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.
From the Permian extinction’s devastation arose a reptilian sheep called . Fossil hunters of early Triassic sediments have been frustrated for many years, as nearly are , because it was about the Permian extinction’s only land animal survivor. There has been about why it survived when almost nothing else did. No single animal ever dominated Earth’s land masses as thoroughly as did during the early Triassic. was probably a burrower (many have likened to a pig because of that burrowing), which may have provided the shelter needed to survive the Permian holocaust. It may also have been a and could eat most surviving plants. But some think that its survival, when almost every other species died, was due to luck. Luck is a surprisingly common proposed explanation for evolutionary events and outcomes, and some creatures seemed to be in the right place at the right time while others were in the wrong place at the wrong time. The spread of was also aided by two other facts: the land masses , so could simply walk to dominance of Earth; and few predators capable of eating a survived. One (being semi-aquatic may have also helped species survive the Permian extinction), as did , but not much else did. was a , as were the dominant land animals before the Permian extinction.
The (c. 201 to 145 mya) and (c. 145 to 66 mya) periods spanned the Golden Age of Dinosaurs. The human fascination with dinosaurs is primarily due to their great size. They were Earth’s largest land animals ever, by far. Huge predators hunted even larger herbivores. Prosauropods, or , were and were the early Jurassic’s dominant herbivorous dinosaurs, but their four-legged descendants, , supplanted them by the mid-Jurassic and sauropods became Earth’s largest land animals ever. Some species may have weighed more than , which would have rivaled the , which is generally considered to be the largest animal that ever lived. The blue whale achieved weight primacy, but the sauropods’ vast dimensions are still awe-inspiring. Some were up to and could reach . Some of the largest sauropods ever lived in the late Jurassic, when they were most numerous, but huge sauropods . A prominent hypothesis is that their tremendous size was a strategy for digesting lower-quality food sources; they could digest food for a longer period as it wound its way through their digestive systems. Their size also discouraged predation and . But their highly efficient air sac breathing system may have been the main reason why they could get so large, particularly in the record-low oxygen Jurassic Period, at least according to .
Two major events happened soon after appeared, and their sequence seems to support the Cooking Hypotheses. The first of which was the migration of from Africa ; they spread to and by 1.8 mya (perhaps 1.6 mya in the case of Java), and . It was the , and may have become the first multi-continental member of the human line, and certainly the first widespread one. Favorable climates and a lower Himalaya range and Tibetan Plateau may have encouraged that migration. Unlike Miocene apes that began to migrate from Africa 16.5 mya, there was no unbroken forest to sustain journey to East Asia. Those migrants would have to sleep on the ground for much of the journey and were not adapted for sleeping in trees, . From today’s viewpoint, it may seem that they were adventurers, but as will also become obvious with the spread of , in one individual’s lifetime, there was probably only modest movement, expanding into the next uninhabited valley or two. Such an expansion happened one valley at a time, one generation at a time, to make it across a continent in a few thousand years for those that could adapt to changing biomes. Migrating at the same latitude would not have presented great climatic issues. As those migrations happened during the ice age, they were along southern Eurasia. There is no evidence yet that ever made it to Australia, probably because of the ocean crossing required for passage.
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When humans began to raze forests and use the resultant soils to raise crops, they were working their way down through the food chain, no longer harvesting ecosystem detritus but destroying entire ecosystems literally at their roots for short-term human benefit. That practice eventually turned forest ecosystems into deserts. As this essay will survey, that was a rampant problem in all early civilizations. Eventually, humans learned to reach even further back into the ecological horizon as they began burning energy stores that were hundreds of millions of years old; was first and second. They were burned a million times as fast as they were created. In all instances, humans were releasing sunlight energy that had been captured and stored by organisms. In the 20th century, when humans began using nuclear fission, they were going even further back in time and harvesting energy stored via billions of years ago. With each new energy source, humans were harvesting older, more concentrated energy sources, which released far more energy than the previously used source. In each instance, humans plundered the energy source to exhaustion. Humans have not lived in “harmony” with nature since they learned to control fire.
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As smoke cleared and dust settled, literally, from the , the few surviving mammals and birds crept from their refuges, seeds and spores grew into plants, and the began, which is also called the Age of Mammals, as they have dominated this era. The Cenozoic’s first period is the , which ran from about 66 mya to 23 mya. As this essay enters the era of most interest to most humans, I will slice the timeline a little finer and use the concept of epochs. The Paleogene’s first epoch is called the (c. 66 to 56 mya).
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Most plants produce seeds, which would have largely survived the catastrophe and began growing when conditions improved. Ferns came back first, in what is called a , as ferns are a . Crocodiles, modern birds (which included ), mammals, and amphibians also survived, and all could have found refuge in burrows, swamps, and shoreline havens, lived in tree holes and other crevices that they were small enough to hide in, and all could have eaten the catastrophe’s detritus. In general, freshwater species fared fairly well, especially those that could eat detritus. Also, the low-energy requirements of ectothermic crocodiles would have seen them survive when the mesothermic/ dinosaurs starved. The primary determinants seem to have been what could survive on detritus or energy reserves and what could not, and what could find refuge from the initial conflagration. While there may have been some evidence of dinosaur decline before the end-Cretaceous extinction (it was gradually growing colder), and the may have caused at least some local devastation, the complete extinction of non-avian dinosaurs, ammonites, marine reptiles, and others that would have been particularly vulnerable to the bolide event’s aftermath has convinced most dinosaur specialists that the bolide impact alone was sufficient to explain the extinction and no other hypothesis explains the pattern of extinction and survival that the bolide hypothesis does. In general, the key to surviving the end-Cretaceous extinction was being a marginal species, and all of those on center-stage paid the ultimate price. The end-Cretaceous extinction's toll was nearly 20% of all families, half of all genera, and about 75% of all species, and marked the end of an era; the Mesozoic ended and made way for the Age of Mammals, also called the , which used to have the .