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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere 10 THE CRETACEOUS EXTINCTION AND THE RISE OF LARGE MAMMALS So much has been written about the extinction of the dinosaurs that finding anything new to say is a tall task. By mid-2006, the understanding has remained virtually unchanged for a decade or more: a large asteroid smashed the Cretaceous world out of existence in a short-term reign of fire and toxicity. Even weeks after the impact, the damage had been done. The dinosaurs were all dead, as were all ammonites, rudistid clams, mosasaurs, and about 50 percent of the rest of the species that had been blissfully living in the healthy and robust late Cretaceous world. The dinosaurs disappeared from a world warm and far more oxygenated than it had been when the Mesozoic Era had begun. By the end of the Cretaceous, the world, in terms of its vegetation and atmospheric makeup, had taken on a much more modern cast. But it was not yet the atmosphere of our world. Carbon dioxide was still immensely higher in concentration than now, and—more relevant to the story here—atmospheric oxygen was still lower, although not by much. But the biggest similarity between those times, some 65 million years ago soon after the asteroid impact, and the world of today was that dinosaurs existed in neither. It was the disappearance of the dinosaurs that allowed mammals to fill the emptied world—and fill it they did. Yet here, too, the story of changing oxygen provides a new why for some of the most interesting details of that history of “mammalization”
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere of the planet. This chapter looks at how the explosion of mammals filling the new, dinosaur-free world was itself shaped by the last significant rise in oxygen. THE HISTORY OF CENOZOIC MAMMALS As we have seen, the ancestors of the mammals, the Paleozoic and early Mesozoic therapsids, were the dominant land animals until the Permian extinction, and even through the Triassic they maintained moderate diversity, if no longer being the most diverse and numerous of land animals as their Paleozoic forbearers had been. The first true mammals are found in rocks of late Triassic age—at about the time of the first dinosaurs, in fact. But while they may have both appeared at the same time, these two groups then went on to very different fates. The first dinosaurs were already relatively large animals for their time, about a meter long, but soon after they evolved into much larger sizes even before the end of the Triassic. But the first mammals may have been a tenth of the size or less of the first, meter-long dinosaurs, and they then stayed small—for a very long time. Why? This obvious question is all the more perplexing because some of the immediate ancestors of the first mammals, even forms that lived alongside them, such as the advanced cynodonts of the late Triassic, were the same size or larger than the early dinosaurs. The first mammals could have been larger, but they were not. Does this size limitation tell us something about their metabolism and constraints in the teeth of the low-oxygen high-heat interval? Let’s recount the ecological position of the first true mammals. It is the late Triassic and early Jurassic. The world is hot. And the world has oxygen levels as low as 10 percent and certainly less than 15 percent for tens of millions of years. Mammals then, as now, are presumed to be warm-blooded and furry. The biggest mystery is how they reproduced. Almost all mammals today are placental, where development takes place in the female to the point that the newborn is capable of existence outside the mother; a few are marsupial, where early birth prior to full development requires a period of time in a secondary pouch within the mother; and a very few, such as the platypus and
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere echidna, lay eggs. Unfortunately, we have no record of mammal eggs, and the fossil record is utterly opaque in telling when the first placental mammals occurred. But there is indirect evidence that is telling. DNA work (based on the molecular clock) indicates that the divergence of major groups of placental mammals—such as divergence into insectivores, carnivores, and artiodactyls, among many other major groups, happened between 100 million and 60 million years ago. Thus, some of the major divisions of mammals predated the Cretaceous extinction. But prior to that extinction all of these forms were small, and not just in the late Cretaceous. All Mesozoic mammals were small, from the first in the Triassic until those of the latest Cretaceous. One possibility for this small size was so as not to compete with the dinosaurs. No dinosaurs occupied the rodent niche in the Mesozoic, and, as we know all too well today, there is a good living to be had if one is rat-sized or smaller. Under this scenario, mammals did not compete with dinosaurs for ecological reasons. But it may be that other reasons were involved. Perhaps in the hot, low-oxygen world (at least of the late Triassic until the end of the middle Jurassic), a larger, warm-blooded, highly active mammal—an animal that needed to eat much more than a cold-blooded form—was just too energetically expensive to exist. Here, as in Chapter 9, is the idea that dinosaurs were a really different kind of beast than anything we know today and, at least until the Cretaceous, were the only kind of animal that worked really well in the peculiar early and mid-Mesozoic conditions on Earth. OXYGEN AND THE SIZES OF MAMMALS We know that the disappearance of the dinosaurs unleashed a torrent of evolution, producing, in short order, many mammalian taxa. And for the first time, true mammals of larger size evolved. Thus, after the Cretaceous extinction, after the smoke had cleared and the dinosaurs were no longer around, large mammals did begin to appear. But it took a while. Santa Barbara paleontologist John Alroy has meticulously studied the sizes of mammals through time. Through dint of hard work in the library and among numerous dusty museum drawers, he tabulated average size for over 2,000 kinds of mammals from the late Meso-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere zoic and Cenozoic of North America. His work showed an increase in body size through the Cenozoic. But details of the overall size increase indicate that it happened at two different times and rates, rather than as a smooth and continuous increase. The first size increase occurred in the first few million years after the Cretaceous extinction and seems to have been a response by mammals to filling in now-emptied ecological niches. When the herbivorous dinosaurs died out, there was nothing around to browse higher bushes and trees and it became advantageous to grow larger. Large size also lends protection against predation and with the loss of the large, medium, and small dinosaur carnivores, a host of mammal groups began to enlarge. But a much larger increase in size happened much later, in an interval of 50 million to 40 million years ago in the Eocene Epoch. What caused this kind of size increase? One possibility is that it was at least enabled by a rise in oxygen. The Berner oxygen curve suggests there was a rapid increase in atmospheric oxygen soon after the Cretaceous mass extinction of 65 million years ago. At the same time, mammal size increased. Coincidence? Probably not. As for dinosaurs, it may have been that rising oxygen had something to do with mammalian reproduction. A 2005 study by geochemist Paul Falkowsky suggested that the first appearance of “placental mammals” (the vast majority of mammals today, all of which have live births and nurse their young) could not take place until a critical level of rising oxygen was reached. Their argument was that prior to that time, there was insufficient oxygen within the placenta of pregnant female mammals to nurture developing embryos. This happened in the late Cretaceous. A key aspect to understanding the evolution of mammals is discovering when the placental form of reproduction first occurred, for there may be a crucial and potentially limiting aspect to placental reproduction related to oxygen. The mother’s arterial blood (which is oxygenated) mixes with venous blood (which is enriched in carbon dioxide) in the placenta. Fetal blood picks up its oxygen load from this admixture, and thus the fetus is exposed to oxygen levels that are lower than that of the mother’s arterial blood. If the oxygen level of the mother’s blood is already depressed because of living at higher altitude
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere or, in the Mesozoic, living at lower atmospheric oxygen levels, the life of the fetus would be endangered. Today our best model for understanding the life of animals in the lower-oxygen past comes from the study of animals at high altitude. Mammals can readily survive up to 28,000 feet; humans can certainly live, if only for a short time, atop Mt. Everest. But no mammal reproduces above 14,000 feet, which corresponds to the oxygen levels of the early Jurassic. And these are animals that have had 65 million to 100 million years to refine the placental system. The first evolved placentas would surely have been less efficient in delivering oxygen. This would seemingly suggest that the placental system of reproduction was not possible until oxygen blood levels had risen to, perhaps, Cretaceous levels—above 15 percent and perhaps approaching 20 percent. It thus looks as if the oxygen-level increase of the late Cretaceous—an oxygenation event that at least in part helped spark the major diversity increase in dinosaurs—also allowed the first evolution and successful implementation of a new kind of reproductive pattern, placental development (itself but one kind of live birth). OXYGEN AND THE RISE OF HUMANS The change in oxygen levels over time seems to have provoked major changes in evolution. What about one of the biggest of all changes and the most important to us? Can any aspect of the changing atmosphere be interpreted in a new way so as to explain the evolution of our own species? In one sense it can. Johns Hopkins paleontologist Steve Stanley has dubbed us “Children of the Ice Ages,” and many paleoanthropologists think that the rise of high intelligence and culture was a means of dealing with the challenges imposed by the glaciation of the past 2 million years, the recent ice ages. But what about oxygen levels? That angle has never been examined. Almost all models of the atmosphere for the past 10 million years indicate that oxygen levels were higher than now, with atmospheric oxygen levels as high as 28 percent even 5 million years ago. Could it be that higher oxygen levels stimulated, or made easier, the transition to larger brains? Nervous system tissue needs more oxygen than any other
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere kind of tissue, and the rapid enlargement of brain volume perhaps outstripped the circulation system in the brain necessary to sustain all the new neurons. Higher oxygen would give some cushion for error, it seems to me. But this is still sheer speculation. THE END OF HISTORY? We have now finished our journey through time. The 540-million-year trip is over with this chapter. We have seen evidence of major changes in oxygen levels through time. Will oxygen levels continue to change in the future? That is the subject of the final chapter.
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