian ankylosaurids and xenarthran glyptodonts.
The animal was rather large, measuring 2.5 metres (8.2 ft) in length, making it the second-largest known nonmarine turtle or tortoise, surpassed only by Megalochelys atlas from Asia, which lived in the Pleistocene. It lived in Australia, Lord Howe Island, New Caledonia and Vanuatu.
Meiolania turtles fed on plants. Because the remains known by 1925 were found close to beaches, it was thought to be aquatic. It is now known to have been terrestrial.
The genus was erected in 1886 based on remains found on Lord Howe Island, which Richard Owen assigned to the two species M. platyceps and M. minor (now a synonym of the latter).These were the first good meiolaniid remains, and were used to show that the first known remains of a related animal, a species from Queensland now known as Ninjemys oweni (which was assigned to Meiolania until 1992), did not belong to lizards as initially thought, but to turtles.Woodward sank Niolamia argentina into Meiolania, but this was not accepted by later authors.
In New Caledonia, M. mackayi was described from Walpole Island in 1925. It was smaller and less robust than M. platyceps.Meiolania remains are also known from the Pindai Caves, Grande Terre, and from Tiga Island.
M. brevicollis was described in 1992 from the Camfield Beds of northern Australia, and differed from M. platyceps in having a flatter skull and other horn proportions.
Remains of M. damelipi have been found on the island of Efate in Vanuatu, associated with settlements from the Lapita culture.
It is thought that M. damelipi was hunted to extinction by the Lapita people about 3,000 years ago, based on the presence of bones at the bottom of a garbage midden at an archeological site.The remains were primarily leg bones, indicating the turtles were butchered elsewhere.The bones were not present in younger layers of the mound, suggesting the turtles disappeared within 300 years of first human contact.

 

Titanochelon
image

Several giant tortoises called Europe home. The last, Titanochelon, was also the largest ever to evolve on the continent. Some Titanochelon tortoises had shells approaching 6.6ft (2m) in length. This makes them considerably larger even than today"s Galбpagos tortoises.
Until recently, palaeontologists mostly assumed that Titanochelon had disappeared from western Europe about 3.3 million years ago. They may have clung on in a few places – there are remains, perhaps 2.4 million years old, from the Greek island of Lesbos, which was connected to mainland Europe at the time – but this seems to have been the overall trend.
However, the tortoise remains described by Pйrez-Garcнa and his colleagues in January 2017 suggest a rethink is in order.
At a site called Fonelas P-1 in the Baetic Mountains of south-east Spain, palaeontologists came across a thighbone and toe bone, both of which clearly belonged to Titanochelon.
Predators may well have been one of the factors that drove Titanochelon to evolve such large bodies
The bones are just two million years old. That means giant tortoises survived on the European mainland for 1.3 million years longer than palaeontologists thought.
The bones were unearthed in an ancient hyena den. However, Pйrez-Garcнa suspects that their presence there reflects scavenging rather than predation.
"Carrion was very abundant in that warm region, with abundant water resources," he says.
In fact, Pйrez-Garcнa says that the pressure from predators may well have been one of the factors that drove Titanochelon to evolve such large bodies.

 




Гласувай:
0
0



Няма коментари
Търсене

За този блог
Автор: valentint
Категория: Политика
Прочетен: 222829
Постинги: 161
Коментари: 25
Гласове: 13
Архив
Календар
«  Март, 2019  
ПВСЧПСН
123
45678910
11121314151617
18192021222324
25262728293031