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16.12.2015 16:44 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part1 Mammals ch.1 Carnivores -Mesonychids
Автор: valentint Категория: Забавление   
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Mesonychids (Mesonychia)
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Mesonychia is a group of archaic hoofed mammals present in the Paleocene and Eocene of Asia, Europe, and North America.They are generally considered to be cursorial, Hyaena-like, scavenging carrion-feeders. Mesonychids are important as a group because of their distinctive morphological and functional specializations, because of their broad geographic distribution, and because they may be related to the origin of whales.
The great temporal range of mesonychians invites consideration of evolutionary changes they exhibit. Mesonychids are placed in their own order, Mesonychia. Mesonychia include two families, Mesonychidae and Andrewsarchidae.Hapalodectidae with one genus Hapalodectes, to the order Mesonychia is placed in the Mesonychia.Two Asian genera, Dissacusium and Yantanglestes, and one dubious species,Hukoutherium shimemensis, are separately placed in the Mesonychia.
Mesonychids were medium- to large-sized mammals. Body weights of 22 mesonychid species are estimated to range from 7 kg to 193 kg, with a median of 36 kg.
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1. Sinonyx 2. Andrewsarchus 3. Ankalagon 4. Synoplotherium 5. Dissacus
Study of Pachyaena gracilis and Pachyaena ossifraga suggests that mesonychids were not sexually dimorphic.
Postcranial study of five mesonychid genera, Ankalagon, Dissacus, Sinonyx, Pachyaena, and Mesonyx, show that mesonychids achieved greater cursorial capability over time through proportional and structural changes in the limbs.These proportional and structural changes increased the functional lengths of limbs, minimized rotational and mediolateral movements of limb joints, and increasingly limited limb movement to a parasagittal plane.
The mesonychids were an unusual group of condylarths with a specialized dentition featuring tri-cuspid upper molars and high-crowned lower molars with shearing surfaces. They were once viewed as primitive carnivores, like the Paleocene family Arctocyonidae, and their diet probably included meat and fish. In contrast to this other family of early mammals, the mesonychids had only four digits furnished with hooves supported by narrow fissured end phalanges.
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They first appeared in the Early Paleocene, undergoing numerous speciation events during the Paleocene, and Eocene.Mesonychids fared very poorly at the close of the Eocene epoch, with only one genus, Mongolestes,surviving into the Early Oligocene epoch. Mesonychids probably originated in Asia, where the most primitive mesonychid, Yangtanglestes, is known from the early Paleocene. They were also most diverse in Asia where they occur in all major Paleocene faunas. Since other carnivores such as the creodonts and condylarths were either rare or absent in these animal communities, mesonychids most likely dominated the large predator niche in the Paleocene of Asia. Throughout the Paleocene and Eocene, several genera, including Dissacus, Pachyaena and Mesonyx would radiate out from their ancestral home in Asia and into Europe and North America, where they would give rise to new mesonychid genera. These animals would have migrated to North America via the Bering land bridge.


The largest mesonychid was Andrewsarchus mongoliensis

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It is known only from one skull which was 83 cm (33 in) long and 56 cm (22 in) wide. Andrewsarchus mongoliensis  is an extinct mammal that lived during the Eocene epoch, roughly between 45 and 36 million years ago. It had a long snout with large, sharp teeth and flat cheek teeth that may have been used to crush bones. Because Andrewsarchus is only known from a single skull, whether it was an active predator or a large scavenger is open to debate, as is its exact time range.
Andrewsarchus is known only from one enormous skull (32.8 in/83 cm long and 22 in/56 cm wide) and fragments of bone. If Andrewsarchus was proportioned in the same manner as Mesonyx obtusidens, it had a length from the snout to the back of the pelvis of about 11 feet (3.4 m) and a height from the ground to the shoulder or middle of the back of about 6 feet (1.8 m). Thus in round numbers it is possible that it may have been three times the size of Synoplotherium (Dromocyon) vorax or of Mesonyx obtusidens and possibly the largest land-dwelling carnivorous mammal known.Its chief rival for this title is the South American short-faced bear Arctotherium, which is estimated to have weighed up to 1,700 kg (3,700 lb). The cranium is about twice the length of that of a modern Alaskan brown bear (Ursus arctos middendorffi), but with a lower length-to-width ratio, and about triple the length of an American wolf"s (Canis lupus occidentalis). With modern brown bears or polar bears weighing between 450 kg (~1,000 lb) and 675 kg (~1,500 lb) and only an extreme specimen of a wolf weighs up to 77 kg (170 lb), this plausibly puts Andrewsarchus in the 1,000 kg (2,200 lb) size range. This weight appears close to the practical size limit of carnivorous land mammals, possibly relating to available food as well as metabolic requirements.There has as yet been no post-cranial material found. As it is not known if Andrewsarchus had a robust or gracile build, the weight of the average animal is in dispute. If the build was robust, some specimens of the animal might have weighed up to 4,000 pounds.
Andrewsarchus is named for the famous explorer and fossil hunter Roy Chapman Andrews. It was discovered in June 1923 by Kan Chuen Pao, a member of Andrews" expedition, at a site in the Gobi Desert in Inner Mongolia known as Irdin Manha [variants: Erdeni-Mandal and Erdenemandal ("jeweled mandala")] on the third Central Asiatic Expedition that was led by Andrews and sponsored by the American Museum of Natural History. The skull is now on display in the American Museum of Natural History in New York; the lower jaw was not found.
It was classified in the clade Mesonychia due to the similarity in structure between its teeth and skull with those of other mesonychid species known from complete skeleton, however, much of this was based only on Osborn"s original publication, and more recent studies have found it to have no special mesonychid affinities, instead grouping with various artiodactyl clades. Indeed one study (Spaulding et al.) has not only found them to be closer to entelodonts, but as kin to Whippomorpha in their Cetancodontamorpha.
The appearance and behavioral patterns of Andrewsarchus are virtually unknown and have been topics of debate among paleontologists ever since it was first discovered. All that is known about Andrewsarchus comes chiefly from the single meter-long skull found in Late Eocene sediments in what is now Mongolia.
New theories indicate that the teeth of Andrewsarchus may have been blunt and uncharacteristic of predators. Its diet could have been more omnivorous than carnivorous, consisting of carrion, bones, rooted plants, or mollusks rather than freshly killed meat. As a scavenger, Andrewsarchus may have gained access to freshly killed carcasses by using its formidable size to scare away other smaller predators and scavengers. Until more fossil evidence that may provide insight into these areas of uncertainty is uncovered any reconstructions remain highly speculative.
Andrewsarchus possessed some of the strongest jaws ever seen in a land mammal, able to bite through large bones if needed. To judge from its immense jaws, and the coastal location of the fossils, Andrewsarchus may have fed on beached primitive whales, shellfish and hard-shelled

Mongolonyx
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Mongolonyx is an extinct genus of carnivorous mesonychid mammal that lived during the Middle Eocene in Mongolia. It became extinct during the Oligocene. It was described by Szalay and Gould.
There are two types.Mongolonyx robustus - a more archaic look.Known for poorly preserved skull (almost no teeth, no dorsal part, but with the lower jaw).Mongolonyx dolichognathus)- A later species, belongs to the formation of the Irdyn Manga.Found lower jaw and upper jaw fragment.
The genus is clearly diagnosed and has a number of characteristic morphological features. So, from Mesonyx it is distinguished by more significant overall size,as well as relatively more obtuse and large molars.His teeth are also relatively larger and more massive than those of Dissacus, Harpagolestes and most other members of the family. Unlike Dissacus and Pachyaena, Mongolonyx lacks cingulum on the upper teeth and does not have any traces of metaconid on the lower molar trigonides. This genus is distinguished from Synoplotherium, Mongolestes and Harpagolestes by a more straight lower jaw with an elongated and relatively narrow symphysis.
The Mongolonix is were large carnivores. The length of their skull exceeded 60 cm, which is quite comparable with the skull of the large Alaskan grizzly.Based on the proportions of Mesonyx, known for more or less complete skeleton, it can be assumed that these mesonichids reached approximately 2.8 m in length (not including the tail) and more than 1.4 m at the withers.


Mongolestes
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("Mongolian robber") is an extinct genus of mesonychid known from the "Ulan Gochu" formation of Inner Mongolia and likely originated in Asia.Mongolestes was the last representative of Mesonychia, having died out completely in the early Oligocene, and outlasting all other
mesonychids,being Mongolestes is distinct from other mesonychids in several dental features, including very large teeth and the loss of M3, and a mandibular symphysis that is steeper.
In the early Oligocene ends a long history of ancient flesh-eating monsters - Mesonychia, who «hold" in the steel jaws of the power of the supreme predators. The last representative of this dynasty - Mongolestes, crowning one of the branches of the Mesonychidae family, living out the remainder of time allotted to his squad in Asia.
Mongolestes were about 33.9 - 30 million years ago. From these animals found so far are relatively few - the jaw and teeth, but the remarkable size of the finds. The sources of measurements are approximate, but even by conservative estimates, they were no less large lions. For example, when the lower jaw is 36.4 cm - the skull it is was about 50 cm in length. The bones of the skeleton has not yet been found, but since Mongolestes (with Mongolonyx and Harpagolestes - such as large predators) was in close relationship with Mesonyx, whose skeleton is restored, it could well have similar proportions of the body. On this basis, Mongolestes could easily reach 1.10 - 1.20 at the withers and about 2.2 - 2.4 in length without the tail.
Crooked teeth were like garden secateurs. This form of the jaw with backward curved premolars contributed to the retention of crack bones.
In his family Mongolestes had the largest teeth relative to the jaws. With a shortened snout and stiff up the front of the lower jaw it was a "bulldog" in Mesonychidae. In practice, Mongolestes had one of the most powerful bone-crushing dentoalveolar apparatus of carnivorous mammals of their size class. Compare with him, perhaps, can only creodont Hyainailouros and some Amphicyonidae, such as Ischyrocyon.
In general, the dull massive cheek teeth, worn teeth and dental-maxilla a general view of the device indicates a specialization Mongolestes to consumption carrion and the remnants of the meal predators. Of course, they robbed the smaller hunters. It is able to attack the weakened animal disease, injuries, old and young fledgling. In general, the way of life, as it should be carnivorous their size class. This is quite predictable, because of increasing in size predators, there is always a temptation, so to say, "go the easy way" by going from an active hunt for more easily rob and the consumers of the corpses. However, it is virtually a dead end in development, because the larger the animal, the more food it requires. And if he is a dedicated carnivore, much more vulnerable to any fluctuations in the well-established environmental conditions that result in a change in the number of herbivores. The rarity of findings may indicate that for every Mongolestes was assigned a large area, but because their populations were not particularly high, which does not promote rapid recovery of the number of thinning the number of predators in the drought and lack of fodder.
Mongolestes lived mainly in woodlands and open areas. It is noteworthy that with them around the same time in Asia, lived a large creodonts Hyaenodon gigas and Megalopterodon mongoliensis. However, these relatively lightweight predators could not withstand this "tank with teeth.

 

 



 




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