9. Sam's Heritage

Seismosaurus and the other spectacular dinosaurs of the Jurassic Period were the product of millions of years of evolution. Their reign came in the middle of the Age of Dinosaurs, which began in the Triassic 95 million years before Sam's time and lasted for another 85 million years, when dinosaurs finally became extinct at the end of the Cretaceous. During Sam's life, lasting perhaps a hundred years from hatching to that day 150 million years ago when Sam died of predation, choking, or some other cause, tectonic forces were barely perceptible in the middle of the North American continent. But the motion of the continents during the waning millennia of the Jurassic profoundly affected the world of the dinosaurs, nearly bringing the giant sauropods like Sam to extinction. Those events are almost lost to history, their traces scarcely recognizable and now remote from the desert of New Mexico.

In a broader sense, we now understand that the earth was restless and unstable during the Mesozoic. North America may have been relatively calm when Sam lived, but elsewhere radical changes were happening. Jurassic geography was profoundly different from the layout of continents and oceans today. Nor were the tectonic events at the end of the Jurassic the only time when dinosaurs and other life were affected by the continual restructuring of the earth's crust. Indeed, tectonic activity may be the ultimate driving force behind all evolutionary change, beginning even with the origin of life.

Dinosaur ancestors arose in the remote past of the Paleozoic Era, when North America and South America were joined with Europe, Africa, and Asia in the giant supercontinent of Pangaea. Antarctica, Australia, and India were then still connected to Pangaea, too, with only minor hints that in the Mesozoic they would become islands. Pangaea, containing all the continental land mass of the Paleozoic world, lay on one side of the globe and the World Ocean on the other in a lopsided configuration that held few indications of changes to come. The subsequent restructuring of Pangaea in the Mesozoic Era was to drive a corresponding restructuring of life on earth, changing forever the nature of ecosystems on land and in the sea. In their own way, dinosaurs played a major role in the terrestrial ecosystem-in-flux.

The changes in life on land during the late Paleozoic and the Mesozoic do not read like a genealogy, one species following another in unbroken succession. That is what we should expect if our record of life on earth were complete, but alas, the book of life has many missing pages. Instead, the changes read from the fossil record are more like the evolution of music or culture: each generation builds on the successes and failures of past generations, recreating the heritage in its own way. Some of the changes seem to flow naturally, but occasionally the radical addition of a novelty wracks the seemingly complete and harmonious order.

Sam's heritage began long before the Jurassic Period, indeed, long before dinosaurs appeared. The origins of all dinosaurs and their ecosystems lay not in the Mesozoic Era, but more remotely in the waning stages of the Paleozoic Era, when much of Pangaea was dominated by reptiles whose ecosystems were complex and diverse. Reptiles prospered in the late Paleozoic, in the Carboniferous and Permian periods, and the ecosystems seem to have gradually become more complex through time. Changes in the faunas of the late Paleozoic led to improved success at dry-land existence, allowing reptiles to occupy deserts and uplands more successfully than before. These changes were important but not profound, for the animals and plants of the late Paleozoic had not been tested by major tectonic events like those that would follow in the Mesozoic.

Dinosaurs had not yet arisen, but their ancestors resided somewhere in a great profusion of reptilian species that dominated the closing epochs of the Paleozoic Era. The sail-back reptiles like Dimetrodon typify this time, but smaller and less remarkable animals dominated the terrestrial habitats. These were the mammal-like reptiles, such as Cynognathus, all living in a complicated food web that was surprisingly modern despite its antiquity (roughly 250 million years ago). Plant eaters were as large as pigs and cows, and highly specialized. Predatory reptiles were the primary and secondary consumers, feeding on the flesh of other reptiles. This complex ecological setting of the terrestrial vertebrates established an array of specialized niches in the Permian. The Permian vertebrate faunas of North America, Central Asia, and South Africa were nearly all reptilian, prospering in habitats that were relatively hostile to amphibians.

Even when all the land mass was locked into the supercontinent Pangaea, faunas found today in the fossil record of North America, Central Asia, and South Africa must have lived in regions separated by great distances. Yet they were remarkably similar. Tectonic forces had not yet created major barriers (like mountains and seaways) to dispersal. Permian reptiles flourished on land with unparalleled success. Their evolutionary history showed no sign of the pending disaster.

At the end of the Permian, and marking the end of the entire Paleozoic era, a disaster of untold proportions shook the biosphere. More than 90 percent of all species on earth went extinct. The niches occupied by the Permian reptiles were wracked by the sudden and catastrophic extinction that came closer to obliterating all life on earth than any other event since life became abundant in the Precambrian. The most profound change ever experienced by vertebrate animals was this calamitous extinction at the end of the Permian.

This Permo-Triassic extinction episode came without warning, a cataclysm that cleared many niches of their inhabitants but failed to destroy the potential of the biosphere to rebuild complexity. Abruptly the habitats became vacant on land and in the sea. The extinction was so abrupt and profound that this biological event itself marks not just the boundary between two geologic periods (Permian and Triassic) but the boundary between two geological eras: the Paleozoic Era and the Mesozoic Era. Newly evolved life forms that followed the extinction were drastically different, but the recovery was not immediate. Millions of years passed after the Great Extinction before the complexity was reestablished.

Following the Great Extinction and throughout the Triassic Period, tectonic convulsions shook the supercontinent. A rift developed in the southern part of Pangaea that eventually opened to form first the South Atlantic Ocean. Then the North Atlantic opened, like the jerks of a ragged and uneven zipper. Mountain-building generated by friction of the mobile land masses created barriers to atmospheric circulation and mixing of animal populations. Australia, India, and Antarctica separated from Pangaea and became island continents, their animals isolated like passengers in a raft.

Roughly 225 million years ago in the middle of the Triassic, after 20 million years of biological experimentation following the Great Extinction, dinosaurs first appeared. Their emergence had little immediate effect. Their exact ancestry is still a mystery, buried with the myriad of reptiles of the early Triassic called archosaurs. The early dinosaurs were small predators--about the size of a chicken. They were largely bipedal, nimble, and fleet of foot. They were, in turn, prey for the crocodile-like phytosaurs, giant floating and basking carnivores that probably overwhelmed their victims by ambush. The phytosaurs had teeth and jaws that must have inspired great feats of escape by the diminutive dinosaurs foraging or hunting along the shore and even venturing occasionally into water. Other archaic reptiles also competed for newly redefined niches in ecosystems based on the photosynthetic capacities of ferns, tree ferns, cycads, and conifers. Some coniferous forests of the Triassic grew to incredible heights. The profusion of giant logs at the Petrified Forest National Park in Arizona is evidence of one such forest.

The ecological setting for these early dinosaurs scarcely resembled the habitats that would follow. The earliest Triassic dinosaurs could easily have become extinct, snuffing out the dinosaur potential at an early stage, for their hold was fragile and tenuous. Their undramatic entrance onto the Mesozoic stage had little immediate effect on Triassic animals and plants. They did not sweep away their reptilian competitors' niches and replace them wholesale; instead the dinosaurs gradually specialized and diversified until by the end of the Triassic, their distribution was worldwide and their position secure--though still not conspicuous.

One specialized group of late Triassic dinosaurs became the ancestors of the giant sauropods of the Jurassic. These were the prosauropods, an aptly named variety of large plant-eating precursors to Apatosaurus, Diplodocus, Brachiosaurus, and their relatives including Seismosaurus. The prosauropods were giants of the Triassic and early Jurassic, with heavy builds and a general loss of bipedal locomotion, which returned them to the quadrupedal habits of their reptilian ancestors. Their relatively long necks and legs permitted them to feed higher in trees than the other herbivores, and they may have been able to balance on the hind legs to assume bipedal stance when feeding. The prosauropods were widespread and survived into the early part of the Jurassic, but their descendants, the sauropods, overshadowed them as the world of dinosaurs expanded and diversified. The prosauropod populations dwindled and eventually became extinct in the early Jurassic, their place in history more dramatic for their progeny than their contributions to their own contemporary landscape. From these gradual and modest beginnings, with roots in the prosauropods of the Triassic and early Jurassic, the sauropods emerged amid a bewildering array of other dinosaurs and reptiles, many gigantic.

At the end of the Triassic, huge geophysical changes once again occurred. The world of the dinosaurs went topsy-turvy in a reorganization that shook the terrestrial landscape to its foundations. Changes in the arrangement of the continents and oceans radically altered atmospheric circulation, the mixing of populations between regions or land masses and between the newly defined oceans and seas. These tectonic events disrupted and probably compressed the habitat gradients on the continents that had been stable for nearly a hundred million years.

As continents rattled and shook at the end of the Triassic, prominent chains of mountains took shape along the seams that once linked the Americas with Europe and Africa, while newly expanding oceans increased the isolation of the separate land masses. Seaways opened and closed in low latitudes, sometimes forming great expanses of oceanic waters. For example, the Tethys Sea persisted as a giant gulf that separated Europe from Africa, and North America from South America as the Atlantic Ocean continued to expand. Vestiges of the Tethys Sea today include the Caribbean and Mediterranean seas. Some of the Triassic land masses became isolated, their stranded populations of plants and animals confined to limited geographic areas. Such isolation could have promoted local episodes of speciation and variation among the land-dwelling inhabitants of the Triassic and early Jurassic. It might have prompted the beginnings of the sauropods out of prosauropod stock.

The sauropod dinosaurs probably originated from this period of intensified evolution, and they soon diversified to become the largest animals ever to live on land. They were present in Asia in the early Jurassic, but by the middle of Jurassic time the sauropods had become diverse and widespread, although not yet cosmopolitan. Their landscape in the middle and late Jurassic was radically different, more with respect to the animals with which they lived than the plants. The giant phytosaurs were gone, replaced by the earliest and equally deadly crocodiles. These aquatic predators seem to have secured their dominance in the shallow water habitats so well that dinosaurs never seriously challenged their occupation of the predatory niche of shorelines and open water. Flying reptiles (the pterosaurs) and the earliest birds fluttered or glided overhead, safe from marauding predators on the ground, successfully traversing open landscapes well beyond the reach of the meat-eaters.

The new niches were redefined and expanded. Dinosaurs capitalized on the additional complexity now possible with their single most important novelty: their size. Stegosaurs, iguanodonts, and ankylosaurs ambled along beside the sauropods, but their immensity (reaching several tons as adults) paled beside the adult sauropods of the Jurassic Period. Nevertheless, these herbivores exacted heavy tolls on their habitats, for their food requirements were immense. These were the usual prey for the giant carnivores of the Jurassic, typified by the awesome predator of the late Jurassic, Allosaurus. These giant meat-eaters in turn menaced at least baby and juvenile sauropods, if not the adults. And surely, just as in the ungulate herds of Africa today, sick and aged individuals from all populations of dinosaurs in the Mesozoic, including the sauropods, fell prey to these top carnivores.

The history of the sauropods, however, is perplexing. It seems out of synchrony with the history of other groups of dinosaurs. With dinosaur populations well established on all other continents by the middle of the Jurassic, the lack of sauropods in North America until near the close of the period almost defies explanation. But, from what we know of the fossil record, that was indeed the situation: sauropods were absent in North America until near the end of the Jurassic. When they appeared, however, the increase in their numbers and their ensuing diversification came astonishingly fast. Sauropod fossils appear first in North America at the bottom of the Morrison Formation. Everything we know about Jurassic sauropods in North America, the quintessential dinosaurs of the Mesozoic, comes from the Morrison Formation, the same body of rocks that yielded Seismosaurus.

Sauropods thus seem to have entered North America later than anywhere else. And despite their abundance in the Morrison Formation, only the broadest outline of their evolution can be discerned. All of the Jurassic species of sauropods were exceptionally large as adults, an order of magnitude larger than any other contemporary dinosaur. This difference marks an abrupt change in the ecological setting, for now the dominant plant-eaters far outstripped the largest predators in size. Relations between predators and prey surely became reorganized, but the nature of those changes is elusive. At least four families of sauropods entered North America from Eurasia by way of now lost land bridges in the late Jurassic: the Cetiosauridae (for example, Haplocanthosaurus), the Brachiosauridae (Brachiosaurus), the Camarasauridae (Camarasaurus), and the Diplodocidae (for example, Diplodocus, Apatosaurus, and Seismosaurus).

Six individuals in a herd of Seismosaurus.

In North America the brachiosaurs (Brachiosaurus and Ultrasaurus) foundered, never reaching the abundance this family achieved in other parts of the globe. Brachiosaur skeletons occur only rarely in the Morrison Formation, but members of this family occur in large accumulations elsewhere, such as the Tendaguru beds of Tanzania in eastern Africa. Likewise, the cetiosaurs (Haplocanthosaurus) never became abundant in North America, where the more advanced sauropods seem to have eclipsed their possibility for success.

Camarasaurus, the most generalized sauropod, is the most abundant of the Morrison dinosaurs. More than any other dinosaur, Camarasaurus typifies the dinosaurs of the late Jurassic in North America. Its effect here was probably more extensive than that of any other dinosaur, for its populations probably outnumbered all other sauropods combined. This genus had several species, all closely related, and all built on the same basic body plan--heavy body, stout legs, relatively short tail and neck, and rather heavy skull reminiscent of a bulldog's head. Although successful in the late Jurassic of North America, the family did not proliferate into many genera and species in the fashion of the Diplodocidae.

The diplodocids, on the other hand, were diverse. Bones of these giants are common in the Morrison Formation, reflecting their abundance and diversity. Within this family belong the spectacular giants (and supergiants): Diplodocus, Apatosaurus, Barosaurus, Supersaurus, and Seismosaurus. They were the most highly specialized of the sauropods in North America, perhaps the most highly specialized sauropods ever. These dinosaurs had exceptionally long necks and tails, small (horse-size) and delicate skulls, and remarkably complicated adaptations for locomotion in their pelvis, sacrum, and vertebrae.

The great diversification of sauropods in North America that produced the giants and supergiants of the late Jurassic is problematic. Until recently, most geologists and paleontologists painted a relatively uniform picture of the Jurassic ecosystem: lowland habitats, largely humid and lush with vegetation. However, this assumption has been radically challenged during the past decade. Defying the traditional notion that sauropods had abundant food supplies and never wandered far from water is a new portrait that puts them in a patchy habitat. In contrast to earlier ideas about the Morrison Formation, western North America seems to have been ecologically diverse, ranging from wet lowland or riparian habitats to utterly barren desert. Vast tracts of virtually plantless terrain were dissected by perennial streams where vegetation grew in abundance. These narrow strips or belts of forests provided the sustenance for the plant-eating dinosaurs, especially the sauropods. They probably fed almost constantly when food was available, but they might have been forced to trek across barren land to find new sources.

Immense lakes, some of them alkaline and poisonous, covered large parts of the modern geographic region called the Colorado Plateau. These lakes and the open deserts were barriers to dispersal and migration, restricting the mobility of the giant dinosaurs, and perhaps also promoting geographic isolation and subsequent genetic isolation conducive to speciation. Thus the diversity of the landscape in turn promoted the diversification of the Jurassic giants, including the sauropods.

The large carnivorous dinosaurs of the late Jurassic were efficient meat-eating predators and scavengers that followed herds of sauropods, waiting for opportune moments--just as lions do today with herds of zebra and other grazers in Africa. Young sauropods and the sick or old were vulnerable to predators, but adults were relatively safe from those annoying meat-eaters in the same way that adult elephants in India or Africa have little to fear from carnivores. The most abundant carnivore was Allosaurus, which in many respects was more capable of taking down prey than its more famous (and more recent) relative, Tyrannosaurus. Allosaurus was more lightly built than Tyrannosaurus rex  and was generally smaller, but some individuals may have approached the five-ton weight of a small Tyrannosaurus and reached lengths of more than fifty feet. Moreover, the teeth of Allosaurus were sharper, thinner, and better equipped for slicing flesh than were those of Tyrannosaurus. While Allosaurus and its contemporaries Ceratosaurus, Marshosaurus, and Stokesosaurus may not have been capable of taking large sauropods except in coordinated pack-hunting efforts (if indeed such social organization was possible), these menacing carnivores commanded the attention of all the giants of the late Jurassic.

Contemporary plant eaters of the late Jurassic were the plated dinosaur Stegosaurus, the small bipedal Camptosaurus, and the various nodosaurs and ankylosaurs--armored dinosaurs that foraged low to the ground and avoided the large predators probably through camouflage and the impressive protection afforded by their bony plates. These herbivores were so strikingly different from the sauropods that their habitats must have overlapped seldom, or never--as attested by the fact that they are almost never found in the same excavation sites as sauropods. The only likely times of competition would have been during searches for nesting ground, or when food was scarce, such as during a drought. Otherwise, these smaller (but nevertheless large by human standards) dinosaurs probably spent more time keeping out of harm's way in the moving landscape of legs and tails than they did in direct competition with the real giants of the time.

Pterosaurs flew above and may have interacted with the dinosaurs. These airborne reptiles were not yet large in the late Jurassic when Seismosaurus lived, but they may have been numerous in some places. However, their fossil remains are frustratingly rare in the Morrison Formation; whether that rarity reflects the lack of original abundance in the Jurassic of North America or represents a preservational bias is uncertain. Similarly, birds (Archaeopteryx) had originated by the late Jurassic, but none have been found with the Morrison dinosaurs. Their absence in the fossil record in North America probably reflects preservational bias. Only six specimens of Archaeopteryx have been found, all from Germany.

This paradise, the Jurassic zenith of the sauropods in which Sam lived, could not last forever. Changes in the forests of the late Jurassic, notably the advent of angiosperms (flowering plants), brought profound change to the world of the sauropods. And as the Atlantic Ocean expanded, South America, Antarctica, and Australia became more isolated, further restricting dispersal from one land mass to another.

For reasons still unexplained, the sauropods almost perished at the end of the Jurassic Period. Sauropod numbers and diversity diminished so dramatically that their existence for the next 80 million years during the Cretaceous Period was inconsequential. They were relicts from a lost world. Consider: in North America alone, thousands of whole or partial skeletons of sauropods have been documented in the Morrison Formation of the late Jurassic; fewer than a dozen or so have been documented from the entire Cretaceous of North America.

What caused this Sauropod Crisis at the end of the Jurassic? No one knows for sure. Indeed, few scientists have taken a serious interest in the dwindling of the sauropods, an event in their history that almost brought them to extinction. But there is one very plausible answer: the great sauropods may have been done in by flowers.

The first burst of color in the Mesozoic came with the advent of the angiosperms, the flowering plants. This event of the early Cretaceous was possibly the most important evolutionary invention since reptiles conquered the land with specially adapted eggs that could be laid out of water. This was, in essence, the sexual revolution of the Mesozoic, for the plants that employed flowers to propagate engaged other organisms in the reproductive stages of their life history.

Before the angiosperms, the ferns, cycads, and conifers had dominated the Mesozoic. Their reproductive structures produce a naked seed, with little stored nourishment, but in plenitude. Dispersal of the small seeds of conifers, like the seeds and spores of countless plants since invasion of dry land in the Paleozoic, was accomplished by little more than wind. But wind pollination and wind dispersal were no guarantee of reproductive success. Whatever the cause of the crisis on the Jurassic landscape, new plants with covered seeds, the angiosperms, emerged with increasing prominence. The key was reproductive success, probably promoted by the simultaneous diversification of pollinators, the advanced insects.

Plants with flowers could advertise free meals in the form of nectar or pollen, attracting pollinators that would incidentally improve the likelihood of fertilization of the developing eggs which lay deep within the inflorescence. Assured of successful pollination by the unwitting insects that carried pollen from one flower to another, the angiosperms invested heavily in the life history of the developing egg by adding copious supplies of stored energy, producing the covered seed ("angio sperm") that carried its own food supply sufficient to nourish germination and sustain the newly emerged plant for weeks.

The earliest angiosperms, some of which resembled the modern magnolias of the southeastern United States, collectively introduced a novelty into the plant world: the dependence on animals for fertilization, and to a lesser extent, for dispersal. This innovation, in turn, promoted increased variation in the genetic vigor of the plants, for now plants could have their eggs fertilized by pollen from members of the same species from (relatively) distant populations. Specialization followed, with the flowering plants diversifying in a dramatic expansion of new families and genera.

This adaptive strategy prompted one of the most enduring revolutions in the terrestrial ecosystem, supplanting the conifers and their associated faunas while opening a myriad of new adaptive niches for animals and plants alike. Insects proliferated, and in turn became the major food supply for the Cretaceous mammals, which similarly diversified. Eventually the insects became important as food for birds and fishes as well, promoting the respective adaptive radiations of these vertebrates in the Cenozoic Era. The novelty of flowering plants stimulated this wholesale reorganization of life on land in an ever-expanding story of competition and specialization.

Conifer forests suffered, becoming isolated and restricted to geographic regions far from the premium landscapes that the flowering plants so fully exploited. (Remember that in the Cretaceous, worldwide climate was likely considerably warmer and moister than that of today. The cool-temperate, subarctic, and dry regions that support conifers in abundance today would have been rare in the Cretaceous.) By the end of the Cretaceous, conifers had lost their dominance, and the flowering plants had overwhelmed the terrestrial ecosystems. The dinosaurs that were closely tied to the conifers dwindled, too, suffering dramatic losses in abundance and diversity, while other dinosaurs seem to have capitalized on the complexity of the Cretaceous forests. Whereas those dinosaurs expanded in diversity and abundance with the diversification of the flowering plants, the sauropods nearly perished. They were left out of the new wave of experimentation among the dinosaurs, instead hanging on in archaic habitats that became increasingly isolated. The sauropods were overtaken by more progressive dinosaurs of the Cretaceous Period, their time of dominance long since passed.

Footprints of Brontopodus birdi  from Nashville, Arkansas. These footprints of a sauropod from the early Cretaceous document that sauropods survived the end-Jurassic extinction of many kinds of sauropods. The tracks were made by a large adult the size of Apatosaurus. The genus Brontopodus was named on the basis of these footprints alone, but they were probably made by the dinosaur whose bones (discovered elsewhere) have been named Pleurocoelous.

Although the history of the sauropods cannot be directly and unequivocally associated with the demise of the conifers, the pattern is certainly persuasive: as the conifers dwindled, so did the sauropods--almost to extinction as early as the transition from the Jurassic to the Cretaceous. Although they survived in isolated populations around the world throughout the 80 million years of the Cretaceous Period, the sauropods were inconsequential and, except for their size, inconspicuous. They never truly recovered the losses they suffered at the Jurassic-Cretaceous transition. Their decline was so profound that they were absent from North America for at least half (40 million years) the Cretaceous Period.

The last sauropod genus in North America is Alamosaurus, an emigrant from South America whose remains are known only from Utah and New Mexico and only at the end of the Cretaceous Period, 65 million years ago. Several other sauropod genera managed to hold on till the end of the Cretaceous on other continents, notably South America and India, but these genera were from the same family (Titanosauridae) as Alamosaurus. The remains of these genera are nowhere abundant, and their populations were probably sparse.

The sauropods, thus, became extinct at the end of the Cretaceous 65 million years ago along with all the other dinosaurs and a myriad of other life forms--but they were on their way out long before. The end-Cretaceous extinction was not as severe or as profound as the Permo-Triassic event that set the stage for the dinosaurs. Their exit may have been dramatic, especially if they were ushered out by the impact of an extraterrestrial body. Or, it may have been relatively gradual, occurring over several million years. In either case, the demise of the sauropods at the end of the Cretaceous was the final whimper in a long and spectacular history.

In that long history, Seismosaurus may have been one of the (if not the) supreme examples of a dinosaur order that brought forth the biggest animals ever to walk on earth. The particular animal, Sam, which I was privileged to excavate, may or may not be the only example of this magnificent genus that science will ever come to know. But what we know of this individual tells us that Sam's kind was of spectacular dimensions.