
Other unlucky accidents oft-times happen in these seas, as, when men swim in the bearing ocean, the greedy Hayen, called Tuberon or Shark.
Armed with a double row of venomous teeth, pursue them, directed by a little rhombus or musculus, variously streaked and coloured with blue and white, that scuds to and fro to bring the shark intelligence.
As we watched the Hayens, some mind went panniculus.
They may run, swim or fly. They may weigh less than a penny or more than a dozen school buses. From humans to whales to bats, the placental mammalsβso named for the placenta that nourishes the fetus during developmentβare mind-bogglingly diverse.
(The placental mammals are one of three major groups of mammals; the other two are the egg-laying monotremes and the pouched marsupials.)
For years researchers have been attempting to piece together when the placentals originated and when the groupβs modern orders, such as the primates and the bats, first emerged. Now a major new analysis of thousands of anatomical features of modern and extinct mammals, as well as molecular sequences from living species, is helping them to do just that.
The study also hints at what the ancestral placental mammalβthe one that ultimately gave rise to creatures as disparate as tree sloths and sea lions–looked like.
Previous attempts to reconstruct the evolutionary history of mammals yielded conflicting scenarios.
But, Fossil evidences suggested that the placentals burst onto the scene shortly after a massive asteroid slammed into the earth around 65 million years ago and snuffed out the dinosaurs.
Studies that instead used molecular sequence data to get at the question of when placentals originated and diversified painted a different picture, indicating that the group appeared as early as 100 million years ago, when dinosaurs were still thriving.
This date is far older than the oldest known placental fossils, but it helped to explain the diversification of the group: the supercontinent Gondwana was fragmenting at that time and the breakup would have provided opportunities for populations to become separated and evolve in isolation into new forms.
Early Evolution;
The new study, which looked at more than 4,500 traits in 86 fossil and living mammal species, controverts the early origin model, concluding instead that the placentals originated after the mass extinction event, with the first members of modern placental orders evolving some two million to three million years later–after the continental breakup.
Early Evolution;
A phylogenetic tree showing the relationships among cetacean families. [1]
The evolution of cetaceans is thought to have begun in the Indian subcontinent , from even-toed ungulates 50 million years ago, over a period of at least 15 million years.
MYA-
Cetaceans are fully aquatic
marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the
Eocene or earlier, sharing a closest common ancestor with hippopotamuses.
Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Discoveries starting in the late 1970s in
Pakistan revealed several stages in the transition of cetaceans from land to sea.
Mysticeti and Odontoceti;
The two modern parvorders of cetaceans β
Mysticeti (baleen whales) and Odontoceti (toothed whales) β are thought to have separated from each other around 28-33 million years ago in a second cetacean radiation , the first occurring with the
archaeocetes . [2] The adaptation of animal echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales . The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).
The ADRs-
Ancestorβdescendant relationships (ADRs), involving descent with modification, are the fundamental concept in evolution, but are usually difficult to recognize.
Two endemic, distinctive types of short-finned pilot whale , Tappanaga (or Shiogondou ) the larger, northern type and Magondou the smaller, southern type, can be found along the Japanese archipelago where distributions of these two types mostly do not overlap by the oceanic front border around the
easternmost point of Honshu.
Conclusion;
It is thought that the local extinction of long-finned pilot whales in the
North Pacific in the 12th century could have triggered the appearance of Tappanaga , causing short-finned pilot whales to colonize the colder ranges of the long-finned variant.
Whales with similar characteristics to the Tappanaga can be found along Vancouver Island and northern USA coasts as well.
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