Marine Mammals and Fossil Records

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The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions. Five of these clades are still extant, whereas two are extinct. the degree to which each of these clades has adopted an aquatic lifestyle. Both Cetacea and Sirenia originated at about the same time and were fully aquatic by the end of the Eocene epoch. Each of these evolutionary histories is different from the others based on which changes in each anatomical system occurred. Despite this finding, they all have one change in common at the very beginning. The earliest members of each of these clades of marine mammals show adaptations of the feeding apparatus for feeding in water. Orcinus Orcas, more commonly known as killer whales, are the most widely distributed cetacean species in the world. Everything about them describes the ultimate predator. In actuality, they are dolphins and not whales as their name suggests. Nearly everything about the Orcinus Orca shows some type of mechanism for adaptation. Their characteristic fusiform body shape prevents drag and is designed for a swift and graceful glide through the water with a swimming potential of nearly 30 mph. In addition to their streamlined bodies, the Orcinus orca's forelimbs are adapted for swimming. An Orca uses its two paddle like pectoral flippers to steer. These flippers closely resemble the major bones of the forelimbs of land mammals, albeit modified into a paddle shape to fit their aquatic lifestyle. The tail flukes are boneless, dense connective tissue, and help the Orca stop. A dense triangular dorsal fin helps the Orca maintain stability as it propels at high speeds, though it isn’t crucial. Living in mostly frigid waters, the endothermic Orcas need to main... ... middle of paper ... ... survivor of both its family and genus. As their prey are aquatic, polar bears are excellent swimmers, even using only its front limbs, an aquatic adaptation found in no other four-legged mammal. Global warming poses a threat to their icy arctic habitat but maybe it is part of the transition from a terrestrial to marine habitat. They just might adapt into the water again. Also, another clue that something might happen is that they spend most of their time in the water feeding. This is no great surprise, because a great deal of mammalian evolution has been linked to changes in feeding ecology. The earliest representatives of each clade all show morphological features that indicate they were feeding while in the water, suggesting that feeding ecology is a key factor in the evolution of marine mammals. (Uhen M.D. 2007) Works Cited THE ANATOMICAL RECORD Uhen (2007)

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