A New History of Life

by Peter Ward

  Some 200 million years ago, only 50 million after the great paroxysm of the Permian extinction, the Triassic period came to an end in another bloodletting. As we saw in the last 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 diversified 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 known, and not the topic of the sort of revisionist history that is the goal of this book. Instead, let us look at the record of oxygen during the Jurassic, and compare that to the numbers and kinds of dinosaurs in the ancient Jurassic Park.

  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 the 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. This controversy is rekindled every several years by some new finding that brings it to the surface once again. Thus its preeminence, even superseding the question as to whether or not the dinosaurs were warm-blooded. Way down on the list of questions about dinosaurs is the inverse of the dinosaur extinction question, not why did they die out, but why did they evolve in the first place? We certainly know when they first appeared in the second third of the Triassic period (some 235 million years ago), and we certainly know what these earliest dinosaurs looked like: most were like smaller versions of the later and iconic T. rex and Allosaurus. Bipedal forms that quickly became large. What has not been largely known or even considered among those who do know about it 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.

  Why were there 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 that was highly successful for the Earth during its Triassic period. 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 dinosaurs 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 goes on to suggest 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: “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 as well as other land vertebrates remained at a relatively low diversity, a finding that Sereno and others continue to find perplexing. But now this question can be answered. The history of animal life on Earth repeatedly showed a correlation between atmospheric oxygen and animal diversity as well as body size: times of low oxygen saw, on average, lower diversity and smaller body sizes than times with higher oxygen. It appears that these same relationships held for dinosaurs. The 2006 work by Ward, Out of Thin Air, was the first source explicitly relating dinosaur body plan and later giantism: the oxygen levels.

  If dinosaur diversity was indeed dependent on atmospheric oxygen levels, the extremely low atmospheric oxygen content of the Late Triassic readily explains the long period of low dinosaur diversity after their first appearance in the Triassic.

  Low-oxygen times killed off species (while at the same time stimulating experimentation with new body plans to deal with the bad times). Support for this thesis can found by comparing the latest atmospheric oxygen estimates for the Triassic through the Cretaceous with the most complete compilation of dinosaur diversity through the same sampling period. The latter, published in 2005 by paleontologist and sedimentologist David Fastovsky and colleagues, showed that dinosaur genera stayed roughly constant from the time of the first dinosaurs in the latter half of the Triassic and the first half of the Jurassic. It is not until the Late Jurassic that dinosaur numbers started to rise significantly, and this trend then continued right up to the end of the Cretaceous, with the only (and slight) pause in this rise coming in the early part of the late Cretaceous. By the end of the Cretaceous (in the Campanian age, 84 to 72 million years ago) there are hundreds of times more dinosaurs than during the Triassic to late Jurassic. So what was the cause of this great increase? The relationship suggests that oxygen levels played a role in dictating dinosaur diversity. Through the late Triassic and first half of Jurassic, dinosaur numbers were both stable and low, as was atmospheric oxygen when compared to today’s values. Gradually, oxygen rose in the Jurassic, hitting 15 to 20 percent in the latter part of the period. It is only then that the numbers of dinosaurs really began to increase. Oxygen levels steadily climbed through the Cretaceous, and so too did dinosaur numbers, with a great rise in dinosaur numbers found in the late Cretaceous, the true dinosaur heyday. The dramatic rise in oxygen at the end of the Jurassic was also the time that the sizes of dinosaurs increased, culminating in the largest-known dinosaurs appearing from the late Jurassic through the Cretaceous.

  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 period the flowering plants had largely displaced the conifers that had been the Jurassic dominants. The rise of angiosperms created more plants and sparked an insect diversification. More resources were available in all ecosystems, and this may have been a trigger for diversity as well. Yet the relationship between oxygen and diversity and between oxygen and body size has played out over and over in many different groups of animals, from insects to fish to reptiles to mammals. Why not in dinosaurs as well?

  Dinosaurs evolved during or immediately before the late Triassic oxygen low (between 10 and 12 percent, equivalent to an altitude of fifteen thousand feet today), a time when oxygen was at its lowest value of the last 500 million years. We have already seen that many other animals changed body plans in response to extremes of oxygen, and so too with the dinosaurs. The dinosaur body plan is radically different from earlier reptilian body plans, and appeared 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. The initial dinosaur body plan (evolved by the first saurischian dinosaurs such as Staurikosaurus and the somewhat younger, dinosaur-looking beast named Herrerasaurus) was in some part in response to the low-oxygen conditions of the time, and from that it can be concluded that 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 re
spiratory limitations imposed by Carrier’s constraint. The Triassic oxygen low thus triggered the origin of dinosaurs through formation of this new body plan.

  It is totally misunderstood what it means to inhabit a world where there is only 10 percent oxygen at sea level, as it would have been for the last 20 million years of the Triassic period. That is the level of oxygen found atop Mount Rainier, in Washington State. It is the amount of air atop the highest Hawaiian volcanoes, where the giant Keck Observatory peers out into space, a place where astronomers learn very quickly about the loss of energy and mental acuity that comes from such low oxygen levels. The principle of uniformitarianism fails us in this, because the use of high altitude to better understand low oxygen levels fails on so many levels, because at altitude, it is not just the oxygen that is lower in concentration, but all gases. One of these gases is water vapor, and this has a real effect on bird eggs at altitude. There had to have been major adaptations to such low oxygen, however, as the most important constraint on land animal evolution at this time. And indeed there were. One such we have named “dinosaur,” for the first dinosaurs, all bipedal with new kind of lungs and respiration, became the most efficient land animals of all time in low-oxygen settings. Those that have survived, the creatures we call birds, maintain this excellence.

  The fossil record shows that the earliest true dinosaurs were bipedal and came from more primitive bipedal archosauromorphs slightly earlier in the Triassic. These archosauromorphs were the ancestors of the lineage giving rise to the crocodiles as well, and may have been either warm-blooded or heading in that way. We see bipedalism as 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?

  Even modern-day lizards cannot breathe while they run (and they have had hundreds of millions of years to potentially get this right). This is due to their sprawling gait. Modern-day mammals show a distinct rhythm by synchronizing breath taking 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 slightly bows downward and then straightens out with running, and this slight up-and-down bowing is coordinated with air inspiration and 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.

  If a creature runs 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 running from predators—even in the amount of time looking for food and how food is looked for—would have surely 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. Predators that actively seek out prey require high speed and endurance. 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?

  In the Triassic, the crocodile lineage and the dinosaur lineage shared a quadrupedal common ancestor. This beast may have been a reptile from South Africa named Euparkeria. This group is technically called the Ornithodira, and from its earliest members this group began to evolve toward bipedalism. This is shown by their anklebones, 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, 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 be the very first of its kind; a still-terrestrial form looks like a fast runner that perhaps began gliding between long steps using arms with skin flaps. The oldest undoubted flying pterosaur was Eudimorphodon, also of the Late Triassic.

  While these ornithodires edge toward flight, their terrestrial sister group headed toward the first dinosaur morphology. The Triassic Lagosuchus was a transitional form, being between a bipedal runner and a quadruped. It probably moved slowly on all fours, but reared up on its hind legs for bursts of speed—the bursts necessary to bring down prey, for this was a predator. But it still had forelimbs and hands that had not yet attained the dinosaur type of morphology, so it is not classified as a dinosaur. Its successor, the Triassic Herrerasaurus, meets all the requirements and is classified as a dinosaur—the first. But, as we shall see below, it may have lacked one attribute that its immediate descendants would rectify: a new kind of respiratory system that could handle the still-lowering oxygen content of Earth’s atmosphere.

  This first dinosaur was fully bipedal,1 and it could grasp objects with its hands, having a thumb like we do. This five-fingered hand was distinct from the functionally three-toed foot (there were five actual toes, but two were so vestigial that only three toes touched the ground while running or walking). Because it was not totally bipedal, evolution no longer had to worry about maintaining a hand that had to touch the ground for locomotion. So with a free appendage no longer necessary for locomotion, what to do with it? The much later and more famous T. rex reduced the size of the forearm to the point that some have suggested it was nonfunctional. Not so for these first dinosaurs, however. While their posture was that of the later carnivorous dinosaurs so familiar to us, their hands were obviously used—probably for catching and holding prey while on the run.

  So this is the body plan of the first dinosaurs, from which all the rest evolved; bipedal, elongated neck, grasping hands with a functional thumb, and a large and distinctive pelvis for the massive muscles and necessary large surface area for these muscles used in walking and running. These early bipeds were relatively small, and before the end of the Triassic they again split into two groups, which remained the most fundamental split of the entire dinosaur clan. A species of these bipedal, Triassic dinosaurs modified its hipbones to incorporate a back-turned pubis, compared to the forward-facing pubis of the first dinosaurs. As any schoolboy knows, this change in pelvic structure marks the division of the dinosaurs into the two great divisions: the ancestral saurischians, and their derived descendants, with whom they would share the world for about the next 170 million years: the ornithischians.

  Of interest here, of course, is how dinosaurs breathed.2 It has been discovered that their respiratory system was quite different from the cold-blooded reptiles we find today, but very similar to that of the warm-blooded birds. The lungs of modern-day amniotes (reptiles, birds, and mammals) are of two basic types (although we will see that there are more than two respiratory systems, which include lungs, circulatory system, and blood pigment type). Both kinds of lungs can be reasonably derived from some single kind of Carboniferous reptilian ancestor that had simple saclike lungs. Extant mammals all have alveolar lungs, while extant turtles, lizards, birds, and crocodiles—all the rest—have septate lungs. Alveolar lungs consist of millions of highly vascularized, spherical sacs called alveoli. Air flows in and out of the sacs. It is therefore bidirectional.

  We mammals use this system, and our familiar breathing—in, out, in, out—is quite typical. Our air has to be pulled into these sacs, and then expelled again as oxygen switches place with carbon dioxide. We do this by a combination of rib cage expansion (powered by muscles, of course) and contraction of the large suite of muscles collectively called the diaphragm. Somewhat paradoxically, contraction of the diaphragm causes the volume of the lungs to increase. These two activities—the interacting rib expansion/diaphragm contract
ion—create a reduction in air pressure within the lung volume, and air flows in. Exhaling is partially accomplished by elastic rebound of the individual alveoli: when they inflate they enlarge, and soon after they naturally contract due to the elastic properties of their tissue. The many alveoli used in this kind of lung allow for a very efficient oxygen acquisition system, which we warm-blooded mammals very much need in order to maintain our active, movement-rich lifestyles. But the fact that air goes in and out of the same tube is very inefficient and reduces oxygen uptake relative to the amount of energy expended to get it.3

  In contrast to the mammalian lung, the septate lung found in reptiles and birds is like one giant alveolus. To break it into smaller pockets that increases surface area for respiratory exchange, a large number of blade-like sheets of tissue extend into the sac. These partitioning elements are the septa, which give these kinds of lungs their name. There are many variations on this basic lung design among the many different kinds of animals that use it. Some kinds of septate lungs are partitioned into small chambers; others have secondary sacs that rest outside the lung, but are connected to it by tubes. As in the alveolar lung, airflow is bidirectional in most—but, as recently discovered, not all, and the exceptions to this rule, recently found, have profoundly changed our understanding of not only the paleobiology of early reptiles but their fate during the Permian mass extinction.