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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere 9 THE JURASSIC: DINOSAUR HEGEMONY IN A LOW-OXYGEN WORLD Some 200 million years ago, only 50 million years after the great paroxysm of the Permian extinction, the Triassic Period came to an end in another blood letting. As we saw in the previous chapter, of the many lineages of land life that suffered through this extinction, it was only the saurischian dinosaurs that came through unscathed. The mass extinction ending the Triassic Period was not just a phenomenon on land. It also wiped out most stocks of chambered cephalopods, but in the lower Jurassic they rediversified in three great lineages: nautiloids, ammonites, and coleoids. Scleractinian reefs flourished once again, and large numbers of flat clams colonized the seafloor. Marine reptiles belonging to the ichthyosaur and the new plesiosaur stocks again were top carnivores. On land the dinosaurs flourished, and mammals retreated in size and numbers to become a minor aspect of the land fauna but showed a significant radiation into the many modern orders near the end of the Cretaceous. Birds evolved from dinosaurs in the latter parts of the Jurassic. This is all well known and not the topics of the sort of revisionist history that is the goal of this book. Instead, let’s look at the record of oxygen during the Jurassic and compare that to the numbers and kinds of dinosaurs in the ancient Jurassic Park.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere THE JURASSIC BESTIARY As we saw in Chapter 8 the first dinosaurs were the bipedal saurischians, and they soon spawned another group, the ornithischians. The ornithischians, which began as relatively small, carnivorous bipeds, quickly evolved into herbivores and stayed that way, with both quadrupeds and bipeds. But they remained a relatively small part of the terrestrial fauna during the first half of the Jurassic. The reason for their rarity relative to the number of saurischians is probably related to oxygen levels. Unlike the saurischians, which showed bone pneumatization, ornithischians never evolved this trait. If bone pneumatization is a consequence of the air sac respiratory system, it can thus be inferred that ornithischians never used this kind of lung. Dinosaur expert Robert Bakker disagrees, suggesting that all dinosaurs used the air sac system, but that it was less developed in ornithischians, having only the abdominal sacs and not the sacs that fit into cervical bones in the neck. It seems more likely that the ornithischians had no air sacs at all. The Jurassic world was not solely a dinosaur habitat, of course. We know that the Jurassic and the succeeding Cretaceous Period were times of innovations on land, in the sea, and also in the air, and it was the expansion into the air by two major groups of vertebrates that might have been the most radical of all Jurassic evolutionary changes. Three distinct kinds of flyers were in the skies: pterodactyls, pterosaurs, and birds, with the last evolving from saurischian bipeds during the latter parts of the Jurassic. While no one disputes that true birds first appeared in the Jurassic, there is still controversy about when these earliest birds took to the air. Most experts believe that the first birds could fly. Others, such as John Ruben, consider that true flying did not occur until the early Cretaceous. Regardless, true birds were on the scene in the Jurassic, and once again it appears that this extreme evolutionary novelty was a consequence, or was stimulated by, lower-than-now oxygen conditions that existed until near the end of the Jurassic itself. Finally, the Jurassic (continuing into the Cretaceous) was the time when several lineages of true mammals evolved, all at small size, while in the seas the ichthyosaurs competed with both long- and short-necked plesiosaurs.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere Thus, the dinosaurs were not alone on the Jurassic stage. But they were certainly dominant on land. Let’s break their history down into distinct stages and then attempt to explain why these stages came about. SIX STAGES IN THE HISTORY OF DINOSAUR FAUNAL MAKEUP While at first it seems that there were many kinds of dinosaur body shapes, in fact there were really but three: bipeds, short-necked quadrupeds, and long-necked quadrupeds. All three shared a common characteristic with birds and mammals—a fully upright (rather than sprawling) posture. Let’s look at these shapes in the context of atmospheric oxygen and ambient temperature when they originated and when they thrived. Any history of life must come to grips with and explain the extreme gigantism of the Jurassic dinosaurs. Iconic of these dinosaurs are the sauropods, the long-necked, long-tailed saurischians that are synonymous with dinosaur in most of popular culture. This body plan resulted in the largest sizes of land life ever evolved, and this begs the question, why? Why this shape and why so big? Why did other groups of dinosaurs not converge on this shape? Let’s look for possible explanations for that dinosaur body plan and gigantism in relation to the prevailing oxygen content. The history of dinosaurs can be summarized as follows: Middle Triassic The earliest dinosaurs appeared in the last third of the Triassic but remained at low diversity for their first 15 million years. The majority of forms were bipedal, carnivorous saurischians. Toward the end of the period, quadrupedal saurischians (sauropods) evolved. Ornithischians diverged from the saurischians before the end of the Triassic but made up a very small percentage of dinosaur species and individuals. For much of the Triassic, dinosaur size was small, from 3 to 10 feet in length, while the earliest ornithischians (such as Pisanosaurus) were 3-feet-long bipeds that had a new jaw system specialized for slicing plants. Late Triassic In the latest Triassic the first substantial radiation of dinosaurs occurs. It took place among saurischians, with the evolution
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere of both more and larger bipedal carnivores and the first gigantism among early sauropods (such as Plateosaurus of the Upper Triassic). Early to Middle Jurassic Continuation of the trend of the latest Triassic, where saurischian bipeds and quadrupeds dominated faunas, characterized this phase. During this time, however, the ornithischians, while remaining small in size and few in number, diversified into the major stocks that would ultimately dominate dinosaur diversity later in time, in the Cretaceous. These stocks included the appearance of heavily armored forms (such as the thyreophorans). These were quadrupeds and included the first stegosaurs of the middle Jurassic. A second group was the unarmored neornischians (which included ornithopods, hypsilophondontids, iguanodons, and duck bills), marginocephalians (the ceratopsians, which did not appear until the Cretaceous), and bone-headed pachycephalans. But it was the sauropods that were most evident in numbers. They split into two groups in the latest Triassic, the prosauropods and true sauropods, and in the early and middle Jurassic the prosauropods were far more diverse than sauropods but went extinct in middle Jurassic time, leading to a vast radiation of sauropods into the late Jurassic. Finally, the bipedal saurischians also showed diversity and success in the lower and middle Jurassic. In the latest Triassic time they split into two groups (the ceratosaurs and tetanurans). The ceratosaurs dominated the early Jurassic, but by middle Jurassic time the tetanurans increased in number at the expense of the ceratosaurs. They too split into two groups, the ceratosauroids and the coelophysids. The latter group eventually produced the most famous dinosaur of all, the late Cretaceous Tyrannosaurus rex, although its middle Jurassic members were considerably smaller. Their most important development in the Jurassic was evolution of the stock that gave rise to birds. Upper Jurassic This was the time of the giants. The largest sauropods came from late Jurassic rocks, and their dominance continued into the early part of the Cretaceous. Keeping pace with this large size were the saurischian carnivores, with giants such as Allosaurus typical. Thus, the most notable aspect of this interval was the appearance of sizes far larger than in the early and middle Jurassic. This was not only among the saurischians. During the late Jurassic the armored ornithischians also increased in size, most notably among the heavily ar-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere mored stegosaurs. The diversification of ornithischians at this time—with the appearance of stegosaurs, ankylosaurs, nodosaurs, camtosaurs, and hypsilophontids—radically changed the complexion of the dinosaur assemblages. Early to Middle Cretaceous While the dominants for the early part of this interval remained large sauropods, as the Cretaceous progressed, a major shift occurred: ornithischians increased in diversity and abundance until they outnumbered saurischians. Sauropods became increasingly rare as many sauropod genera went extinct at the end of the Jurassic. Upper Cretaceous Dinosaur diversity skyrocketed. Most of this diversification came through large numbers of new ornithischians: ceratopsians, hadrosaurs, and ankylosaurs, among others. Only a small number of sauropods were present. So how to make sense of this pattern? In his review of the dinosaurs, Paul Sereno noted the difference between any of the separate dinosaur diversifications and mammals after the Cretaceous. One great difference, of course, is that the Cretaceous-Tertiary extinction was unique in being so catastrophic and so short in duration. The end-Triassic mass extinction, for example, was a far more extended event. Nevertheless, Sereno’s observations are important, and in his view, crying out for explanation. For example, he noted: The radiation of the nonavian dinosaurs, by comparison to the Paleocene mammals was sluggish and constrained. Taxonomic diversification took place at a snail’s pace; standing diversity, which may have totaled 50 genera or less during the first 50 million years (the late Triassic and early Jurassic) increased slowly during the Jurassic and Cretaceous, never reaching mammalian levels; maximum body size for herbivores and carnivores was achieved more than 50 million years after the dinosaurian radiation began, only 8 to 10 distinctive adaptive designs evermore and few of these would have been apparent after the first 15 million years of the dinosaur radiation. Sereno thus found no reasonable explanation for this early dinosaur history, and most other workers have yet to even take the emergence into account in any meaningful, hypothesis-driven way. Here, however, we can propose several hypotheses that readily explain the history above, and the pattern of subsequent dinosaur history. For
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere any reader who has made it this far, the explanation will be no surprise: the history of dinosaurs was largely dictated by changing oxygen levels. HYPOTHESES FOR THE OBSERVED HISTORY OF DINOSAUR DIVERSITY, DISPARITY, AND SIZES Here we can propose the following hypothesis based on dinosaur morphological and taxon abundance in the context of a long-term rise in Mesozoic oxygen levels. These explanations can answer the questions posed by Sereno about the observed history of dinosaurs. They are as follows. Hypothesis 9.1: The Carnian through Hettangian interval (late Triassic to earliest Jurassic) was a time of low-oxygen levels and this coupled with very high carbon dioxide levels and hydrogen sulfide poisoning—not asteroid impact—was the major cause of the Triassic-Jurassic mass extinction. Support for low-oxygen levels over this interval comes from marine stratal evidence of progressive anoxia (black shales, laminated beds, and trace fossils [such as chondrites] characteristic of low-oxygen sea bottoms), from the modeling of GEOCARBSULF, and from studies of fossil plants across this time interval by University of Chicago paleobotanist Jenny McElwaine, who showed morphological evidence of rising carbon dioxide in her fossil plants collected over this time interval. The combination of low oxygen, high global temperatures, and, based on the recovery of new biomarkers, perhaps hydrogen sulfide was the killing mechanism. While there were successive asteroid impacts over this time interval, one of which (the Manicouagan event of 214 million years ago, or 14 million years before the Triassic mass extinction) was large, the impacts played little or no role in the extinctions according to scientific observations of many stratal sections of this time interval. Hypothesis 9.2: Ornithischian dinosaurs did not possess as effective a respiratory system as did saurischians. However, they were competitively superior to herbivorous saurischians with
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere regard to food acquisition. With the rise of oxygen to near present-day levels in the Cretaceous, ornithischians became the principal herbivores because of this superiority, leading to the extinction of many saurischian herbivores through competitive exclusion. While Robert Bakker has argued that ornithischians and saurischians alike had an air sac respiratory system, the lack of bone pneumaticity in any of the ornithischians indicates that if they used an air sac system it would have been rudimentary. What they did possess was a series of tooth adaptations that may have been far superior to the teeth in herbivorous saurischians. While we have seen that the Jurassic to Cretaceous interval marked a relatively rapid and significant rise in atmospheric oxygen, other events were taking place, including a radical change in flora. Dinosaurs evolved in a gymnosperm-dominated world—with conifers, but with ferns, cycads, and gingkoes as well. But in the early part of the Cretaceous a new kind of plant appeared, a flowering plant. With this new kind of reproduction and other adaptations, these plants, the angiosperms, underwent a rapid adaptive radiation. They out-competed the earlier flora nearly everywhere on Earth to the extent that by the end of the Cretaceous, some 65 million years ago, the angiosperms made up as much as 90 percent of vegetation. This transition in available food types would have affected the herbivores, and the kind of herbivores available as food would have directly affected carnivore body plans. Killing a late Jurassic sauropod would have been very different from killing a late Cretaceous hadrosaur. Herbivory is dependent on the correct kind of teeth for the available plants. The sauropods may have lived on pine needles, their huge barrel bodies being, essentially, giant fermenting tanks for digestion of a relatively indigestible food source. The appearance of broadleafed plants, the angiosperms, would have required different teeth and biting surfaces than those optimal for slicing pine needles off trees. Thus, the transition from the sauropod-dominated faunas of the Jurassic to the ornithischian-dominated faunas of the Cretaceous was surely related in some part to the change in plant life. But respiration may have played a part as well, and perhaps if oxygen had not risen above 15 percent, the ornithischian takeover would not have taken place.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere JURASSIC-TRIASSIC DINOSAUR LUNGS AND THE EVOLUTION OF BIRDS In Chapter 8 we looked at the questions of dinosaur metabolism and respiration. The proposal there was that the first dinosaurs were of a kind of animal never seen and not alive today. Through upright posture and an evolving air sac system, they developed respiratory efficiency (the amount of oxygen extracted from air per unit time, or per unit energy expended in breathing) superior to any other then-extant animal. But these early forms may have lost (or never gained in the first place) endothermy, replacing it with a more passive homothermy or even ectothermy, which was attained with larger size. That was their trick—using ectothermy to reduce oxygen consumption while at rest and a superior lung system to allow extended movement without going into rapid anaerobic (and thus poisonous) states. But what of the later dinosaurs? We know that birds, a group of dinosaurs first appearing in the Jurassic, eventually had both endothermy and a very different kind of lung than in any extant reptile. It seems probable that the large and small saurischians parted company, with smaller forms evolving endothermy later in the Jurassic as oxygen levels rose rapidly. John Ruben’s group stakes out a very different and conservative position that the first true birds had both ectothermy and reptilian, not air sac, lungs. Most other dinosaur and bird specialists are not so sure. Some believe that endothermy and air sac lungs of some kind were present in Archaeopteryx, while others indicate that based on bone pneumaticity the air sac system was present in the bipedal carnivores that gave rise to birds. With perhaps the exception of the always-fascinating tyrannosaurids, no group of dinosaurs has received more attention in recent times than the basal birds. Vigorous debate centers on their body covering and, most importantly, on when flight first evolved and why. The first birds appeared about 150 million years ago, and the famous first bird remains Archaeopteryx. That is just before the start of the Cretaceous. Oxygen had been rising for 50 million years at that time. Gigantism in dinosaurs was common. The immediate ancestors of the birds were fast, ground-running dinosaurs that may have used their forelimbs for a type of predation, a motion that was preadapted for a wing
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere stroke in a flyer, according to Berkeley paleontologist Kevin Padian. The fossil record suggests that the ancestors of the first bird were the bipedal carnivorous saurischians known as troodontids or perhaps the dromaeosaurids, forms that appear to have been already feathered (there is much controversy about this). Could Archaeopteryx fly? Padian thinks so. But there is debate about when true flight took place. Could the late Jurassic “birds” really fly, at a time when their competition in the air would have been the diverse and successful pterodactyls? The fossil record does show that by the lower Cretaceous there was a bird (Eoaluolavis) that had evolved a “thumb wing,” an adaptation that allows greater maneuverability at slower speeds. Thus, within a few million years after Archaeopteryx, fairly advanced flight was present. New discoveries from China have revealed an unexpected high diversity of birds by the early part of the Cretaceous. Flight was an adaptation that stimulated a rapid evolution of new forms. What new information can be added? Flight is highly energetic. Birds use a great deal of energy to fly, and that, added to their relatively small size and endothermy, makes them great users of oxygen. So the air sac system serves them well. But what of the late Jurassic, when oxygen may have been somewhat lower than now? Could it be that the even lower oxygen of the early and middle Jurassic delayed flight? What about other known flyers? The pterodactyls had long been in the air by the evolution of the first bird, but pterodactyls may not have been as energetic in their flying or might have had an air sac system, for they also show bone pneumaticity consistent with the presence of an air sac system. Hence, there are questions about lung type not only in the first birds but also in other flyers, their immediate ancestors, and in the bipedal, saurischian dinosaurs like T. rex that came along in the Cretaceous. DINOSAUR REPRODUCTION AND OXYGEN LEVELS Alas, the total extinction of dinosaurs 65 million years ago (unless birds are considered dinosaurs, now accepted by many) will forever make it impossible to answer many pressing questions about their biology. So it is natural that we try to answer these questions using their
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere nearest living relatives, the birds and reptiles. After lung type and metabolic type, some of the most interesting questions relate to reproductive strategy, and this too can be examined in the light of changing oxygen levels. Birds show little variation in their reproduction. Extant birds, our best window to the dinosaurs, all lay eggs with a porous calcareous shell. There are no live births in birds, in contrast to extant reptiles, which have many lineages using live birth. There is also great variation in egg morphology between birds and some reptiles. While the eggshell in birds and reptiles consists of two layers, an inner organic membrane overlain by an outer crystalline layer, the amount of crystalline material varies greatly, from a thick, calcium carbonate layer like that in birds to almost no crystalline material at all, so that the outer layer is a leathery and flexible membrane. Even the mineralogy of the crystalline layer varies, from calcite in birds, crocodiles, and lizards to aragonite (a different crystal form of calcium carbonate) in turtles. Eggs are thus divided into two main types: hard or crystalline and soft or parchment (some scientists further subdivide the parchment eggs into flexible [used by some turtles and some lizards] and soft [parchment, used by most snakes and lizards]). Not surprisingly, the fossilization potential of these different hardness categories of eggs differs markedly. There are numerous fossil hard eggs known (many from dinosaurs), a few flexible eggs, and no undisputed soft eggs preserved. Because of the great interest in dinosaurs there has been much speculation about their reproductive habits (the thought of two gigantic Seismosaurus mating rather boggles the imagination), and there are still many mysteries. One of the seminal discoveries about dinosaurs was that they laid large calcareous eggs, with calcite crystals making up the mineral layer, a finding from the first expedition to the Gobi Desert by an American Museum of Natural History expedition in the 1920s. Since then, thousands of Cretaceous dinosaur eggs have been found around the world, and even the nesting patterns have been discovered, the most notable being the nest discoveries in Montana by Jack Horner. But are these Cretaceous finds characteristic of dinosaurs as a whole? This question remains unresolved and controversial. While most scientists assume that all dinosaurs laid hard-shelled eggs, this is
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere far from proven, and, as we shall see below, there is indirect evidence that some early dinosaurs may have utilized parchment eggs or even live births. What is the egg record for dinosaurs? Almost all dinosaur eggs come from the Cretaceous, and the nature of their crystal form and size, as well as the number and pattern of pores in the egg, show a wide variety. There are certainly plenty of eggs found from the Cretaceous, but, while known, there are far fewer Jurassic dinosaur eggs and almost none known from the Triassic. There are several possibilities for this. Perhaps there is some preservation bias, with pre-Cretaceous eggs as common as those of the Cretaceous, but the lesser extent of Triassic and Jurassic dinosaur beds compared to the vast expanse of Cretaceous-aged beds has caused this difference. Another possibility is that pre-Cretaceous eggs fossilized much less readily than those from the Cretaceous. This would certainly be the case if pre-Cretaceous eggs were leathery like those of extant reptiles, rather than calcified like birds. And if, like the marine ichthyosaurs, some dinosaurs utilized live birth rather than egg laying there would certainly be fewer eggs to find. As in so many other aspects of the history of life, the level of atmospheric oxygen may have played a major role in dictating mode of reproduction. Fossil eggs from Cretaceous deposits attributed to dinosaurs (what else could have laid such large eggs?) have a calcium carbonate covering like a chicken egg (but thicker). But unlike chicken eggs, which are smooth, the dinosaur eggs were usually ornamented with either longitudinal ridges or nodular ornamentation. Presumably, ornamentation allowed the eggs to be buried after emerging from the female, with the ornament allowing airflow between the eggs and the burial material. The ability to bury eggs may have aided their fossil preservation potential and perhaps helps explain why there are so many Cretaceous eggs and so few other kinds. The heavy calcification would also help the eggs withstand the overpressure of burial in soil or sand. Also the complex behavior involved in nest making and orienting the eggs in burial mounds is now known for the late Cretaceous—but not before. What are the advantages of calcareous eggs? They are strong, harder for predators to break into, and aid in development. As the embryo grows inside the egg, some of the calcium carbonate is dissolved
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere ancestor—but would be what we would predict if hard-shelled egg laying evolved numerous times by separate lineages of dinosaurs. If we add the additional (and different) eggshell morphologies found in extant reptiles and birds, there are a combined 12 separate eggshell microstructures that have evolved. Perhaps each of these is an adaptation to a different kind of stress that each egg undergoes: a turtle egg in a deep burrow, for instance, faces a very different series of challenges than does a bird egg in a nest high in a tree. But the more likely is that hard eggs separately evolved in multiple lineages—including dinosaur lineages. With this (admittedly tenuous) evidence at hand, a different scenario can be proposed from the Geist-Rubin hypothesis that archosaurs evolved the hard eggshell in the Permian. The extreme environmental conditions that Geist and Rubin noted—high temperature and thus desiccation—and the other deleterious factor that they didn’t—low-oxygen values—would have stimulated evolutionary change from the system used by animals that had first evolved in a high-oxygen and perhaps lower-temperature world. Seemingly, a better response to heat and low-oxygen (which is further magnified by the heat) would be live birth. The evolution of live birth thus may have come about in response to lowering global oxygen values in the late Permian. Unfortunately, evidence for this is again an absence of evidence. Nevertheless, it is what we have to work with: despite the enormous number of therapsid bones found in South Africa, Russia, and South America, a fossil egg or nest has never been found in these rocks. Therapsids may have already evolved live births by this time, a trait carried on by their descendents—the true mammals that were first found about the same time as the first dinosaurs appeared on the scene. What can we learn about oxygen and eggs in living organisms? While data are surprisingly scanty, it appears that oxygen can diffuse through the wall of a modern parchment egg more readily than it can in a calcareous egg. Parchment eggs can also be kept within the mother’s body for extended periods of time, where it is maintained in an oxygen-rich environment. It may be that many lineages of dinosaurs evolved the calcareous egg in the late Jurassic as a response to rising oxygen and that the formation of calcareous eggs, which are
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere then buried, was a reproductive strategy that was not viable in the late Permian through middle Jurassic environments of lower atmospheric oxygen. So, let me more formally pose this: Hypothesis 9.3: The low-oxygen and high-heat conditions of the late Permian into the Triassic stimulated the evolution of live birth and of soft eggs that would have been effective at allowing oxygen movement into eggs and carbon dioxide out. On the other hand, the higher-oxygen levels (and continued high temperatures) of the late Jurassic-Cretaceous interval stimulated the evolution of rigid dinosaur eggs and egg burial in complex nests. Only time will tell if new discoveries from late Triassic through middle Jurassic strata will add significant new information to the topic of dinosaur (and mammalian!) birth strategy. Like characteristic metabolisms, the contrasting patterns of live births versus egg laying are fundamentally important—and ones that have received surprisingly scant attention by evolutionary biologists. Solving this problem—by learning the time of origin and the distribution of one kind of birth strategy or the other—should be a major research topic of the near future but, sadly, may prove to be intractable because of the non-preservation of parchment eggs. JURASSIC-CRETACEOUS IN THE SEAS Let’s now move from land to sea. The Jurassic and Cretaceous oceans would have been dangerous places to swim in. The major Triassic marine predators just increased in number in the Jurassic and added a new and efficient kind of fish- and cephalopod-eater, plesiosaurs. In the Cretaceous yet another kind of tetrapod predator appeared as well: mosasaurs displaced plesiosaurs and ichthyosaurs as the top carnivores in the sea. Ammonites continued to flourish but evolved large numbers of uncoiled shell shapes in addition to the traditional planispiral shapes of the Jurassic and lower Cretaceous. A diverse calcareous plankton including coccoliths and foraminifera changed the nature of
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere planktonic communities, while a new form of gastropods, the entirely carnivorous neogastropods, joined a host of other shell-breaking predators to totally transform the benthos in what has been called the Marine Mesozoic Revolution. A response to this increase in predation was the evolution of infaunal siphonate clams (with heterodont dentition) and stalkless crinoids (the comatulids). All of these events are well known and documented and thus are not the targets of revisionist history. But the reasons behind the specific designs of some of these same organisms are another matter. The rising oxygen of the Jurassic and Cretaceous following the Triassic nadir caused an increase in the number of species, but, as we have seen, it was low oxygen that stimulated new kinds of body plan. Let’s look at four types of aquatic body plans that are related to oxygen levels in the seas and that also made the Mesozoic oceans very different places than our present-day oceans. 1. The evolution of low-oxygen-tolerant bivalves. The oxygen low of the early through middle Triassic produced a new and poorly habitable ocean. As we have seen, animals do very badly in low oxygen. Atmospheric oxygen levels affect oceanic oxygen levels and quite often even serve to magnify the effects. Many ocean bottoms of the Mesozoic were completely anoxic, and most were at least hypoxic. Rare was an ocean community of these times that had oxygen levels like those on the bottoms of modern-day oceans. Just as in the Cambrian Explosion, where animals were stimulated to produce new kinds of body plans based around respiratory systems, so too did animals of the Triassic seas show a multitude of new adaptations. As we have seen, the land fauna experimented with a variety of lung types. The same kind of exploration took place in the oceans. The bivalved mollusks were one group that evolved a new kind of body plan, and even physiology, in response to the nearly endless expanse of nutrient-rich but low-oxygen bottoms. The very lack of oxygen on the ocean bottoms made them, in one sense, wonderful places to live. Vast quantities of reduced carbon, in the form of dead planktonic and other organisms, fell to the seafloor and were buried there. On an oxygenated bottom this material would soon be consumed, by filter- or deposit-feeding organisms and scavengers, and used for food. But the low-oxygen conditions kept these or-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere ganisms out, and not even the usual bacteria that decompose dead creatures on the sea bottom were around. As we have seen, this is one reason that oxygen levels plummeted in the Triassic. But the clams figured a way out of this. A few kinds, living on the seafloor of the ocean bottoms that had at least some oxygen, fed not on the falling organics but on methane-containing compounds coming up from some fraction of the organic-rich sediment. Methanogens are a group of bacteria that thrive in low- or no-oxygen conditions, and even several inches down into the sediment on a sea bottom with some oxygen, they would have penetrated into an oxygen-free zone—thus it was an ideal environment for methanogens. As methanogens metabolize, they release methane as a by-product. The Mesozoic clams may have had other bacteria in their gills that could exploit the methane and other dissolved organic material, or they may simply have fed on the bacteria. A somewhat similar mechanism is found today in the deep-sea vent faunas, where giant tubeworms and clams use these chemicals as food. But the difference is that the modern vent faunas are oxygenated. The animals down there do not even need gills. The clams of the Triassic and Jurassic were not so lucky. These kinds of clams are found in huge numbers in Triassic and Jurassic sediments. In the latest part of the Triassic, when oxygen reached its lowest levels, the number of these clams was so great that they formed rocks themselves with their shells—a kind of rock known as a coquina. Two Triassic taxa that did this were Halobia and, especially, Monotis. Both lay on the surface of the sediment and were immobile. There was no burrowing (like the majority of today’s clams) or movement on top of the sediment (like modern-day cockles). They were more akin to mussels—they just sat there. And like mussel beds, they were often abundant. In recent Triassic work, many scientists have now sampled and seen these kinds of beds from all over the world in rocks of this age, and the shock of seeing so much life packed into rocks is always striking. For tens of feet of stacked strata there is nothing but an endless packing of shells. There must have been billions of these clams lining the bottoms of the sea, presenting a spectacle that has no parallel today. But the most unusual aspect of the clam beds is
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere that there is virtually nothing else in there with them. There may be the occasional ammonite or other mollusk, but these are rare indeed. The clam beds are essentially monotypic—composed of a single species. It is very unusual to see monotypic assemblages in the modern-day marine sea bottom, especially those in the tropics. But the Triassic world was so warm that virtually all the world was tropical, and yet there is never much diversity to these beds, which are found worldwide. The flat clams began to diminish in the shallows first in the mid-to late Triassic but held on in deeper water until the end of the Cretaceous. The abundant Cretaceous clam Inoceramus lived in many environments, but giants of this kind are found in deep-water deposits. Some fossils are as large as 6 feet across, and like the Triassic clams they seem to have been neither carnivores nor herbivores, but instead they were chemosynthetic, using chemical compounds coming up from the reduced carbon–rich mud on which they lay. Ironically, it appears that they were eventually driven into extinction by rising oxygen levels. With the appearance of an ever-more oxygenated sea bottom in the upper Cretaceous, as atmospheric oxygen rapidly rose, the conditions that had succored the flat clams disappeared. 2. The evolution of low-oxygen-tolerant cephalopods. There are many places in the world where marine strata of the latest Triassic age are overlain by Jurassic strata. At such outcrops one can walk through time, and if the strata are continuous, the dramatic events of the late Triassic and early Jurassic are present for all to see. This interval of time and rock preserves evidence of the great Triassic mass extinction, one of the so-called Big Five mass extinctions, a dubious honor of species death. As you walk through upper Triassic beds you are first in strata packed with fossils of the flat clam Halobia; then you move into younger rocks with the even more abundant Monotis, the clam described just above. Then these clams disappear in turn, over only several feet of strata, leaving a long barren interval of rock and time, the last stage of the Triassic, an interval perhaps 3 million years in length known as the Rhaetian stage. Finally, after this thickness virtually without fossils, a new group suddenly appears—the ammonites. While there are ammonites to be found in the upper Triassic, they are never abundant. Most seem to have gone extinct when the clams
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere did, perhaps 3 million years before the final phase of this end-Triassic mass extinction. This situation changed drastically with the onset of the Jurassic. Most famously at the beach of Lyme Regis of England, but also in southern Germany and at many other localities worldwide, the earliest Jurassic ammonites appear in huge numbers, and they diversify over only a few short meters of strata as well. This is not like the Triassic flat clams where one species is all you get. These ammonites of the first part of the Jurassic are diverse and abundant. It is a fossil collector’s dream, and it tells us that the great drop in oxygen was finally over and that a slow rise in oxygen was finally underway. But the ammonites were not telling us that oxygen levels similar to today were suddenly in place. The ammonites appear because the surface of the early Jurassic seas began to have a modicum of oxygen, and the ammonites took full advantage. They did so because they were among the best animals on Earth for dealing with low oxygen. Chapter 3 presented a new potential pathway for the evolution of the first cephalopods. The nautiloids are still with us today—but up until the end of the Cretaceous, the nautiloids from the Devonian on were far outnumbered by one of their descendants, the ammonites. Because of the overall similarity of the chambered shells in both nautiloids and ammonoids, we presume they may have had somewhat similar modes of life. Nautiluses today live in highly oxygenated water over most of their range. But here and there they also live in hypoxic bottoms. This was a great curiosity when first discovered, because it was conventional wisdom that all cephalopods need high-oxygen conditions. Not so the Nautilus. My decade of studying them proved that they are very tough and resistant when taken out of the water. They can sit out 10 or 15 minutes with no ill effects. When they are in water, they quickly replenish oxygen in their blood through one of the relatively largest and highest-powered pump gills ever evolved. If ever an animal was adapted for low oxygen, this is it. British zoologist Martin Wells, who measured oxygen consumption of various captive nautiluses in New Guinea, finally proved this. When Nautilus is confronted with low oxygen, it does two things. First, its metabolism slows way down. Second, it apparently uses some of the gas in its air-filled chambers for emergency respiration.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere The mass appearance of ammonite fossils in lower Jurassic strata suggests that, like the nautiluses, the ammonites were superbly designed to extract maximum oxygen from minimal dissolved volumes of the oh-so precious gas. They do so because of a powerful pump gill system that was capable of moving sufficient volumes of low-oxygen water across the gill surface to yield the necessary number of oxygen molecules from seawater to live. To formalize this: Hypothesis 9.4: Jurassic-Cretaceous ammonite body plans evolved near the Triassic-Jurassic boundary in response to worldwide low oxygen. Their new body plan (compared to the ammonoids that came earlier) involved a much larger body chamber relative to the phragmocone, which may have allowed for much larger gills. Because of this they had to use thinner shells, and this required more complex sutures. The sutures also allowed faster growth by increasing rate of chamber liquid removal. Within the large body chamber was an animal that could retract far into this space and that had very long gills relative to its ancestors. What support is there for this hypothesis that basal Jurassic ammonites were low-oxygen specialists? We know that early Jurassic ammonites are found in great numbers in otherwise animal-free strata, and we know their body chamber length increased at this time. Unfortunately, ammonite soft parts are still unknown, and we do not know if they had four gills (like Nautilus) or two (like modern-day squid and octopus). But from the very unstreamlined shells of the majority of early Jurassic forms, it is clear that these animals were not fast swimmers. It is far more likely that they floated slowly or swam gently near the surface, using their air-filled shell like a zeppelin. Their pump gill forced huge volumes of water across their lungs in short periods of time, allowing them to live where most animals could not. The ammonites went on to stay very common right up to the end of the Cretaceous. My work in Spain and France in the late 1980s showed that they were killed off, very suddenly, as a consequence of the Cretaceous Chicxulub asteroid. But by the end they were living in higher-oxygen waters and their shapes changed subtly, allowing a more active and vigorous life style.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere 3. The evolution of crabs. Another new body plan that may have arisen as a consequence of and as an adaptation for low oxygen is that of the lobsters and crabs. While the overall shrimp-like body form of crustaceans is found in Paleozoic rocks, crabs are a relatively new invention. A crab is simply a shrimp-like form in which the abdomen is tucked under the body. Fusion of the head and thorax into a heavily armored and calcified plate makes the crab a difficult nut to crack for its predators. And placement of the abdomen under this armor plating is design genius. It is the abdominal regions that are most susceptible to breakage in any predatory attack, and by eliminating this chink in their armor, the crabs rapidly rose to marine prominence. Their large claws allow them to crack open mollusk shells, among other prey; they are shell-breaking predators. Prior to this, few predators were able to break into shelled organisms. Crabs and others evolved the morphological means to render many previously impregnable skeletons vulnerable. Thus, the accepted reason for the crab’s body plan, novel as it is, relates to defense (tucking of the abdomen, thickening and increasing calcification of the head-thorax region) and offense (evolving a strong pair of jaws). But here is another: crab design came about in some part as a primary adaptation for increasing respiratory efficiency. Hypothesis 9.5: The crab’s body plan evolved for multiple reasons but one was that it increased respiratory efficiency by putting the gills in an enclosed space under the cepahlothorax (the head-thorax) and then evolving a pump to move water over the now enclosed gills. The crab gill design is a marvelous way to increase water passing over the gills. Crabs evolved from shrimp-like organisms, and in these ancestors a progression toward the crab gill system can be seen. In shrimp the gills are partially enclosed beneath the animal. While covered dorsally, the gills are attached to segments and are open to water underneath. 4. The evolution of the calcareous plankton. The formation of calcium carbonate—limestone, coming in the two mineral species calcite and aragonite—is affected by several factors, most importantly tem-
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere perature, pH, and the concentration of calcium and carbon dioxide levels in water. But there is another factor that is important as well—oxygen. In our world, most calcium carbonate formation is mediated or undertaken for adaptive reasons by organisms. Calcium carbonate is the most commonly used mineral in skeletal formation, and the majority of animals, and many protozoans and plants, have skeletons of one kind or another. While there are substantial numbers of organisms using silicon for skeletons, in numbers and biomass produced, they are far exceeded by those using calcium carbonate. All of these organisms also need oxygen for life, so in water containing low levels of oxygen there is little or no calcium carbonate produced. It was the substantial rise of oxygen levels from the Jurassic through the Cretaceous that increasingly favored the formation of calcareous skeletons by plankton. The amount of calcium carbonate produced at any time on Earth has substantial effects on the atmosphere and chemistry of the oceans. When atmospheric carbon dioxide levels are high, the rate of limestone formation increases—but only if there is sufficient oxygen to allow the organisms making the skeletons to flourish. The only exception to this is among the group of plants that produce carbon dioxide skeletons, such as the single-celled planktonic forms known as coccolithophorids. Today the largest amount of global calcium carbonate formation comes from oceanic organisms, both animal and plant. The coccoliths are the most important plants, but also of major importance is a group of protists known as foraminifera. The latter are relatively large amoeba-like creatures with different types existing in both the bottom sediment and free floating in the plankton. The skeletons of both accumulate on the bottom, and in portions of the ocean they produce thick deposits on the ocean floor. As this calcium-rich layer is eventually subducted during plate tectonics, it is heated and combines with other minerals. A by-product of this reaction is the formation of more carbon dioxide, which enters the atmosphere through volcanic processes. The calcium ends up in other minerals, but the carbon circulates between organic and inorganic phases. The death, sinking, and ultimate burial of these two planktonic groups have a second geological effect. So high is the volume of living
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere flesh in foraminifera and, especially, coccoliths that they increase the rate of burial of organic carbon. This in turn affects atmospheric oxygen levels. If large volumes of organic (and thus reduced) carbon are quickly buried, it causes a rise in atmospheric oxygen. This may have been an important cause of the rise of oxygen in the Mesozoic. Both groups of carbonate-bearing plankton appeared during the Mesozoic. The coccolithophorids evolved in the Triassic but began to appear in geologically important concentrations during the Cretaceous and after. By late Cretaceous times they were so abundant that they produced a characteristic rock type: chalk. Similarly, the planktonic foraminifera appeared in great numbers in the Cretaceous. Foraminifera do not thrive in very-low-oxygen conditions, and the great rise in planktonic foram abundance in the Cretaceous must have been abetted by the rise in oxygen that characterized the latter two periods of the Mesozoic. THE END OF THE ERA The long dinosaur summer probably seemed like an endless summer to the mammals—or might have, if only they had brains large enough to be reflective with. But as most were the size of rats, the Mesozoic mammals probably spent most of their time figuring out how to dodge predators and still get the next meal. Unlike most summers, the Summer of Dinosaurs did not gradually wind down into a cooler autumn. Instead, it turned from summer to the depth and death of winter almost instantaneously. How long was the Chicxulub asteroid in an Earth-crossing orbit before it had its rendezvous with our planet, 65 million years ago? In Chapter 10 we will briefly take a new view of trends in the successors of the dinosaurs—mammals.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
Representative terms from entire chapter: