A New History of Life (40 page)

The air-sac system
is
better than the mammalian system. It has been estimated that a bird is 33 percent more efficient in extracting oxygen from air than a mammal at sea level. But at higher altitude this differential increases: a bird at five thousand feet in altitude may be 200 percent more efficient at extracting oxygen than a mammal. This gives the birds a huge advantage over mammals and reptiles living at altitude. And if such a system were present deep in the past, when oxygen even at sea level was lower than we find today at five thousand feet, surely such a design would have been advantageous, perhaps enormously so, to the group that had it in competing or preying on groups that did not.

We know that birds evolved from small bipedal dinosaurs that were of the same lineage as the earliest dinosaurs, a group called saurischians. The first bird skeletons come from the Jurassic (although there is now some controversy about just how “birdlike” the earliest species, such as the famous
Archaeopteryx
, really were, and we will return to this). But the air sacs attached to bird lungs are soft tissue, and would fossilize only under the most unusual circumstances of preservation. Thus we do not have direct evidence for when the air-sac system came about. But we do have indirect evidence, enough to have stimulated the air-sac-in-dinosaurs group to posit that all saurischian dinosaurs had the same air-sac system, as do modern birds. And like
birds they were also warm-blooded. The evidence comes from holes in bones, places where these air sacs may have rested.

All credit for the first to make the audacious suggestion that dinosaurs had a bird launch system goes to Robert Bakker. It had been known since the late 1800s that some dinosaur bones had curious hollows in them, just as bird bones do. For decades this discovery was either forgotten or attributed to an adaptation for lightening the massive bones, for many of these bones with holes, later called pneumatic bones, came from the largest land animals of all time, the giant sauropods of the Jurassic and Cretaceous. The pneumatic bones were found mainly in vertebrae. Birds have similar pneumatic vertebrae, and while it can be said that some of the bird bones were light to enhance flying, it was also clear that some of the air sacs attached to bird lungs rested in hollows in bones. Thus, in birds, bone pneumaticity was an adaptation for stashing away the otherwise space-taking air sacs. The bodies of animals are filled with necessary organs, and putting the air sacs in hollowed-out bones make a lot of evolutionary sense. But Bakker made the leap and suggested that the pneumatic bones in his beloved fossil sauropods had evolved for a similar purpose, and were direct evidence that sauropods had and used the air-sac system.

Bakker’s larger purpose was to try to add further evidence that dinosaurs were warm-blooded rather than make any claim about this being an adaptation to low oxygen. Birds, with their enormous energy and oxygen demands related to flying, were thought to have evolved the air-sac system as a way of satisfying the metabolic demands of their endothermy.

Following Bakker, other dinosaur workers took up the call, and the specific case of air sacs being present in sauropods was made by paleontologist and dinosaur specialist Matt Wedel in 2003, while similar arguments for bipedal species were made at about the same time by dinosaur specialist Greg Paul. In 2002 he suggested that the first of the so-called archosaurs, the primitive late Permian through early Triassic reptilian group (that we have called archosauromorphs), which would eventually give rise to crocodiles, dinosaurs, and birds,
had air sacs. Examples of this group, which included the quadruped form Proterosuchus (described above as one of the earliest Triassic archosaurs), would have had a reptilian septate lung. Inspiration may have been aided by a primitive abdominal pump-diaphragm system (more primitive, perhaps, than this system as found today in modern crocodiles). What was unknown at the time, however, was that the crocodiles and their ilk back then had a far better respiratory system than was then agreed upon, thanks to their innovation of making air move one way through the respiratory system.

This discovery did not take place until 2010, and certainly affects our view of the relative evolutionary fitness of crocodiles, dinosaurs, and mammals. In fact, all of the Triassic reptiles seemed to have been better “breathers” than we mammals.

Successively, however, the evolution of the air-sac system may have fairly rapidly progressed in the lineage leading to dinosaurs at least. Alas (for them, anyway), the crocodiles ceased major innovation to their respiratory system with the newly evolved flow-through anatomy; they never experimented with pneumaticity in bones, and air sacs.

By the time the first true dinosaur is seen in the Middle Triassic, there may have been part of the air-sac system in place. The most primitive theropods from this time (the first dinosaurs) do not show bone pneumatization; the lung itself may have become inflexible and relatively smaller, both characteristics of extant bird lungs. With the Jurassic forms such as
Allosaurus
, the air-sac system may have been essentially complete but still much different from the bird system, modified as it has been for flying (for even the modern-day flightless birds came from fliers in the deep past) with large thoracic and abdominal air sacs.

By the time
Archaeopteryx
had evolved in the middle part of the Jurassic, there may have been a great diversity of respiratory types among the dinosaurs, some with pneumatized bones and some without. There also may have been a great deal of convergent evolution going on. For instance, the extensive pneumatization in the large sauropods studied with such care by Wedel may have arisen somewhat independently from the system found in the bipedal saurischians.

A final note about air sacs: While universal in saurischian dinosaurs, there is still no evidence of air sacs in the other giant dinosaur group, the ornithischian dinosaurs, including the well-known duckbills, iguanodons, and horned ceratopsian dinosaurs—not coincidentally (for three groups) all from the Cretaceous, not the Jurassic. The lack of an air-sac system in this group meshes well with their distribution in time. During the Jurassic times of very low oxygen they were minor elements of the fauna. It was not until the great oxygen rise of the Late Jurassic through the Cretaceous that this second great group of dinosaurs became common.

Perhaps the earliest dinosaurs were something like lions: sleeping twenty hours a day to conserve energy as dictated by the low oxygen, but when hunting, doing so actively, more actively than any of their competitors, which would have included the nondinosaur archosaurs (such as the early crocodiles), the cynodonts, and the first true mammals. All they needed to be was better than the rest. All evidence suggests that they were.

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, there are, in fact, many kinds of organisms that can generate heat in their bodies without external heat sources. These include large flying insects, some fish, large snakes, and large lizards. 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.

Dinosaurs were not alone on the Jurassic stage, for our own ancestors were present, at very small size, as were other land animals and sea animals, including turtles on land and in the sea, as well as long-necked plesiosaurs and crocodiles. But the dinosaurs were certainly dominant on land. While at first it seems that there were many, many kinds of dinosaur body shapes, in fact there were really but three. All three shared a common characteristic with birds and mammals: a fully upright posture. The three kinds of dinosaurs were bipeds, short-necked quadrupeds, and long-necked quadrupeds. Each
had a different time of origin and time of maximum abundance. Five distinct and successive assemblages of dinosaurian “morphotypes” (body plans) seem apparent to us. They are as follows:

  1.   
Late Triassic
. The earliest dinosaurs appeared in the last third of the Triassic but remained for their first 15 million years at low diversity. 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 is small, from one to three meters, and the earliest ornithischians (such as
Pisanosaurus
) were meter-long bipeds that had a new jaw system specialized for slicing plants. In the latest Triassic the first substantial radiation of dinosaurs occurs. It takes place among saurischians, with the evolution of both more and larger bipedal carnivores, and the first gigantism among early sauropods (such as
Plateosaurus
of the Late Triassic).

  2.   
Early to mid Jurassic
. Saurischian bipeds and long-necked quadrupeds dominated faunas. During this time, however, the ornithischians, while remaining small in size and few in number, diversify into the major stocks that will ultimately dominate dinosaur diversity in the Cretaceous. These stocks include the appearance of heavily armored forms (such as the thyreophorans). These are quadrupeds, and include the first stegosaurs of the middle part of the Jurassic. A second group is the unarmored neornischians (which include ornithopods—hypsilophodontids, iguanodons, and duckbills—and marginocephalians—the ceratopsians, which do not appear until the Cretaceous—and bone-headed pachycephalans). But it is the sauropods that are 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.
    The bipedal saurischians also showed diversity and success in the early and middle Jurassic. In latest Triassic time they had 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 in two, the two groups being 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 the evolution of the stock that gave rise to birds.

  3.   
Late Jurassic
. This was the time of the giants. The largest sauropods come from late Jurassic rocks, and their dominance continues 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. And this was not only among the saurischians. During the late Jurassic the armored ornithischians also increased in size, most notably among the heavily armored stegosaurs. The diversification of ornithischians at this time with the appearance of stegosaurs, ankylosaurs, nodosaurs, camptosaurs, and hypsilophontids radically changes the complexion of the dinosaur assemblages.

  4.   
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 become
increasingly rare as many sauropod genera go extinct at the end of the Jurassic.

  5.   
Late Cretaceous time
. 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.

No evolutionary history can ever be pinned on one factor. Dinosaur morphology changed from predator-prey interactions, competition among themselves and others of their world, perhaps even climate change driven in large part by the incredible rises and falls of sea level during the Jurassic and Cretaceous—at one point a sea level rise so large that North America became two separate minicontinents, separated by a large if shallow north-south-running sea. Nevertheless, oxygen levels must have played a part.

The time of the first dinosaur grouping, the late Triassic assemblage, was a time of low oxygen levels, and this coupled with very high carbon dioxide levels—not asteroid impact—was the major cause of the Triassic-Jurassic mass extinction. The combination of low oxygen and high global temperatures was the killing mechanism. Yet studies of the number of land vertebrate taxa before and after the T-J mass extinction clearly show that saurischian dinosaurs survived this mass extinction event better than any group of vertebrates, and one important reason may have been because of their superior respiration system, because of their air-sac lungs, which gave them competitive superiority over other terrestrial animals with different lungs.

Ornithischian dinosaurs, on the other hand, did not possess as effective a respiratory system as did saurischians. However, they were competitively superior to herbivorous saurischians with regard to food acquisition, larger heads, stronger jaws, and better teeth. 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 the Jurassic to Cretaceous interval marked a relatively rapid and significant rise in atmospheric oxygen, other events were taking place. One of these was the breakup of the once-global continent of Pangaea into smaller continents. Another, and perhaps more significant for the distribution and taxonomic makeup of the later Mesozoic dinosaur faunas, was the radical change in flora. Dinosaurs evolved in a gymnosperm-dominated world—with conifers, but ferns, cycads, and ginkgos as well. But in the early part of the Cretaceous a new kind of plant appeared, a flowering plant.