400 Million Years of Shark Evolution

If you went back in time and looked at the first, unremarkable prehistoric sharks of the Ordovician period--about 420 million years ago--you might never guess that their descendants would become such dominant creatures, holding their own against vicious marine reptiles like pliosaurs and mosasaurs and going on to become the apex predators of the worlds oceans. Today, few creatures in the world inspire as much fear as the Great White Shark, the closest nature has come to a pure killing machine--if you exclude Megalodon, which was 10 times bigger. Before discussing shark evolution, though, its important to define what we mean by shark. Technically, sharks are a suborder of fish whose skeletons are made out of cartilage rather than bone; sharks are also distinguished by their streamlined, hydrodynamic shapes, sharp teeth, and sandpaper-like skin. Frustratingly for paleontologists, skeletons made of cartilage dont persist in the fossil record nearly as well as skeletons made of bone--which is why so many prehistoric sharks are known primarily (if not exclusively) by their fossilized teeth. The First Sharks We dont have much in the way of direct evidence, except for a handful of fossilized scales, but the first sharks are believed to have evolved during the Ordovician period, about 420 million years ago (to put this into perspective, the first tetrapods didnt crawl up out of the sea until 400 million years ago). The most important genus that has left significant fossil evidence is the difficult-to-pronounce Cladoselache, numerous specimens of which have been found in the American midwest. As you might expect in such an early shark, Cladoselache was fairly small, and it had some odd, non-shark-like characteristics--such as a paucity of scales (except for small areas around its mouth and eyes) and a complete lack of claspers, the sexual organ by which male sharks attach themselves (and transfer sperm to) the females. After Cladoselache, the most important prehistoric sharks of ancient times were Stethacanthus, Orthacanthus, and Xenacanthus. Stethacanthus measured only six feet from snout to tail but already boasted the full array  of shark features: scales, sharp teeth, a distinctive fin structure, and a sleek, hydrodynamic build. What set this genus apart were the bizarre, ironing-board-like structures atop the backs of males, which were probably somehow used during mating. The equally ancient Stethacanthus and Orthacanthus were both fresh-water sharks, distinguished by their small size, eel-like bodies, and odd spikes protruding from the tops of their heads (which may have delivered jabs of poison to bothersome predators). The Sharks of the Mesozoic Era Considering how common they were during the preceding geologic periods, sharks kept a relatively low profile during most of the Mesozoic Era,  because of intense competition from marine  reptiles like ichthyosaurs and plesiosaurs. By far the most successful genus was Hybodus, which was built for survival: this prehistoric shark had two types of teeth, sharp ones for eating fish and flat ones for grinding mollusks, as well as a sharp blade jutting out of its dorsal fin to keep other predators at bay. The cartilaginous skeleton of Hybodus was unusually tough and calcified, explaining this sharks persistence both in the fossil record and in the worlds oceans, which it prowled from the Triassic to the early Cretaceous periods. Prehistoric sharks really came into their own during the middle Cretaceous period, about 100 million years ago. Both Cretoxyrhina (about 25 feet long) and Squalicorax (about 15 feet long) would be recognizable as true sharks by a modern observer; in fact, theres direct tooth-mark evidence that Squalicorax preyed on dinosaurs that blundered into its habitat. Perhaps the most surprising shark from the Cretaceous period is the recently discovered Ptychodus, a 30-foot-long monster whose numerous, flat teeth were adapted to grinding up tiny mollusks, rather than large fish or aquatic reptiles. After the Mesozoic After the dinosaurs (and their aquatic cousins) went extinct 65 million years ago, prehistoric sharks were free to complete their slow evolution into the remorseless killing machines we know today. Frustratingly, the fossil evidence for the sharks of the Miocene epoch (for example) consists almost exclusively of teeth--thousands and thousands of teeth, so many that you can buy yourself one on the open market for a fairly modest price. The Great White-sized Otodus, for example, is known almost exclusively by its teeth, from which paleontologists have reconstructed this fearsome, 30-foot-long shark. By far the most famous prehistoric shark of the Cenozoic Era  was Megalodon, adult specimens of which measured 70 feet from head to tail and weighed as much as 50 tons. Megalodon was a true apex predator of the worlds oceans, feasting on everything from whales, dolphins, and seals to giant fish and (presumably) equally giant squids; for a few million years, it may even have preyed on the equally ginormous whale Leviathan. No one knows why this monster went extinct about two million years ago; the most likely candidates include climate change and the resulting disappearance of its usual prey.

U.s. A Multicultural Country - 1045 Words

In terms of diversity, the U.S. contains a lot of history, which makes them what they are now. Both the U.S. as well as Latin America were colonies, achieved independence movements and were directly influenced by countries like Spain and France. The difference relies that in the U.S., these European countries remained for a longer period of time, making the U.S. a multicultural country. According to the Guinness Book of World Records, Queens, a major area of New York City is considered to be â€Å"the most diverse place on the planet† (Colangelo). There is also the Diversity Visa Program, in which the U.S. randomly allocates 55,000 visas around the world. In this program, people from six geographic regions can register for the lottery and there†¦show more content†¦There are mostly no prejudices against religion and skin color, most of the Latin people gather together and find ways to support each other, no matter where they are, they like to keep in touch and help th e ones with less experience. Overall this supportive characteristic of Latin people should be applied to the U.S. culture in order to create a more unified country and improve the way their society develops. This could help stop riots, mass shootings and violence acts towards people from different religion which has been increasing in the U.S. in the last 5 years. The sentiment of integration and inclusion is something the U.S. hasn’t developed yet. In the case of Latin culture, whenever they feel they are not being respected and that their differences are not taken into consideration, their sentiment of integration towards the American culture decreases and there is when riots and protests trigger. But on the other side, when these people are taking into account as part of the U.S. many things including economy, safety and legal issues have positive effects. Although, at some point people from Latin America are running away from their country because of political and economi c problems, there should be agreements, which are the reforms in the immigration policies in order to stop this cycle and create better andShow MoreRelatedAssignment 2 : A Multicultural Society807 Words   |  4 PagesAssignment 2: The Application Paper Keitha Williams October 18, 2017 Argosy University Assignment 2: The Application Paper A multicultural society is where a diverse group of races, cultures, and different ethical background coexist together in a country, state, city, community or neighborhoods. Basically, a ground of people from different backgrounds living together in the same community. 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The Story of an Hour Analysis Free Essays

On â€Å"The Story of an Hour† â€Å"The Story of an Hour,† a short story by Kate Chopin, details the events of the short hour when Mrs. Mallard finds out her husband has died, only to see him walking through the door. Chopin makes it clear through the structure of the story that the irony of these events is no coincidence; there is conflict when people are oppressed and their soul has no joy or freedom. We will write a custom essay sample on The Story of an Hour Analysis or any similar topic only for you Order Now It is clear in the exposition that Mrs. Mallard is afflicted with heart trouble, implying she is weak, both physically and emotionally. Her sister Josephine and her husband’s friend, Richard, know how fragile she is so, â€Å"great care was taken† to break the news of Mr. Mallard’s death. Following the news of her husband’s death, Mrs. Mallard, â€Å"wept at once†¦in her sister’s arms,† showing her dependence on others. As the immediate feeling of grief spent itself, she went up to her room to be alone with her thoughts. Although she is at first weak, someone who is completely emotionally unstable would not be so willing to go be alone in her room to grief. There is obviously some want to be self dependent and free. The story’s rising action builds through a series of scenery descriptions. After Mrs. Mallard has gone up to her room, she faces an open window. The blue sky â€Å"showing here and there† and â€Å"countless sparrows† twittering outside were mentioned, symbolizing happiness and peace. Mrs. Mallard is then described by the narrator as a woman with â€Å"a dull stare in her eyes,† as she fixed her gaze on the blue sky, as if finally realizing that they were there. The details of the rising actions transition into the climax, where Mrs. Mallard feels an emotion â€Å"approaching to possess her. † After the realization that â€Å"there would be no one to live for† for the rest of her life Mrs. Mallard continues to whisper under her breath, â€Å"Free! Body and soul free! † This moment is filled with joy; however a turn of events could be predicted to come soon as she prayed â€Å"that life might be long. † At the falling action of the story, Mrs. Mallard leaves the room with the open window with a â€Å"feverish triumph in her eyes. † She is unquestionably a confident and new woman as she walks down the stairs with her sister. The story concludes with the denouement in which Brently Mallard enters the front door, unharmed. The shock of his homecoming is extremely evident through Josephine’s â€Å"piercing cry† and Richard quickly trying to block him from the view of his wife. Mrs. Mallard apparently has a heart attack and dies â€Å"of the joy that kills. † By the time Mrs. Mallard realizes that because her husband has died she will now be able to experience the joy of freedom, â€Å"the face of this possession† has unexpectedly walked in through the door. Although most would expect the weak Mrs. Mallard to die when hearing the news of her husband’s death, it is seeing him alive and realizing that she will continue to be his possession that causes her heart and her soul to die. How to cite The Story of an Hour Analysis, Papers