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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere 8 THE TRIASSIC EXPLOSION The animals and plants of the Triassic Period make up a most interesting assemblage of organisms. This chapter looks at this crossroad in time. Coming out of the most devastating of all mass extinctions, the early Triassic world was empty of life. At the same time, all modeling suggests that a long interval of the Triassic was a time when oxygen levels were lower than today. In Chapter 2 it was proposed that times of low oxygen, especially following mass extinction, foster disparity: the diversity of new body plans. In the Triassic these two factors combined to create the largest number of new body plans seen since the Cambrian. This chapter proposes that it is to that seminal Cambrian time that we can most accurately compare the Triassic. We should give this time and its biotic consequences a new name. THE TRIASSIC EXPLOSION The middle Triassic was a time of amazing disparity on land and in the sea. In the latter, new stocks of bivalved mollusks took the place of the many extinct brachiopods, while a great diversification of ammonoids and nautiloids refilled the oceans with active predators. Fully a quarter of all the ammonites that ever lived have been found in Triassic rocks—a time interval that is only 10 percent of their total time of existence on Earth. The oceans filled with their kind, in shapes and
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere patterns completely new compared to their Paleozoic ancestors—and why not? As shown above, this kind of animal was the preeminent, low-oxygen adaptation among all invertebrates. A new kind of coral, the scleractinians, began to build reefs, and many land reptiles returned to the sea. But it is on land that the most sweeping changes in terms of body plan replacements—and body plan experimentation—took place. Never before and never since has the world seen such a diverse group of different anatomies on land. Some were familiar Permian types: the therapsids that survived the Permian extinction diversified and competed with archosaurs for dominance of the land early in the Triassic, but this ascendance was short lived. Many kinds of reptiles were locked in a competitive struggle with them and with each other for land dominance. From mammal-like reptiles to lizards, earliest mammals to true dinosaurs, the Triassic was a huge experiment in animal design. Why was this? The conventional answer is that the Permian extinction removed so many of the dominant land animals that it opened the way for more innovation than at any other nonextinction time, perhaps any other mass extinction time as well. It was the most devastating of the mass extinctions. Perhaps, as well, it was simply that many terrestrial animal body plans finally came to an evolutionary point of really working efficiently rather than the sprawling posture of the early reptiles and amphibians. Even as late as the end of the Permian and into the Triassic, groups as mature as the dicynodonts and cynodonts were still trying to attain the most efficient kind of upright posture—rather than the less efficient, splayed-leg orientation of the land reptiles—with all of the ramifications and penalties in respiration that this entailed. But perhaps there is more than this. Body plans were being stimulated into creation by intense selective pressures, and dominant among these was the need to access sufficient oxygen to feed, breed, and compete in a low-oxygen world. There is an old adage about nothing sharpening the mind faster than imminent death. The same might be said about evolutionary forces when faced with the most pressing of all selective pressures—attaining the oxygen necessary for the high levels of animal activity that had been evolutionarily attained in the high-oxygen world of the Permian, when nothing was
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere easier to extract from the atmosphere. The two-thirds drop in atmospheric oxygen certainly lit the fuse to an evolutionary bomb, which exploded in the Triassic. Thus, the diversity of Triassic animal plans is analogous to the diversity of marine body plans that resulted from the Cambrian Explosion. It also occurred for nearly the same reasons and, as will be shown, was as important for animal life on land as the Cambrian Explosion was for marine animal life. As we saw in Chapter 3, the Cambrian Explosion followed a mass extinction (of the Ediacaran fauna), and it was a time of lower oxygen than today. The latter stimulated much new design. Finally, the Cambrian Period itself ended in a mass extinction—mainly of trilobites that we know of but also among many of the more exotic arthropods known from the Burgess Shale, such as Anomalocaris. In similar fashion, on land the Triassic Explosion followed a mass extinction, was a time of lower oxygen, and ended in a mass extinction. Before this extinction, mammals had evolved, as had true dinosaurs, but many of the other kinds of body plans disappeared and dinosaurs, were the dominant land animals. In this end-Triassic mass extinction the dinosaurs suffered least of all. Why dinosaurs? This chapter will look at those questions. VOYAGE Let’s begin by looking back to the middle part of the Triassic period. In this middle-late Triassic world, 215 million years ago, on land at least we seem to have arrived among a veritable smorgasbord of animal body plans. Many quite different kinds of vertebrates inhabit this world. Dog-like creatures walk beneath the conifer- and tree-fern dominated vegetation. They are cynodonts, carnivorous varieties, but there are massive herbivores belonging to the same group as well. They are all very mammalian in appearance and behavior, except in one aspect. They move little and seem to tire easily. The carnivores mostly lay in wait, and the herbivores browse stolidly. The cynodonts are not the only mammal-reptiles here, for rhino-sized dicynodonts also browse the low brushy vegetation. Their odd, name-giving tusks extending from a parrot-like beaked mouth make them look like nothing of our
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere world, and seeing them harkens memories of the late Permian world prior to the great Permian mass extinction. All are panting heavily and give the impression of animals having just engaged in strenuous exercise. But most have been motionless; yet they pant for good reason. The level of atmospheric oxygen at this time is equivalent to being higher than 10,000 feet in our world. Except that here we are not atop any mountain: the low swamps and nearby arm of the sea attest to our being at sea level. Other herbivores are here too, and they are clearly from groups long-diverged away from the mammal-like reptile lineage. It is soon apparent that they are more numerous than the mammal-like reptiles. One of the oddest is another beaked herbivore, from the reptile group known as rhynchosaurs, and near it is a heavily armored quadruped, an aetosaur, looking something like an armadillo, only much larger and better armored. Soon we notice other quadruped reptiles, all fairly primitive archosaurs. Many are 5 to 10 feet in length, and they too move little; when they do, the movement is labored and the panting rapid. Large size and armor evolve for one reason, to avoid being eaten—but the cost is high. Moving a heavy body about extracts a great metabolic cost. Yet there is method in this seemingly morphological madness, for it is clear that the carnivores here are in abundance. A number of reptiles are visible with heads like that of a crocodile but with bodies obviously evolved for rapid movement on land. Some rise up on their back legs, but they are still quadrupeds for rapid movement. They seem better suited for prolonged movement than the other designs seen till until now, but they are no greyhounds. They prowl but in labored fashion. Until now all the animals we have seen have been quadrupeds, but it is not long before we encounter our first bipedal animal, and soon we find that this world certainly has its share of animals that walk bipedally. All of them seem far more at ease in this atmosphere, one that bothers even us humans, supposedly advanced mammals that we are. We have finally encountered our first dinosaurs. Some are small and carnivorous; others, the prosauropods, are relative giants, the largest animals on land and the largest animals ever evolved up to this point. There seem to be many varieties of the smaller
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere bipedal forms, and we are thankful that most of the obvious carnivores are relatively small, for they zip about, running rings around the other vertebrates. But soon a much larger form, 12 feet in length, strides into view, a staurikosaur, the top carnivore among the dinosaurs. We watch this animal, puzzled at how different it seems from the nondinosaurs. Despite its size, it is very active. This dichotomy extends as well into the swampy and freshwater habitats, where untold numbers of crocodile-like phytosaurs loll on the banks. They too seem not disposed to frolic in any fashion. It is only the dinosaurs that move about with speed, grace, and purpose. In the oceans we see a major change. Largely gone are the brachiopods, the bivalved invertebrates so dominant in Paleozoic oceans. In their place is another kind of bivalve—the more familiar clams. Few burrow. Most rest on the surface of the sea bottom, sometimes in huge numbers. Swimming above the bottom are many varieties of fish, among them a host of ammonites. This latter group just missed total extinction in the Permian extinction. From the few spared species, however, a host of new species has evolved until now. In the latter parts of the Triassic period, they are even more numerous than anytime before. There are also coral reefs again, but like the bottom communities of invertebrates found on the sandier and muddier bottoms, the reefs are composed of an entirely new suite of corals. Gone are the tabulate Reconstruction of a ceratite ammonite, a group found only during the Triassic. These forms evolved from the few ammonites that survived the Permian extinction, an event that decimated the cephalopods.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere and horn corals, they are replaced by the scleractinians, forms that will persist to our time. As interesting as the invertebrates and even the fish are of this world, it is the larger vertebrates in the sea that really give pause. Diverse reptilian stocks have obviously returned to the ancestral ocean. Some, the stocky placodonts, resemble large, clumsy seals as they swim down and root through the clam beds with their peg-like teeth. But the stars of the show are the ichthyosaurs, reptiles that have so thoroughly evolved for a swimming habit that they have lost their legs entirely and are the most fish-like of originally terrestrial vertebrates that the world has ever known or will know until the eventual evolution of dolphins, far, far into the future from this Triassic time. While many are small, there are larger forms too: Mixosaurus is 30 feet long but it is dwarfed by the monstrous Shonisaurus, some of which reach a length of 60 feet. This huge ichthyosaur rivals sperm whales for the title of world’s largest aquatic predator of all time, and it preys at will on hosts of fish and smaller reptiles. It has dinner-plate-sized eyes, the largest eyes ever evolved either before or after this. The marine world, at least, despite its strange and frightful beasts, strikes a sense of some familiarity. But even here we begin to notice strange behavior: the diving reptiles, like the placodonts, come to the surface frequently to breath, as do even the ichthyosaurs. The low oxygen takes its toll. TRIASSIC REBOUND The oxygen story for the Triassic is stunning. Oxygen dropped to minimal levels of between 10 and 15 percent and then stayed there for at least 5 million years, from 245 million to 240 million years ago. The officially designated early Triassic time interval was from 250 million to about 245 million years ago. During this time there was little in the way of recovery from the Permian extinction. There is also a very curious record of large-scale oscillations in carbon isotopes from this time, indicating that the carbon cycle was being perturbed in what looks like either methane gas entering the oceans, or a succession of small-scale extinctions taking place. All evidence certainly paints a picture of a stark and environmentally challenging world for animal life.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Microbes may have thrived, especially those that fixed sulfur, but animals had a long period of difficult times. But difficult times are what best drive the engines of evolution and innovation and from this trough in oxygen on Earth emerged new kinds of animals, most sporting respiratory systems better able to cope with the extended oxygen crisis. On land two new groups were to emerge from the wreckage: mammals and dinosaurs. The former would become bridesmaids in waiting while the latter would take over the world. THE DIFFERENT FATES OF THE TRIASSIC THERAPSIDS AND DIAPSIDS As we saw in Chapter 7, the Permian extinction nearly annihilated land life. The therapsids were hit hard. Much less is known about the diapsids, for at the end of the Permian they were a rare and little seen group in the areas, such as the Karoo or Russia, that have yielded rich deposits with abundant dicynodont (therapsid) faunas. In the Karoo at least, only small fragments of diapsids have come from our uppermost Permian study sections, although two skulls now being prepared as I write this may turn out to be forms that would give rise to the great diapsids and dinosaur dynasty. Roger Smith found them in highest Permian rocks on our last joint collecting trip from the same section where the lowest Triassic diapsids was found. Are they the same species? We will soon know. If we are still poorly informed about their Permian ancestry, there is no ambiguity about the success of the earliest Triassic diapsids. In the Karoo, in strata only a few meters above the beds that seem to mark the transition from Permian to Triassic, there are relatively common remains of a fairly large reptile known as Proterosuchus (also known as Chasmatosaurus). This was definitely a land animal with a very impressive set of sharply pointed teeth. It was also definitely a predator, but like a crocodile, its legs were splayed to the sides (if somewhat more upright than the crocodilian condition). But this condition was to rapidly change in the diapsids to a more upright orientation as the Triassic progressed, and more gracile and rapid predators soon replaced the early diapsids such as Proterosuchus. While the need for speed was surely a driver toward this better
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere locomotory posture, just as important may have been the need to be able to breathe while walking. Like a lizard, Proterosuchus may still have had a back-and-forth sway to its body as it walked, and this sort of locomotion causes compression on the lung area due to what is known as Carrier’s Constraint—the concept that quadrupeds with splayed out legs, such as most (but not all!) lizards, cannot breathe while they run, because their sinuous, side-to-side swaying impinges on the lungs and rib cage, inhibiting inspiration. For this reason, most lizards and salamanders cannot breathe while walking, and Proterosuchus may have had something of this effect, although not as pronounced as in modern-day salamanders or lizards. A solution is to put the legs beneath, but this is only a partial solution. To truly be free of the constraint that breathing puts on posture, extensive modifications to the respiratory system and the locomotory system had to be made. The lineage that led to dinosaurs and birds found an effective and novel adaptation to overcome this breathing problem: bipedalism. By removing the quadruped stance, they were freed of the constraints of motion and lung function. The ancestors of the mammals also made new innovations, including a secondary palate (which allows simultaneous eating and breathing) and a complete upright (but still quadruped) stance. But this was still not satisfactory and a new kind of breathing system was evolved. A powerful set of muscles, known as the diaphragm, allowed a much more forceful system for inspiring and then exhaling air. Thus, by the middle Triassic, some very different respiratory designs were in play, with natural selection and competition as the arbiters. We know what kind of lungs the mammals had. But what about the diapsids—and their most famous members (and descendents of the early kinds such as Proterosuchus)—the dinosaurs? By the end of the middle Triassic they had burst upon the scene. How did they breathe? Therein lies a controversy. What kind of lungs did the earliest dinosaurs evolve? What changes to this lung design came about in their descendents? This has been the source of controversy for more than two decades now. But before we enter the debate, it should be noted that whichever lung system was found in the Triassic dinosaurs, it evolved for a self-similar
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere reason: oxygen levels reached their lowest point in the Triassic, coincidentally at the time when a majority of Permian land animals were going extinct (leaving many empty niches and thus wholesale evolutionary and ecological opportunity), and many vertebrates responded to these two factors by rapidly producing a host of new kinds of vertebrate body plans—and respiration systems as well. The most famous of the new Triassic body plans was a bipedal form that we call dinosaurs. WHAT IS A DINOSAUR? Because of its general interest and rather sensational aspects, perhaps the most commonly asked question about dinosaurs is the manner of their extinction. The 1980 hypotheses by the Alvarez group that Earth was hit 65 million years ago by an asteroid and that the environmental effects of that asteroid rather suddenly caused the Cretaceous-Tertiary mass extinction in which the dinosaurs were the most prominent victims, keeps this question paramount in people’s minds. The fact that this controversy is rekindled every several years by some new finding brings it to the surface once again. Thus its preeminence even supercedes the question of whether or not the dinosaurs were warm-blooded. Way down on the list of questions about dinosaurs is the inverse of the extinction question—not why they died out, but why they evolved in the first place. We know when they first appeared, in the second third of the Triassic Period (some 235 million years ago), and we know what these earliest dinosaurs looked like: most were like smaller versions of the later and iconic Tyrannosaurus rex and Allosaurus. They were bipedal forms that quickly became large. What has not been largely known or even considered is the new understanding that 230 million years ago was the time when oxygen may have been nearing its lowest level since the Cambrian Period. So here is a new view here: dinosaurs evolved during, or immediately before, the Triassic oxygen low, a time when oxygen was at its lowest value of the last 500 million years—and their body plan is a result of adaptation to low oxygen.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Many other animals changed body plans in response to extremes in oxygen and so too did the dinosaurs, in my view. The dinosaur body plan is radically different from earlier reptilian body plans and appears in virtually a dead heat (and in great global heat) with the oxygen minimum. Perhaps this is a coincidence. But because many of the aspects of “dinosaurness” can be explained in terms of adaptations for life in low oxygen, that seems unlikely. To formalize this: the initial dinosaur body plan (evolved first by saurischian dinosaurs such as Staurikosaurus and the somewhat younger Herrerasaurus) was in some part in response to the low-oxygen conditions of the time: Hypothesis 8.1: The initial dinosaur body plan of bipedalism evolved as a response to low oxygen in the middle Triassic. With a bipedal stance the first dinosaurs overcame the respiratory limitations imposed by Carrier’s Constraint. The Triassic oxygen low thus triggered the origin of dinosaurs through the formation of this new body plan. The fossil record shows that the earliest true dinosaurs were bipedal and came from more primitive bipedal thecodonts slightly earlier in the Triassic. These thecodonts (diapsids) were the ancestors of the lineage giving rise to the crocodiles as well and may have been either warm-blooded or heading that way. Bipedalism was a recurring body plan in this group, and there were even bipedal crocodiles early on. Why bipedalism, and how could it have been an adaptation to low oxygen? Earlier we saw how even most modern-day lizards cannot breath while they run, and this is due to their sprawling gait. Modern-day mammals show a distinct rhythm by synchronizing breathtaking with limb movement. Horses, jackrabbits, and cheetahs (among many other mammals) take one breath per stride. Their limbs are located directly beneath the mass of the body and to allow this the backbone in these quadruped mammals has been enormously stiffened compared to the backbones of the sprawling reptiles. The mammalian backbone bows slightly downward and then straightens out with running, and this slight up-and-down bowing is coordinated with air inspiration and
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere exhalation. But this system did not appear until true mammals appeared in the Triassic. Even the most advanced cynodonts of the Triassic were not yet fully upright and thus would have suffered somewhat when trying to run and breathe. By running on two legs instead of four, the lungs and rib cage are not affected. Breathing can be disassociated from locomotion. The bipeds can take as many breaths as they need to in a high-speed chase. At a time of low oxygen but high predation, any slight advantage either in chasing down prey, or in running from predators—even in the amount of time looking for food, or how food is looked for, would surely have increased survival. The sprawling predators of the late Permian, such as the fearsome gorgonopsians, were, like most predators during and before their time, ambush predators, as all lizards are today. So what must it have been like for the animals of the Triassic when they found that, for the first time, the predators were out searching for them rather than hiding and waiting? Were any smart enough to register surprise? The results: carnage, surely carnage. This is why dinosaurs may have arisen. All dinosaurs descended from bipedal ancestors, even the massive quadrupeds of later in the Mesozoic. In the Triassic, the crocodile lineage and the dinosaur lineage shared a quadruped common ancestor. This beast may have been a reptile from South Africa named Euparkeria. This group is technically called the Ornithodira, and even the earliest members began to evolve toward bipedalism. This is shown by their ankle bones, which simplified into a simple hinge joint from the more complex system found in quadrupeds. This, accompanied by a lengthening of the hind limbs relative to the forelimbs, is also evidence of this trend, as is the neck, which elongates and forms a slight S shape. These early Ornithodira themselves split into two distinct lineages. One took to the air. These were the pterosaurs, and the late Triassic Ornithodira named Scleromochlus might have been the very first of its kind, a still-terrestrial form that looks like a fast runner that perhaps began gliding between long steps using arms with skin flaps. The oldest undoubtedly flying pterosaur was Eudimorphodon, also of the late Triassic. While these ornithodires edged toward flight, their terrestrial sis-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere the first true mammals. All they needed to be was better than the rest. Clearly they were. The question of metabolic type may also be compromised by the terms used to describe various possibilities. Dinosaur icon Jack Horner certainly thinks so. Metabolic complexes may have been far more diverse than our simple subdivision into “endothermy” and “ectothermy.” While modern birds, reptiles, and mammals are put into one of these two categories, Greg Paul notes that there are many kinds of organisms that can generate heat in their bodies without external heat sources. He includes large flying insects, some fish, large snakes, and large lizards in this camp. Such animals are endotherms but not in the mammalian or avian sense. There may have been many kinds of metabolism in the great variety of dinosaurs that existed. BACK TO THE SEA There are other clues than dinosaur bones to the nature of life on Earth and the challenges it faced during the low-oxygen times of the Triassic. Part of the Triassic Explosion was a diversification of reptiles returning to the sea. Many separate lineages did this, and the reasons why this happened may be tied up in the problems posed by the hot low-oxygen Triassic world. Until now we have stressed respiratory adaptations in various animals. As we have seen, the kind of respiration used by an animal has consequences far beyond simply acquiring oxygen. Oxygen is necessary to run metabolic reactions in animals; it enables the chemical reactions that are life itself. But as in a chemistry experiment, several factors control the reactions themselves. One of the most important is temperature. Metabolic rate is the pace at which energy is used by an organism. It is far higher in endotherms than in ectoderms. But even in the same organism, the metabolic rate is directly and importantly influenced by temperature to a surprising degree. In a 2005 review of animal metabolism, physiologist Albert Bennett of the University of California at Irvine noted that one-third to one-half of all energy expenditure by an animal is used simply for staying alive through activities such as protein turnover, ion pumping, blood circulation—and breathing. Other required activities, such as movement, reproduction,
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere feeding, and so forth are in addition to this baseline energy expenditure. If metabolic rates go up, so too does the need for oxygen, for the chemical reactions of life are oxygen-dependant. The key finding is that metabolic rates double to triple with each 10-degree temperature rise. The consequences of this in a world that had less oxygen available than now but warmer average temperatures would have been major. Thus, it has been argued here that the large dinosaurs were ectothermic, thus enjoying the best of all worlds (or at least making the best of a very, very bad world, one with no modern counterpart). The enemies were low oxygen and high heat. There is no direct link between oxygen levels in the atmosphere and temperature. But there is a direct link between temperature and carbon dioxide, the well-known greenhouse effect. And, as we saw in Chapter 2, levels of oxygen and atmospheric carbon dioxide are roughly inverse: when oxygen is high, carbon dioxide is low and vice versa (but not without exception). Thus, there were many times in the past with low oxygen and high carbon dioxide, and thus it was hot. What a double whammy! In a low-oxygen world that is hot, animals lose in both ways—they need more oxygen than in a cool world to run their now faster metabolic reactions but have less oxygen available in the atmosphere! We have seen many solutions to deal with low oxygen. One is obviously the simple solution of staying cool. Some solutions to staying cool, or cool enough, are physiological; some are behavioral. One of these is morphological, physiological, and behavioral: it is to return to the sea, for even in the hottest world of the past, the ocean would be essentially cooler in terms of physiology. For this reason, perhaps, many Mesozoic land animals traded feet for flippers or fins and returned to the sea at a prodigious rate. Hypothesis 8.2: In times of higher global temperature but lower atmospheric oxygen, an increasing proportion of tetrapod diversity is composed of animals that re-evolved a marine life style. Who has not been struck by the wonders of the Mesozoic, at the marvels posed by those tetrapods that returned to the sea? In the Tri-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Reconstruction of the large Triassic ichthyosaur Cymbospondylus. The ichthyosaurs represented the most extreme body plan change for a once terrestrial group that returned to the sea. The low oxygen on land may have been a major impetus of this. assic there were giant ichthyosaurs and seagoing tetrapods such as placodonts; in the Jurassic the ichthyosaurs remained and were joined by a host of long- or short-necked plesiosaurs; and in the Cretaceous the ichthyosaurs disappeared, to be replaced by large mosasaurs. The existence of marine tetrapods is no surprise in our whale-enriched world, but the surprise to me was the sense that there were so many kinds back then. This suspicion was finally confirmed with the important research of marine reptile expert Nathalie Bardet, who in 1994 published a review of all known marine reptile families of the Mesozoic. My reaction back then was a simple “Yes, there were lots of them,” with no further interest at that time. But we returned to this data set when my colleague and research partner Ray Huey suggested to me that the high heat of the early Triassic through Jurassic would have been an evolutionary incentive for some number of reptiles to go back into the sea. We can now test this hypothesis using new data on the number of dinosaurs and the number of marine reptiles (as seen in the previously unpublished graph below). This graph indicated that there is a very interesting and inverse correlation between Mesozoic oxygen levels and the number of marine reptiles. When oxygen was low, the percentage of marine reptiles was high. But as oxygen rose, the proportion of tetrapod families that were fully aquatic markedly dropped. It may not be that the absolute number of marine forms decreased as
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Percent of atmospheric oxygen (dotted lines and black squares) plotted against the percentage of marine Mesozoic tetrapod families (black lines and black circles: those vertebrates that returned to the sea, such as ichthyosaurs, plesiosaurs, and mosasaurs). This graph supports the hypothesis suggested here that the number of vertebrates returning to the sea increased during high-temperature, low-oxygen times. The cooler water would have enhanced survival in a low-oxygen world. This regression analysis of these data is highly significant (R2=.52). Previously unpublished. much as it was that the number of terrestrial dinosaurs markedly increased. The figure demonstrating these results is shown above. FROM LOW-OXYGEN AIR Let’s now return to the question of why there were dinosaurs. This question can now be answered in multiple ways. There were dinosaurs because there had been a Permian mass extinction, opening the way for new forms. There were dinosaurs because they had a body plan
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere that was highly successful for Earth during the Triassic. But perhaps these generalizations do not cut to the heart of the matter. Chicago paleontologist Paul Sereno, who has unearthed some of the oldest dinosaurs and has made their ascendancy a major part of his study, looks at the appearance of dinosaurs in another way. In his 1999 review The Evolution of Dinosaurs, he noted: “The ascendancy of dinosaur on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by therian mammals at the end of the Cretaceous.” Sereno suggested that the evolutionary radiation following the evolution of the first dinosaurs was slow and took place at very low diversity. This is quite unlike the usual pattern seen in evolution when a new and obviously successful kind of body plan first appears. Usually there is some kind of explosive appearance of many new species utilizing the new morphology of evolutionary invention in a short period of time. Not so with the dinosaurs. Sereno further noted that the dinosaurian radiation, launched by 1-meter long bipeds, was slower in tempo and more restricted in adaptive scope than that of therian mammals. For millions of years, then, dinosaurs and other land vertebrates remained at relatively low-standing diversity, a finding that Sereno and others continue to find perplexing. In my view, this question can now be answered. Earlier we showed that there appears to be a correlation between atmospheric oxygen and animal diversity: times of low oxygen saw, on average, lower diversity than times with higher oxygen. It appears that the same relationship held for dinosaurs. To formalize this: Hypothesis 8.3: Dinosaur diversity was strongly dependent on atmospheric oxygen levels, and the long period of low dinosaur diversity after their first appearance in the Triassic was due to the extremely low atmospheric oxygen content of the late Triassic. Support for this hypothesis comes from our previous analysis of animal diversity in times of low oxygen versus high oxygen. Low-oxygen times apparently stymied the formation of many individuals (while at the same time stimulating experimentation with new body plans to deal with the bad times). This relationship has been demon-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere strated for marine animals, and it seems to hold for dinosaurs and other vertebrates as well. Let’s examine further evidence in support of this hypothesis in the next section. ON THE NUMBER OF DINOSAURS Support for this hypothesis can be found by comparing two new data sources: the latest GEOCARBSULF results for the Triassic through Cretaceous and a new compilation of dinosaur diversity through the same sampling period. Atmospheric oxygen percentage plotted against number of dinosaur genera. This figure supports the hypothesis that higher oxygen supported a higher diversity of dinosaurs. Part of the reason for this may be due to the fact that rising oxygen levels opened up more habitable areas at altitude, a prediction from Huey and Ward, 2005. Previously unpublished.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Recent work by David Favkosky and colleagues, published in 2005, has provided our best estimate yet of the number of dinosaurs during the three periods of the Mesozoic. They show dinosaur genera staying roughly constant from the time of the first dinosaurs in the late middle and late Triassic through most of the Jurassic. While surely mitigated by collecting biases, the sheer number of identified dinosaur skeletons probably ensures that their overall trend is real. It is not until the latter part of the late Jurassic that dinosaur numbers started to rise significantly, and this trend continued inexorably to the end of the Cretaceous, with the only (and slight) pause in this rise coming in the early part of the late Cretaceous. This slight drop may be due to the very small number of known localities of this age yielding dinosaurs. By the end of the Cretaceous (in the Campanian Stage) there were many more dinosaurs than during the Triassic to upper Jurassic. What was the cause of this great increase? The figure above certainly suggests that changing oxygen levels were coincidental with changing dinosaur diversity. This is probably more than coincidence. Through the late Triassic and first half of Jurassic, dinosaur numbers were both stable and low. While originating in the latter part of the Triassic, they stayed relatively few in number until a moderate rise at the end of the period—a rise that seemed to coincide with the end-Triassic mass extinction itself. Gradually, if the oxygen results from GEOCARBSULF are even approximately correct, oxygen rose in the Jurassic, hitting 15 percent or more in the latter part of the period. It was then that the number of dinosaurs really began to increase. It was also at this time that the sizes of dinosaurs increased, culminating in the largest dinosaurs that ever evolved appearing from the latest Jurassic through the Cretaceous. Oxygen levels steadily climbed through the Cretaceous and so too did dinosaur numbers, with a great rise found in the late Cretaceous, the true dinosaur heyday. There were surely many other reasons for this Cretaceous rise. For instance, in mid-Cretaceous times the appearance of angiosperms caused a floral revolution, and by the end of the Cretaceous the flowering plants had largely displaced the conifers that had been the Jurassic dominants. The rise of angiosperms created more plants and sparked insect diversification. More resources were available in all ecosystems, and this may have been a trigger for diversity as well.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere TRIASSIC-JURASSIC MASS EXTINCTION Let’s finish this chapter with one last bit of evidence pertaining to the kind of respiration present in late Triassic animals. One of the striking new findings of the latest GEOCARBSULF has been the level of Triassic oxygen. Only several years ago the minimum oxygen levels of the past 300 million years were universally pegged at the Permian-Triassic boundary of 250 million years ago. But that time of low oxygen has been substantially moved and now may correspond more closely than previously thought with the Triassic-Jurassic boundary of 200 million years ago. Thus, we are confronted with the possibility that oxygen was lower in the late Triassic than in the early part of the period—perhaps as low as 13 percent of the atmosphere at sea level, or much less than modern-day levels. This time corresponds to one of the major changes of the Triassic: the winnowing out of most land vertebrates, with the exception of the first dinosaurs, the saurischians—creatures with pneumatized bones. This realization comes from new data compiled for this book by one of my grad students, Ken Williford, who very painstakingly went through the literature describing the various Triassic vertebrates and their ranges. The results of this long search are shown in the diagram below. In the figure below, the Triassic-Jurassic mass extinction, one of the five most deadly mass extinctions of the past 500 million years, is shown as the horizontal black line in the middle of the figure. Here the saurischian (dinosaurs) are considered to have had air sacs. The data gathered for this figure showed that every group except the saurischian dinosaurs was undergoing reduction (or at best, maintaining roughly equal diversity) in the time intervals leading up to and after the Triassic-Jurassic mass extinction. The groups with the simplest lungs (amphibians and thecodont reptiles) fared the worst and many groups that had been very successful early in the Triassic, such as the phytosaurs, underwent complete extinction. Both amphibians and thecodonts probably had very simple lungs inflated by rib musculature only. Mammals and advanced therapsids of this time, probably both having diaphragm-inflated lungs, did better, but crocodiles, presumably with abdominal pumps, did poorly. The success of the saurischians may have been due to a multitude of factors (food acquisition, tem-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Diversity of tetrapod genera from the middle part of the Triassic to the middle part of the Jurassic, a time interval of some 60 million years, with hypothesized breathing systems for tetrapod orders. Note that of all the orders shown, only the saurischian dinosaurs increase in diversity leading up to and after the Triassic-Jurassic mass extinction. Their then unique air sac system may have been the prime reason for this. Previously unpublished. perature tolerance, avoidance of predators, reproductive success), but my conclusion is that this group was unique in possessing a highly septate lung with air sacs that was more efficient than the lungs of any other lineage and that in the very low-oxygen world that occurred both before and after the Triassic-Jurassic mass extinction, this respiratory system conveyed great competitive advantages. Under this scenario, the saurischian dinosaurs took over the planet at the end of the Triassic and kept that dominance well into the Jurassic because of superior activity levels, which was related to superior oxygen acquisitions. We now know that, alone among the many kinds of reptilian body plans of the middle to late Triassic, the saurischian dinosaurs diversified in the face of either static, or more commonly reducing, numbers
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere in the other groups. We also know that oxygen reached its lowest levels of the past 500 million years in the late Triassic. Something about saurischians enhanced their survival in a low-oxygen world. In another conversation with John Ruben, I was told that this only shows that the abdominal pump allowed this enhanced survival. But what about the bone pneumaticity shown only by the group that prospered through a very bad time in Earth’s history (at least if you were an air breather). And why did the other groups supposed to have had abdominal pumps do so badly—such as the phytosaurs, which went completely extinct? The ground truth suggests that a long and slow drop in oxygen culminated in the Triassic mass extinction but that this extinction was really a double event, separated by between 3 and 7 million years. There are few places on land where this time interval with abundant vertebrate fossils can be found. We really do not know the pattern of vertebrate extinction as well as we do for the extinction in the sea. We do not know how rapidly the prominent vertebrate victims of the mass extinction such as the phytosaurs, aetosaurs, primitive thecodonts, tritylodont therapsids, and other large animals disappeared. But by the time that the gaudy Jurassic ammonites appeared in the seas in abundance, leaving behind an exuberant record of renewal in early Jurassic rocks, the dinosaurs had won the world. What kind of lungs did they have? Here it can be proposed that they had lungs and a respiratory system that could deal with the greatest oxygen crisis the world was to know in the time of animals on Earth. Let’s formalize this: Hypothesis 8.4: Saurischian dinosaurs had a lower extinction rate than any other terrestrial vertebrate group because of a competitively superior respiration system—the first air sac system. Support for this hypothesis comes from the new anatomical findings of an air sac system in saurischians and the newly presented data (above) that saurischians were actually expanding in number across this mass extinction boundary. They emerged—perhaps gasping, but still standing, over the corpses of the simpler lunged—in a world we will look at in the next chapter.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
Representative terms from entire chapter: