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16.12.2015 16:56 - Encyclopedia Largest Prehistoric Animals Vol.1 Vertebrates part1 Mammals ch.1 Carnivores: Pinnipeds - Seals and Walrus
Автор: valentint Категория: Забавление   
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Последна промяна: 09.06.2020 19:12

Pinnipeds image
1. Acrophoca 2. Allodesmus 3. Aivukus 4. Enaliarctos 5. Thalassoleon
One popular hypothesis suggested that pinnipeds are diphyletic (descended from two ancestral lines), with walruses and otariids sharing a recent common ancestor with bears and phocids sharing one with Musteloidea. However, morphological and molecular evidence support a monophyletic origin.Nevertheless, there is some dispute as to whether pinnipeds are more closely related to bears or musteloids, as some studies support the former theoryand others the latter.Pinnipeds split from other caniforms 50 million years ago during the Eocene.Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of Puijila in early Miocene deposits in Nunavut, Canada. Like a modern otter, Puijila had a long tail, short limbs and webbed feet instead of flippers. However, its limbs and shoulders were more robust and Puijila likely had been a quadrupedal swimmer—retaining a form of aquatic locomotion that gave rise to the major swimming types employed by modern pinnipeds. The researchers who found Puijila placed it in a clade with Potamotherium (traditionally considered a mustelid) and Enaliarctos. Of the three, Puijila was the least specialized for aquatic life. The discovery of Puijila in a lake deposit suggests that pinniped evolution went through a freshwater transitional phase.

Enaliarctos, a fossil species of late Oligocene/early Miocene (24–22 Mya) California, closely resembled modern pinnipeds; it was adapted to an aquatic life with a flexible spine, and limbs modified into flippers. Its teeth were adapted for shearing (like terrestrial carnivorans), and it may have stayed near shore more often than its extant relatives. Enaliarctos was capable of swimming with both the fore-flippers and hind-flippers, but it may have been more specialized as a fore-flipper swimmer.One species, Enaliarctos emlongi, exhibited notable sexual dimorphism, suggesting that this physical characteristic may have been an important driver of pinniped evolution. A closer relative of extant pinnipeds was Pteronarctos, which lived in Oregon 19–15 mya. As in modern seals, Pteroarctos had an orbital wall that was not limited by certain facial bones (like the jugal or lacrimal bone), but was mostly shaped by the maxilla.
The ancestors of the Otarioidea and Phocoidea diverged 33 mya.The Phocidae are likely to have descended from the extinct family Desmatophocidae in the North Atlantic. Desmatophocids lived 23–10 Mya and had elongated skulls, fairly large eyes, cheekbones connected by a mortised structure and rounded cheek teeth. They also were sexually dimorphic and may have been capable of propelling themselves with both the foreflippers and hindflippers.

Phocids are known to have existed for at least 15 million years,and molecular evidence supports a divergence of the Monachinae and Phocinae lineages 22 Mya.The fossil monachine Monotherium and phocine Leptophoca were found in southeastern North America. The deep split between the lineages of Erignathus and Cystophora 17 Mya suggests that the phocines migrated eastward and northward from the North Atlantic. The genera Phoca and Pusa could have arisen when a phocine lineage traveled from the Paratethys Sea to the Arctic Basin and subsequently went eastward. The ancestor of the Baikal seal migrated into Lake Baikal from the Arctic (via the Siberian ice sheet) and became isolated there. The Caspian seal"s ancestor became isolated as the Paratethys shrank, leaving the animal in a small remnant sea, the Caspian Sea.The monochines diversified southward. Monachus emerged in the Mediterranean and migrated to the Caribbean and then the central North Pacific.The two extant elephant seal species diverged close to 4 mya after the Panamanian isthmus was formed. The lobodontine lineage emerged around 9 mya and colonized the southern ocean in response to glaciation.
The lineages of Otariidae and Odobenidae split almost 28 Mya.Otariids originated in the North Pacific. The earliest fossil Pithanotaria, found in California, is dated to 11 mya. The Callorhinus lineage split earlier at 16 mya. Zalophus, Eumetopias and Otaria diverged next, with the latter colonizing the coast of South America. Most of the other otariids diversified in the Southern Hemisphere. The earliest fossils of Odobenidae—Prototaria of Japan and Proneotherium of Oregon—date to 18–16 Mya. These primitive walruses had much shorter canines and lived on a fish diet rather than a specialized mollusk diet like the modern walrus. Odobenids further diversified in the middle and late Miocene. Several species had enlarged upper and lower canines. The genera Valenictus and Odobenus developed elongated tusks. The lineage of the modern walrus may have spread from the North Pacific to the Caribbean (via the Central American Seaway) 8–5 Mya and subsequently made it to the North Atlantic and returned to the North Pacific via the Arctic 1 mya. Alternatively, this lineage may have spread from the North Pacific to the Arctic and subsequently the North Atlantic during the Pleistocene

is a family of pinnipeds. The only living species is the walrus. In the past, however, the group was much more diverse, and includes more than ten fossil genera.
The walrus is a mammal in the order Carnivora. It is the sole surviving member of the family Odobenidae, one of three lineages in the suborder Pinnipedia along with true seals (Phocidae) and eared seals (Otariidae).While there has been some debate as to whether all three lineages are monophyletic, i.e. descended from a single ancestor, or diphyletic, recent genetic evidence suggests all three descended from a caniform ancestor most closely related to modern bears.Recent multigene analysis indicates the odobenids and otariids diverged from the phocids about 20–26 million years ago, while the odobenids and the otariids separated 15–20 million years ago.Odobenidae was once a highly diverse and widespread family, including at least twenty species in the subfamilies Imagotariinae, Dusignathinae and Odobeninae.The key distinguishing feature was the development of a squirt/suction feeding mechanism; tusks are a later feature specific to Odobeninae, of which the modern walrus is the last remaining (relict) species.

The largest Odobenids was Giant Walrus (Pontolis magnus) at 3000 - 5000 сm in length and 2000 - 4000 kg weight
The giant walrus Pontolis was described in 1905 and dates from the late Miocene (7-9 million years ago). The length of its skull was 60 cm, and the size it was contemporary with elephant seals. In general, Pontolis is a third more than the largest modern walrus. These huge pinnipeds have more simple structure of the teeth, the more massive skull, rather large fangs and a more concave and elongated palate than the walrus of the middle Miocene.

Gomphotaria pugnax
Gomphotaria pugnax (pugnacious wedge-seal) was a species of very large shellfish-eating dusignathinewalrus found along the coast of what is now California, during the late Miocene. It had four tusks, with one pair in the lower and upper jaws, and according to wear on the tusks, G. pugnax hammered shellfish open, rather than simply sucking them out of their shells as do modern walruses. In terms of the postcranial skeleton, Gomphotaria and other dusignathine walruses were built more like sea lions than modern walrus. Gomphotaria is a prime example of the extreme diversity that walruses once exhibited.





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Автор: valentint
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