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📖 読解文章

The Extinction of the Megalodon: Causes and Consequences The megalodon, a giant prehistoric shark that lived approximately 23 to 3.6 million years ago, is often portrayed as an apex predator that vanished due to a single catastrophic event. However, recent scientific evidence suggests that its extinction was a gradual process driven by multiple environmental and ecological factors. Three main reasons explain why the megalodon disappeared from the world's oceans. First, a significant drop in global sea levels during the Pliocene epoch reduced the extent of warm, shallow continental seas, which were the megalodon's primary hunting grounds. These shallow waters provided abundant prey, such as whales and large fish. As sea levels fell, these habitats shrank, limiting the shark's access to food and forcing it into deeper, colder waters where it was less efficient. Second, competition with the great white shark (Carcharodon carcharias) played a crucial role. Fossil evidence indicates that great whites emerged around the same time as the megalodon's decline. Great whites were more agile and could hunt in a wider range of temperatures, allowing them to outcompete the megalodon for similar prey, especially juvenile megalodons. Finally, a decline in the diversity and abundance of marine mammals, particularly baleen whales, reduced the megalodon's primary food source. Changes in ocean circulation and productivity led to a decrease in whale populations, and the megalodon, which required enormous amounts of food to sustain its massive body, could not adapt to this scarcity. Consequently, a combination of habitat loss, competition, and prey decline led to its eventual extinction.

🎧 リスニング講義

Now listen to a lecture on the same topic from a marine biology professor. The reading presents a plausible scenario, but recent research challenges each of its points. Let's examine the evidence more closely. First, regarding sea level changes, the timing doesn't match. While sea levels did drop during the Pliocene, the most dramatic declines occurred after the megalodon had already disappeared from the fossil record. In fact, the megalodon's decline began about 5 million years ago, when sea levels were relatively high and shallow seas were extensive. So habitat loss from falling sea levels cannot be the primary cause. Second, the idea that great white sharks outcompeted the megalodon is also problematic. Stable isotope analyses of fossil teeth show that great whites and megalodons occupied different ecological niches. Great whites likely fed on smaller prey and in different water temperatures, so direct competition for the same resources was minimal. Moreover, great whites were present for over a million years before the megalodon's decline, suggesting they coexisted without causing extinction. Third, the claim about declining whale populations is not supported by the fossil record. Marine mammal diversity actually increased during the Pliocene, and many species of baleen whales were larger and more abundant than earlier forms. In fact, some whale species evolved to become faster and more agile, making them harder for the megalodon to catch. This suggests that prey availability was not the issue; rather, the megalodon may have struggled to capture more elusive prey, a factor related to its own physiology rather than prey scarcity. Thus, the extinction was likely due to a combination of factors, but not the ones the reading emphasizes.