Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T. Mass extinctions are periods in Earth's history when abnormally large numbers of species die out simultaneously or within a limited time frame. The most severe occurred at the end of the Permian period when 96% of all species perished. This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species. Many smaller scale mass extinctions have occurred, indeed the disappearance of many animals and plants at the hands of man in prehistoric, historic and modern times will eventually show up in the fossil record as mass extinctions. Discover more about Earth's major extinction events below.
The Triassic began hot and ended hot, and the Jurassic and Cretaceous were also hot, so staying warm was not a significant issue for dinosaurs. stayed cool by becoming aquatic, and for land-based dinosaurs, features such as plates apparently replaced the sails of for both heating and cooling, and like the synapsid sail, those plates may have also been used for display. Also, like the cliché, many large herbivorous dinosaurs lived near cooling swamps, although the issue has been controversial. Cooling swamps and protective water holes that we see in the tropics today were a major aspect of Mesozoic landscapes. But the thermoregulatory aspect that most work is directed toward today is how dinosaurs kept warm. There is compelling evidence that dinosaurs regulated their body temperature in myriad ways, including internal chemistry. All bipedal animals today are endotherms and they all have four-chambered hearts, as dinosaurs did. , dinosaurs living near the poles (, ), and of dinosaur bones all support the idea that , but one of the more intriguing areas is that of . Like tree rings, bones have seasonal growth rings and they have been read for many dinosaur fossils. They have been used to determine dinosaurian life expectancies. could live to be about 30, giant could live to be 50, and smaller dinosaurs, as with smaller mammals, lived shorter lives. The tiny ones only lived three-to-four years and the mid-sized ones lived seven-to-fifteen years. Growth rates also provide thermoregulation evidence. Tyrannosaurs had juvenile growth spurts and largely stopped growing as adults, and sauropods had growth rates equivalent to today’s whales, which are Earth’s fastest growing animals. But there is also evidence of ectothermic dynamics. The great size of dinosaurs would have led to relatively easy ways to stay warm, as large animals have a greater mass-to-surface area ratio, like the way in which . Also, in the generally hot Mesozoic times, staying warm would have been fairly easy, particularly for huge dinosaurs.
Permian–Triassic extinction event - Wikipedia
The possibility that mass extinctions may recur periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation upon impact.
Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez.
Questioning the Impact Theory: What Really Killed ..
Before the era of mass extinction investigation that began in the 1980s, a hundred hypotheses were presented in the scientific literature for the dinosaur extinction, but it was a kind of scientific parlor game. Scientists from all manner of specialties concocted their hypotheses. But even during the current era of scientific study of mass extinctions, much is unknown or controversial and even the data is in dispute, let alone its interpretation. Dynamics may have conflated to produce catastrophic effects, such as increasing atmospheric carbon dioxide concentration warming the land and oceans to the extent that otherwise stable on the ocean floor and in permafrost would be liberated and escape into the atmosphere. That situation is to the , , and extinctions, as well as helping end the . Today, there is genuine fear among climate scientists that , as global warming continues and hydrocarbons are burned with abandon, which could contribute to catastrophic runaway conditions. Wise scientists admit that humanity is currently conducting a huge chemistry experiment with Earth, and while the outcomes are far from certain, the .
The Sixth Mass Extinction?: Cretaceous Extinction Event
There is also evidence that life itself can contribute to mass extinctions. When the eventually , organisms that could not survive or thrive around oxygen (called ) . When anoxic conditions appeared, particularly when existed, the anaerobes could abound once again, and when thrived, usually arising from ocean sediments, they . Since the ocean floor had already become anoxic, the seafloor was already a dead zone, so little harm was done there. The hydrogen sulfide became lethal when it rose in the and killed off surface life and then wafted into the air and near shore. But the greatest harm to life may have been inflicted when hydrogen sulfide eventually , which could have been the final blow to an already stressed ecosphere. That may seem a fanciful scenario, but there is evidence for it. There is fossil evidence of during the Permian extinction, as well as photosynthesizing anaerobic bacteria ( and ), which could have only thrived in sulfide-rich anoxic surface waters. Peter Ward made this key evidence for his , and he has implicated hydrogen sulfide events in most major mass extinctions. An important aspect of Ward’s Medea hypothesis work is that about 1,000 PPM of carbon dioxide in the atmosphere, which might be reached in this century if we keep burning fossil fuels, may artificially induce Canfield Oceans and result in . Those are not wild-eyed doomsday speculations, but logical outcomes of current trends and , proposed by leading scientists. Hundreds of already exist on Earth, which are primarily manmade. Even if those events are “only” 10% likely to happen in the next century, that we are flirting with them at all should make us shudder, for a few reasons, one of which is the awesome damage that it would inflict on the biosphere, including humanity, and another is that it is entirely preventable with the use of technologies .