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How did you come here today?

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  • wokemdengz

    2020-01-31 13:52:19
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  • my husband took me here by car.because the distance is far ,it is convenient and rapid to get here.

    2019-11-29 09:10:12
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  • subways nearly 3hours

    2019-11-14 08:01:54
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  • Reading Set IFrom Fish to Terrestrial Vertebrates One of the most significant evolutionary events that occurred on Earth was the transition ofprobathe Dean Period (408to362available aquatic habitats, including freshwater settings. One of the groups whose fossils areespecially common in rocks deposited in fresh water is the lobe-finned fishThe freshwater Devonian lobe-finned fish rhipidistian crossopterygian is of particular interest tobiologists studying tetrapod evolution. These fish lived in river channels and lakes on largedeltas. The delta rocks in which these fossils are found are commonly red due to oxidized ironminerals, indicating that the deltas formed in a climate that had alternate wet and dry periods. Ifthere were periods of drought, any adaptations allowing the fish to survive the dry conditionswould have been advantageous. In these rhipidistians, several such adaptations existed. It isknown that they had lungs as well as gills for breathing. Cross sections cut through some of thefossils reveal that the mud filling the interior of the carcass differed in consistency and texturedepending on its location inside the fish. These differences suggest a saclike cavity below thesource of oxygen for these fish, but the lungs served as an auxiliary breathing device for gulpingair when the water became oxygen depleted, such as during extended periods of drought. Sothese fish had already evolved one of the prime requisites for living on land: the ability to use airas a source of oxygenA second adaptation of these fish was in the structure of the lobe fins. The fins were thick, fleshy,and quite sturdy, with a median axis of bone down the center. They could have been usedfeeble locomotor devices on land, perhaps good enough to allow a fish to flop its way fiom ssurvival. These fins eventually changed into short, stubby legs. The bones of the fins of oneDevonian rhipidistian exactly match in number and position the limb bones of the earliest knowntetrapods, the amphibians. It should be emphasized that the evolution of lungs and limbs was inno sense an anticipation of future life on land. These adaptations developed because they helpefish to survive in their existing aquatic environmentWhat ecological pressures might have caused fishes to gradually abandon their watery habitatand become increasingly land-dwelling creatures? Changes in climate during the Devonian mayAnothermpetus may have been new sources of food. The edges of ponds and streams surely hadscattered dead fish and other water-dwelling creatures. In addition, plants had emerged intoterrestrial habitats in areas near streams and ponds, and crabs and other arthropods were alsomembers of this earliest terrestrial community. Thus, by the Devonian the land habitat marginalto freshwater was probably a rich source of protein that could be exploited by an animal thatcould easily climb out of water. Evidence from teeth suggests that these earliest tetrapods did notutilize land plants as food; they were presumably carnivorous and had not developed the ability to feed on plants

    2019-10-21 18:07:49
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  • I think the way to be happy is to help others as often as possible,the more often the better,consequently I will easily get happiness.

    2019-09-15 17:44:27
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  • Today, I took my mother's car to come here.Because the traffic jam half an hour in the middle of the way,which made me really upset.Fortunately,I finally came here.

    2018-11-26 18:09:57
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