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17.12.2015 15:09 - Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part1 Mammals ch.9 Desmostylians and Sirenians
Автор: valentint Категория: Забавление   
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Sirenians (Sirenia)
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1. Metaxytherium 2. Protosiren 3. Hydrodamalis 4. Prorastomus  5. Pezosiren
Sirenia are the order of placental mammals which comprise modern sea cows (manatee and dugongs) and their extinct relatives. They are the only herbivorous marine mammals now in existence and the only group of herbivorous mammals to have became completely aquatic. Sirenians have a 50 million year old fossil record (early Eocene-Recent). They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference.Sirenians" closest living relative are Proboscidea (elephants). Tethytheria, are a larger group comprised of Sirenia, Proboscidea, extinct Demostylia and likely the extinct Embrithopoda. Tethyeria appear to have evolved from primitive hoofed mammals known as condylarths, along the shores of the ancient Tethys sea.
The tethyeria, combined with Hyracoidea (hyraces) form an inclusive group called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian orders, with strong support from morphological and molecular research. The ancestry of sirenia is remote from cetacea and pinnipeds, though re-evolving an aquatic lifestyle simultaneously.

The first appearance of Sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian known to date, Prorastomus, was found in Jamaica, not the Old World.
The earliest known sea cows, of the families Prorastomidae and Protosirenidae, are both confined to the Eocene, were about the size of a pig, four legged amphibious creatures. By the time the Eocene drew to a close, came the appearance of the Dugongidae and Sirenians had acquired their familiar fully-aquatic streamliined body with flipper-like front legs with no hind limbs, powerful tail with horizontal caudal fin, with up and down movements which move them through the water, like Cetaceans.
The last of the Sirenian families who made their appearance, Trichechidae, apparently arose from early Dugongids in the late Eocene or early Oligocene. The current fossil record documents all major stages in hindlimb and pelvic reduction from completely terrestrial morphology to the extreme reduction in modern manatee pelvis, providing an example of dramatic evolutionary change among fossil vertebrates.
Since sirenians first evolved, they were herbivores, likely depending on seagrasses and aquatic angiosperms (flowering plants) for food. To the present, almost all have remained tropical, marine and consume angiosperms. Sea cows are shallow divers with large lungs, they have heavy skeletons to help them stay submerged. The bones are pachyostotic (swollen) and osteosclerotic (dense), especially the ribs which are often found as fossils.
Although cheek teeth are relied on for identifying species in other mammals, they do not vary to a significant degree among Sirenians in their morphology, but are almost always low-crowned (brachyodont) with two rows of large, rounded cusps (bunobilophodont). The easiest identifiable parts of sirenian skeletons are the skull and mandible, especially the frontal and other skull bones. With the exception for a pair of tusk-like first upper incisors present in most species, front teeth (incisors and canines) are lacking in all, except the earliest sirenians.
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Dugongids comprise the majority of specimens that compose the known sirenian fossil record. The basal members of this family are placed in the Eocene-Pliocene, and subfamily Haliteriinae. This group includes the fossil genera Halitherium and Metaxytherium.

Metaxytherium gave rise in the Miocene to the Hydrodamalinae, an endemic North Pacific lineage that ended with Steller"s sea cow (Hydrodamalis) - the largest sirenian that ever emerged, with a length of up to 9 meters or more. This species was also the only one which make a successful adaptation to temperate and cold waters, and diet of marine algae. It was completely toothless and its truncated, claw like flippers which it used for gathering plants and fending off from rocks, lacked the presence of phalanges (finger bones). Humans hunted Steller"s Seacow to extinction in the eighteenth century.
Another offshoot of Halitheriinae, the subfamily Dugonginae, appeared in the Oligocene. Most dugongines appear to have been specialists at digging out and eating tough, buried rhizomes of seagrasses; for this purpose many had large, self-sharpening blade-like tusks (Domning, 2001). Modern Dugong is the only survivor of this group, but it has reduced dentition (cheek teeth having only thin enamel crowns, which quickly wear off and leave simple pegs of dentine. For this reason, the dugong likely shifted toward a more delicate diet, consisting of seagrasses and ceased using its tusks for digging.
The Trichechidae have by far more a scant fossil record than dugongids. Their definition has been widened to incorporate Miosireninae, a little-known pair of genera that inhabited NW Europe in the late Oligocene and Miocene. Miosirenines had massively reinforced plates and dentitions that may have been used for crushing shellfish. Such diet in sirenians living around the North Sea seems of little surprise considering modern dugongs and manatee near the climatic extremes of their ranges are known to consume invertebrates in addition to plants.

Manatees are now placed in the subfamily Trichechinae. They first made their appearance in the Miocene, represented by Potamosiren from fresh water deposits in Columbia. Much of trichechine history was likely spent in South America, from where they spread to North America and Africa only in the Pliocene or Pleistocene.
It was during the late Miocene, manatees living in the Amazon basin apparently adapted to a diet of abrasive freshwater grasses, and this innovation is still used by their modern descendants. Interestingly, they continue to add on extra teeth to the molar series their entire lifetime. As worn teeth fall out at the front, the whole tooth row slowly shifts to the front to make room for new teeth erupting in the rear. This horizontal tooth replacement has often been likened, incorrectly, to that of elephants, but the latter are only limited to three molars. Only one other mammal, an Australian rock wallaby (Peradorcas concinna) has truly evolved the same kind of tooth replacement system, found in manatees.


The largest prehistoric
sirenian was Hydrodamalis cuestae at body length is more than 9 m and weight according to some estimates could reach 10 tons
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Hidrodamalis Cuestae (Hydrodamalis cuestae Domning, 1978) - kind of extinct dugongidae sirens (Dugongidae), the probable ancestor of Steller"s sea cow (Hydrodamilis gigas). I inhabited in the Late Miocene and Pliocene over (approx . 5-2 million . L . N . ) Along the coasts of the southern United States and northern Mexico , and in the first half of the Pliocene in Japan
Ancestors Hydrodamalis cuestae are older and smaller size Hydrodamalinae of the genus Dusisiren. The most likely candidate for this role - Dusisiren takasatensis, found in the middle part of the sediments . Honshu (Japan ) , with a Late Miocene age ( approx. 6-10 million . Years ) . According to some morphological characteristics ( reduction of the dentition , the shape of the palate , the structure of the articular surface of the occipital , and others ) it is intermediate between the earlier Dusisiren Jordani and Hydrodamalis cuestae
Genus Hydrodamalis cuestae was described in 1978 by the remnants of the Late Pliocene sediments formation Pismo , California ( USA) .To date, fossils of this animal are known and in other locations of California and dated by the time from the Late Miocene on Late Pliocene ( gemfilly Blanca ) .Found committed in Baja California (north of Mexico ) are Late Pliocene age.The remains of this animal have been found in the Early Pliocene sediments of  Hokkaido (Japan ).Some of the finds from there in 1988, was described by the species name Hydrodamalis spissa Furusawa , 1988 , there were also other residues of . Hokkaido uncertain species identification (Sirenia indet. Illiger 1811) . Today, it found that the Japanese findings also apply to Hydrodamilis cuestae genus.
Maximum Hydrodamalis cuestae body length is more than 9 m, the skull had a length of approx. 79 cm, weight according to some estimates could reach 10 tons.Hidrodamalis Cuestae is the largest known member of the order sirens (Sirenia) for all time of its existence. The animal had a relatively small head, a massive fusiform body, ending with a powerful tail fin. The skeleton was heavy, bone density had a significant and served as a kind of ballast, to prevent the expulsion of the body to the water surface.The front limbs were shortened brush skeleton greatly reduced, many phalanx missing.Animals move,alternately moving the front legs on the bottom surface or gently flapping wide tail fin.
Habitat of these large animals were shallow areas of inland seas, quiet coves, bays and estuaries, where they were more or less protected from the weather and could find a plentiful food - algae and sea grasses.In the mouth, food is ground by means of two special horny plates that were on the palate and on the lower jaw.The teeth of these sirens were completely absent, however, from their embryos laid the alveoli of the upper molars, later filled by connective tissue.With high probability we can assume that Hidrodamalis Cuestae was a gregarious animal and kept in family groups.
Genus extinct at the end of the Pliocene, approx. 2 Mill. Years ago. It seems likely that the disappearance Hydrodamalis cuestae was somehow connected with the beginning of the Ice Age, accompanied by cooling and, as a consequence, changes in habitat of many marine plants and animals, a decrease of fodder areas and the spread of new types of competitors and predators. However, before you complete your disappearance Hidrodamalis Cuestae obviously gave rise to Steller"s sea cow (Hydrodamalis gigas), several more specialized and better adapted to life in the cool shallows of the Pacific north.

 

The second largest prehistoric sirenian was Steller"s sea cow at 8–9 m long (26–30 ft).
image The Steller"s sea cow (Hydrodamalis gigas) is an extinct herbivorousmarine mammal. It was the largest member of the order Sirenia, which includes its closest living relative, the dugong (Dugong dugon), and the manatees (Trichechus spp.). It reached up to 9 metres in length, making it among the largest mammals other than whales to have existed in the holocene epoch. Although the sea cow had formerly been abundant throughout the North Pacific, by 1741, when it was first described by Georg Wilhelm Steller, chief naturalist on an expedition led by explorer Vitus Bering, its range had been limited to a single, isolated population surrounding the uninhabited Commander Islands. Within 27 years of discovery by Europeans, the slow-moving and easily captured Steller"s sea cow was hunted to extinction.
The sea cow grew to at least 8 to 9 m (26 to 30 ft) in length as an adult, much larger than the manatee or dugong; however, concerning their weight, Steller"s work contains two contradictory estimates: 4 and 24.3 metric tons. The true value is estimated to lie between these figures, at around 8 to 10 t. It looked somewhat like a large seal, but had two stout forelimbs and a whale-like fluke.
According to Steller, it "is not the sea cow of Aristotle, for it never comes upon dry land to feed", but it can use its fore limbs for a number of tasks: swimming, walking on the shallows of the shore, supporting himself on the rocks, digging for algae and seagrasses, fighting, and embracing each other.
"It is covered with a thick hide, more like unto the bark of an ancient oak than unto the skin of an animal; the manatee’s hide is black, mangy, wrinkled, rough, hard, and tough; it is void of hairs, and almost impervious to an ax or to the point of a hook."
Its head is small and short compared to the huge body. The upper lip is so large, so broad, and extends so far beyond the mandible, that the mouth appears to be located underneath the skull. The mouth is rather small, toothless, and equipped with double lips, both above and below. When it closes its mouth, the space between the lips is filled up with a dense array of very thick white bristles, 1.5 in (38 mm) long. These bristles take the place of teeth and are used to pull out seaweed and hold food. Mastication is performed by two white bones or solid tooth masses.
It was completely tame, according to Steller. It fed on a variety of kelp. Wherever sea cows had been feeding, heaps of stalks and roots of kelp were washed ashore. The sea cow was also a slow swimmer and apparently was unable to submerge.
The number of sea cows was small and limited in range when Steller first described them; although he had said they were numerous and found in herds, zoologist Leonhard Hess Stejneger later estimated that at discovery there had been fewer than 1,500 remaining, and thus had been in immediate danger of extinction from overhunting. There is evidence that sea cows also inhabited the Near Islands during historic times. Oral tradition on Attu stated that sea cows were still hunted there after their extinction on the Commander Islands.
Fossils indicate Steller"s sea cow was formerly widespread along the North Pacific coast, reaching south to Japan and California in the US. Given the rapidity with which its last population was eliminated, aboriginal hunting likely caused its extinction over the rest of its original range (aboriginal peoples apparently never inhabited the Commander Islands).
The species was quickly wiped out by the sailors, seal hunters, and fur traders who followed Bering"s route past the islands to Alaska, who hunted it both for food and for skins, which were used to make boats. It was also hunted for its valuable subcutaneous fat, which was not only used for food (usually as a butter substitute), but also for oil lamps because it did not give off any smoke or odour and could be kept for a long time in warm weather without spoiling. By 1768, 27 years after it had been discovered by Europeans, Steller"s sea cow was extinct.
It has been argued that the sea cow"s decline may have also been an indirect response to the harvest of sea otters by aboriginal people from the inland areas. With the otters reduced, the population of sea urchins would have increased and reduced availability of kelp, the sea cow"s primary source of food. Thus, aboriginal hunting of both species may have contributed to the sea cow"s disappearance from continental shorelines. In historic times, though, aboriginal hunting had depleted sea otter populations only in localized areas.The sea cow would have been easy prey for aboriginal hunters, who would likely have exterminated accessible populations with or without simultaneous otter hunting. In any event, the sea cow was limited to coastal areas off islands without a human population by the time Bering arrived, and was already endangered. It has been demonstrated that the extinction of the sea cow could have been effected solely by the hunting of the sea cow for meat by fur-trading mariners of the time, and no other factors needed to have contributed.


Another contender was Rytiodus which was 6 m long (20 ft). It was about twice the size as modern sirenians.
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Rytiodus (Meaning Rytina - wrinkled an old name for Steller"s sea cow. ) is an extinct genus of sirenian, whose fossils have been discovered in France, Europe and Libya, North Africa.

With a length of 6 m (20 ft), Rytiodus was about twice the size as modern sirenians, surpassed only by Steller"s sea cow, which was up to 8 m (27 ft) long. Like its closest modern relatives, the dugongs, Rytiodus had a pair of flippers, a streamlined body and a tail fin. Its flattened snout allowed it to feed in shallow coastal waters. Unlike modern sirenians, Rytiodus had short tusks which it may have used to extract food from the sand.


Desmostylians (Desmostylia)
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1. Paleoparadoxia 2. Desmostylus 3. Behemotops 4. Neoparadoxia 5. Ashoroa
Desmostylians were a group of Mid-Tertiary marine mammals which bore superficial resemblance to modern Hippopotami. After enjoying a brief evolutionary radiation during the early and middle Miocene the group became extinct without descendants before the Miocene closed.
Although Desmostylian remains are known from a number of localities around the northern pacific rim they are generally of fragmentary nature. Do to several specialized adaptations Desmostylian habits and phylogenic affinities are highly speculative and many drastically different hypotheses have been proposed over the last century. New finds since the 1960’s have greatly expanded our knowledge of these animals.
The Miocene genera Desmostylus and Paleoparadoxia are now represented by mounted skeletons in both the U.S. and Japan and a newly described member, Behemotops, has pushed the groups known geologic range back into the Oligocene. Additionally, Behemotops offers new clues as to the origin of the Order Desmostylia.Tusk examples from Desmostylus, and cheekteeth from Paleoparadoxia and Desmostylus in the N.S. Brown collection are illustrated.
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Unfortunately for us Desmostylians are extinct so we will never know their ecological niche, functional biology and behavioral habits with certainty any more then we can reconstruct a Mesozoic ecosystem with Dinosaurs and their cohabitants accurately. The surprise is that while the Mesozoic ended some 65 million years ago, Desmostylians survived until perhaps 6 million years ago (Latest Miocene) and were members of an ecosystem far more familiar to us then any of Mesozoic age.
However, like the Dinosaurs Desmostylians have no direct relatives to draw such information from, nor is there any living beast with a set of characters quite like those employed by Desmostylians. The genotype and best known member of the group Desmostylus was first described by O.C. Marsh in 1888 based on a single tooth and for much of the next century additional teeth and broken bits of skull material were the predominate finds. Ancestry for Desmostylians was tenable or nonexistent.
The rare and fragmentary remains of these creatures, in addition to their unique dentition and skeletal features led to numerous habitual and phylogenic interpretations throughout the years, although most workers placed them within the Order Sirenia along with the extant dugongs and Manatees.


The largest desmostylian was Ounalashkastylus tomidai
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New fossils from the Aleutian Islands intensify the mystery surrounding a toothy, hippopotamus-sized mammal unique to the North Pacific. Jaw fossil beast reached 57 centimeters in length, hence the size of the animal approximately equal to the modern hippopotamus, making Ounalashkastylus largest of desmostylia. The oddball creature suction-fed shoreline vegetation, say paleontologists. The Unalaska Island animal is a new genus and species of Desmostylia. The only major order of marine mammals to go wholly extinct, Desmostylia survived a geologic blink -- only 23 million years, from 33 million to 10 million years ago.
The identification of a new species belonging to the marine mammal group Desmostylia has intensified the rare animal"s brief mysterious journey through prehistoric time, finds a new study.
A big, hippo-sized animal with a long snout and tusks -- the new species, 23 million years old, has a unique tooth and jaw structure that indicates it was not only a vegetarian, but literally sucked vegetation from shorelines like a vacuum cleaner, said vertebrate paleontologist and study co-author Louis L. Jacobs, Southern Methodist University, Dallas.
But unlike other marine mammals alive today -- such as whales, seals and sea cows -- desmostylians went totally extinct. Desmostylians, every single species combined, lived in an interval between 33 million and 10 million years ago.
Their strange columnar teeth and odd style of eating don"t occur in any other mammal, The new specimens -- from at least four individuals -- were recovered from Unalaska, an Aleutian island in the North Pacific.



Paleoparadoxia tabatai
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Paleoparadoxia ("ancient paradox") is a genus of large, herbivorous aquatic mammals that inhabited the northern Pacific coastal region during the Miocene epoch (20 to 10 million years ago). It ranged from the waters of Japan (Tsuyama and Yanagawa), to Alaska in the north, and down to Baja California, Mexico. Paleoparadoxia was about 2.2 m  long, weight - 900 kg

Paleoparadoxia is thought to have fed primarily on seaweeds and sea grasses. The jaws and the angle of the teeth resemble a backhoe bucket. Its bulky body was well adapted for swimming and underwater foraging, but not for extended deep-sea living or deep diving. Originally interpreted as amphibious, Paleoparadoxia is now thought to have been a fully marine mammal like their living relatives, the sirenians, spending most of their lives walking across the sea bottom like marine hippos.



 





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