4
DRIFTING CONTINENTS AND GLOBE-TROTTING DINOSAURS
DINOSAURS GOT AROUND. Their fossilized remains occur on every continent and from pole to pole, encompassing a tremendous variety of ancient environments: forests, savannahs, deserts, seashores, river floodplains, and mountain valleys, among others. How did these prehistoric landlubbers disperse across oceanic barriers to populate the globe? Did they make marathon swims (particularly challenging for tyrannosaurs, given their tiny arms), or climb aboard vast mats of floating vegetation? No, we now know that earlier dinosaurs spread over great distances simply by walking, whereas many of their descendants hitch-hiked aboard continental rafts. And it turns out that the processes underlying continental movements have influenced life in other profound ways—in particular, through effects on CLIMATE. This chapter explores a pivotal subplot of the dinosaurian saga: the impact of physical processes on life.
During the early portion of the Mesozoic, including the Late Triassic when dinosaurs appeared, all continents were united in a supercontinent called PANGAEA. From the Jurassic onward, the Mesozoic world witnessed the fragmentation of this monster continent into successively smaller landmasses. Beginning in the mid-Jurassic, Pangaea split into northern and southern blocks. A massive tear in Earth’s continental crust began in the east and proceeded slowly westward, as if the world were being unzipped. The widening rupture filled steadily with water, giving birth to a new ocean, the TETHYS SEA. The now-distinct yet still-gargantuan supercontinents, known as LAURASIAin the north and GONDWANAin the south, continued to fragment until, by the close of the Mesozoic, Earth looked very similar to the present-day Earth. Observed in a speeded-up, computer-simulated sequence, the breakup of Pangaea looks like a giant pane of glass shattering, as if smashed by an enormous asteroid. It turns out, however, that the mechanism behind this continental fragmentation was not extraterrestrial but “intraterrestrial.”
FIGURE 4.1
The sequence of breakup of the supercontinent Pangaea during the Mesozoic, beginning with Pangaea during the Triassic (bottom) and concluding with isolated landmasses in the Cretaceous (top) that closely resemble the arrangement present today. The abbreviation
Prior to the twentieth century, several people, including such luminaries as Francis Bacon and Benjamin Franklin, recognized that the east coast of South America and the west coast of Africa appear complementary, like two connecting pieces of a (very large) puzzle. However, it was an interdisciplinary scientist by the name of Alfred Wegener who, in 1912, first formally proposed the hypothesis of mobile continents, or continental drift, an idea that today we know as plate tectonics. PLATETECTONICS refers to the concept that Earth’s surface consists of large blocks, or “plates,” that move across the planet’s surface. Wegener’s proposed mechanism to account for continental migrations invoked a pair of primary processes. First were tidal effects, essentially the gravitational influence of the moon on Earth. Second was the so-called pole-flight force—a supposed tendency for continents to move away from the poles as a result of centrifugal forces generated by Earth’s rotation. Most investigators at the time (and since) regarded these forces to be far too weak to mobilize landmasses. In addition, for most of the twentieth century, Earth’s crust was generally believed to be too brittle to allow for such continental fluidity.
However, Wegener’s ideas failed initially not only because he lacked a convincing mechanism. In addition, the weight of accumulated evidence was simply inadequate to sway opinions from the traditional view of stable, fixed continents.1 Along with the puzzle-like fits of certain continental margins, Wegener noted several other patterns that appeared to support his radical idea, including the cross-oceanic occurrences of certain kinds of fossil organisms and nearly identical rock sequences along the east coast of South America and the west coast of Africa. Although these same lines of evidence would later be cited as key data in support of the mobile-continent hypothesis, more evidence of patterns, as well as a more convincing mechanism, was needed to trigger a major paradigm shift within the earth sciences. It wasn’t until the late 1960s that technologies advanced to the point that the necessary data could be collected and placed within a theoretical framework.
One key discovery was MIDOCEAN RIDGES and the structure of the rocks that surround them. Like the seam running around a baseball, ridges of basalt traverse Earth’s oceans at a depth of about 2,500 meters (8,000 feet). These gargantuan seafloor swellings can be wider than the state of Texas. Were it not for all the water above, midocean ridges would be visible from the moon. The lavas erupting from these ridges form vast expanses of basalts, youngest at the crests and increasing in age with distance from the ridge. This pattern suggests that the seafloor has actually spread outward from the midocean ridges, and that is exactly what has happened. New OCEANIC CRUST forms along the ridges as the molten MANTLE wells up from below. Over time, the older, displaced crust is pushed farther and farther from its starting point, a phenomenon known as SEAFLOOR SPREADING.
An equivalent process occurs when molten hot spots occur below continents. In this case, however, it is the CONTINENTAL CRUST, less dense than oceanic crust, that wells upward and cracks, creating an elongate rift valley. The Great Rift Valley in Africa, famous for its diverse wildlife and fossilized remains of our hominid cousins, is an example of this process in action. In the future, tectonic forces will literally rend the African continent in two, creating a new ocean in the process. A series of such hot spots may have caused the initial breakup of Pangaea.
The recognition of spreading ridges allowed geologists to determine that Earth’s crust is divided into approximately a dozen major plates that ride atop a semimolten layer beneath. This 1,000-kilometer- (over 600-mile-) thick outer layer of Earth, known as the LITHOSPHERE, is composed of a mixture of continental and oceanic crust plus a portion of the underlying mantle. Whereas virtually all other natural systems on Earth are driven by solar energy, plate tectonics feeds on energy from within the planet. The heat is generated deep within the core by radioactive decay of three elements—potassium, uranium, and thorium—as well as by residual heat from the planet’s formation. The presence of heat at the core sets up a GRADIENT with the cooler surface of the planet, producing heat flow via a process known as CONVECTION.
We are most familiar with convection through boiling water, which creates cells that transport hotter water to the surface and cooler water back down toward the heat source. You might wonder how convection could occur in solid rock. It turns out that the kinds of rocks that occur in the mantle lose their rigidity at very high temperatures and pressures. The heated, supple rock expands, becoming less dense and rising slowly through the mantle until it contacts the lithosphere. Arriving at the crust, the material travels along the crust’s surface as it cools, becoming increasingly dense until it finally begins to sink downward toward the core and begins the cycle anew. Think of a planet-sized lava lamp. Just as in the boiling water example, the result of this circulating matter is convection cells that enable heat to dissipate away from an energy source.
We can think of plate tectonics, the movement of crustal blocks at the surface, as merely a side effect of energy dispersal. As the upwelling heat reaches the planet’s surface, it drives oceanic crust away from the midocean ridges. When relatively heavy oceanic crust eventually collides with lighter, less dense continental crust, the former is shunted back down into the mantle in a process known as SUBDUCTION. Subduction actually assists in driving plate motion, providing a pulling force that augments the pushing force from the spreading centers. Operating along continental margins, often in association with deep oceanic trenches, this “subduction factory” is an effective recycler, converting raw materials such as oceanic crust and sediments into magma and continental crust. Earthquakes, volcanism, and mountain building are geologic offspring of this process. In all, lithospheric plates engage in three general types of behavior that mark their boundaries: they diverge at midocean ridges, they converge in trenches or subduction zones, and they slide past each other along faults. Thus, for example, in western North America, a variety of phenomena—from Mount St. Helens and other volcanoes to mountain ranges like the Rockies and seismic shakers along the San Andreas Fault— result from the collision of two major plates: the Pacific Plate to the west and the North American Plate to the east.
Moving at a blistering pace of about 4 centimeters (1.6 inches) per year, roughly equivalent to the growth of human fingernails, plate tectonics might seem rather pon-derous and ineffectual. But summed over geologic time, this relentless process gobbles up oceanic crust at a gluttonous rate of about 40 kilometers (25 miles) per million years. One complete rock cycle—from extrusion at a midocean ridge to subduction back into Earth’s interior and finally back up to a midocean ridge—takes about 100 million years. As a result, oceanic crust is continually renewed and recycled; the oldest-known crust of this type dates to about 200 million years ago. By contrast, the lighter, more buoyant continental crust is recycled much more slowly and therefore tends to be much older, up to 4 billion years in a few instances. Remarkably, it appears that Pangaea-like supercontinents have formed multiple times in Earth history, apparently cycling on a time scale of about 500 million years. Viewed from a deep time perspective, Earth is not a static body but a dynamic sphere undergoing constant upheaval, both from within and without.
FIGURE 4.2
Simplified diagram showing the inner structure of Earth, as well as the presence of convection currents, or “cells,” in the mantle, and the movement of oceanic and continental crustal plates (lithosphere) at the surface. See text for discussion.
General acceptance of plate tectonics in the 1960s caused a major paradigm shift within the earth sciences, one that fundamentally altered our understanding of dinosaurs and other extinct groups. After all, life could not avoid being caught up in this glacially paced dance of continental cleaving and collision. So it became clear that dinosaurs were a global phenomenon because they originated long before the breakup of Pangaea, allowing ancestral forms to spread over the entire supercontinent. As fragmented continental rafts charted their various courses, the dinosaurian passengers and other fauna and flora on board unwittingly went along for the ride. Once two blocks became separated, the plants and animals on each embarked on separate evolutionary journeys, spawning new forms along the way. Occasionally, two blocks collided or sea levels dropped, establishing land bridges and allowing floral and faunal exchanges between once-separate landmasses. Given this dynamic backdrop, understanding the pattern and timing of continental movements becomes essential to reconstructing the changing world of dinosaurs.
You might predict that the earliest dinosaurs would have been more cosmopolitan— that is, widely distributed—because they would have been able to move throughout much of the supercontinent. In contrast, you might guess that the latest dinosaur faunas would have been more localized or endemic, because many of the landmasses we recognize today had become isolated by the end of the Mesozoic. To a great extent, the record of land fossils supports these predictions, with Triassic and Jurassic forms exhibiting broader geographic distributions (COSMOPOLITANISM) and their Cretaceous descendants restricted to single, smaller landmasses (ENDEMISM). For example, certain varieties of Jurassic sauropods like Brachiosaurus (or at least very close relatives) are known from both the Northern and Southern hemispheres, including Africa, Europe, and North America. Conversely, the Late Cretaceous horned dinosaurs, or ceratopsians, are known only from Asia and North America, with the biggest forms (Triceratops and its ceratopsid kin) so far restricted to North America.
Some important exceptions, however, contradict this trend toward increasing localization, or endemism, in some instances highlighting persistent connections between continents and in others the reconnection of once-separated landmasses. For example, during the Cretaceous, North America appears to have shared brief land connections with Europe and Asia. Much of the supporting evidence comes not from plate tectonics but from paleontology, including studies of dinosaurs. Dinosaur faunas from the latter part of the Early Cretaceous in North America and Europe share a number of faunal similarities, including titanosaur sauropods, iguanodont ornithopods, and strange ankylosaurs. Then, in the Late Cretaceous, an intermittent corridor connected Asia and North America. A variety of closely related dinosaurs inhabited these two landmasses, including theropods—big-bodied tyrannosaurs and various smaller maniraptors—and plant-munching ornithischians—ceratopsians, pachycephalosaurs, hadrosaurs, and ankylosaurs.
Another unexpected example of Late Cretaceous cosmopolitanism—that is, the occurrence of closely related organisms on different continents—relates to fragmentation of the southern supercontinent Gondwana. In this case, the scientific debate has been contentious, leading to the development of rival hypotheses. Later I discuss this example in some detail because it highlights not only the process of science but also the potential for simultaneous illumination of two scientific fields: paleontology and geology/geophysics. It also happens to be a debate in which I have actively participated.
Eduard Suess, a nineteenth-century geologist and contemporary of Darwin, noted that a certain type of fossil fern, Glossopteris, was present in South America, India, and Africa. Suess thought that this pattern pointed to ancient land connections. Based on the fossil evidence, he coined the name “Gondwanaland” for a supposed supercontinent that had at one time included these three landmasses. The name derives from a region called Gondwana in central India, where Suess did some of his key geologic and paleontologic work. With Wegener’s ideas still decades away, Suess did not entertain the notion of mobile continents (at least as far as we know) but, rather, thought that oceans had flooded the vast regions separating these landmasses. Despite the misunderstood mechanism, and the fact that Australia, Antarctica, and Madagascar were also part of the southern landmass, the shortened form of the name, Gondwana, has stuck.
According to most plate tectonic models, the major continental blocks we recognize today had separated from one another by the Early Cretaceous (about 120 million years ago). South America and Africa initially broke away as a unit from the rest of Gondwana, which included landmasses that would eventually separate into Antarctica, Australia, Madagascar, and the Indian subcontinent (including present-day India, Pakistan, Bangladesh, Nepal, and Bhutan). The Indian subcontinent and Madagascar later fragmented as a single block that persisted for several million years into the Late Cretaceous. Finally, Antarctica and Australia went their separate ways, and the Indian subcontinent split off from Madagascar to head northward, ultimately ramming head-long into Asia, causing the rise of the highest and most dramatic mountain range on Earth—the Himalayas. This scenario is based largely on geophysical observations yet has remained poorly tested with fossils.
Some new evidence challenges this traditional Gondwanan breakup scenario. Here again, Madagascar plays a starring role. Madagascar’s isolation in the Indian Ocean dates to about 88 million years ago. Given such a persistent oceanic gulf, it’s not surprising that approximately 80 percent of the current floral and faunal inhabitants of the island are unknown elsewhere. The generation of highly endemic faunas requires two essential ingredients: isolation and time. Madagascar has had plenty of both.
In chapter 1, I recounted the discovery of Majungasaurusskull bones on Madagascar. Even before we had finished unearthing the fossils of this ancient predator, we knew that it shared several specialized bony features with a beast named Carnotaurus, found far away in Argentina. Majungasaurus and Carnotaurus belong to a group of theropods known as abelisaurids, which have the same characteristic wrinkled texture on many of the skull bones. Their closest relatives are smaller-bodied, Velociraptor-like predators called noasaurids, including Knopfler’s vicious lizard, Masiakasaurus knopfleri. Noasaurids and abelisaurids belong to a single evolutionary clan that we call abelisauroids or, more simply, ABELISAURS. To date, abelisaur remains have been found predominantly on Gondwanan landmasses: South America, Africa, Madagascar, and the Indian subcontinent. The only exceptions are a few strays unearthed in southern Europe, which may have shared connections with Gondwana during the Cretaceous. The striking similarity between Majungasaurus and Carnotaurus suggests that they were the evolutionary equivalent of first or second cousins, with a relatively recent common ancestor, perhaps even in the Late Cretaceous.
FIGURE 4.3
Diagrams depicting rival hypotheses of the breakup of the southern supercontinent Gondwana during the Cretaceous. Both hypotheses are shown in a sequence of three images, beginning in the Early Cretaceous (120 million years ago; bottom) and concluding in the Late Cretaceous (80 million years ago; top). Abbreviations: A, Antarctica; Af, Africa; Au, Australia; I, Indian subcontinent; Ma, millions of years ago; SA, South America. The island of Madagascar is the solid black region off the southeast coast of Africa. Note hypothesized differences in land bridges in the two scenarios. See text for discussion.
The abelisaur evidence—together with parallel patterns of kinship in other groups such as titanosaur sauropods, frogs, crocodiles, and mammals—indicated to our group that current knowledge of Gondwanan fragmentation was, well, fragmentary. The closest relatives of these extinct species are not found in nearby Africa, as one might predict, but in India and halfway around the world in Argentina. Based on this evidence, our group postulated that many chunks of Gondwana were linked much longer than previously thought. We argued that the most likely corridor for dispersal would have been through Antarctica, which may have retained connections with “Indo-Madagascar” and Argentina long after Africa became isolated from the rest of Gondwana.
At first glance, a migration route through Antarctica might seem far-fetched, conjuring images of dinosaurs trudging like oversized emperor penguins across snow-covered terrain. But Late Cretaceous Earth was a hothouse world. No polar ice caps existed during most of the Mesozoic, and temperatures even at high latitudes rarely dropped below freezing. In fact, the fossil record reveals a broad diversity of Mesozoic plants and animals living in polar regions.
So what does the combined fossil and geologic data tell us about the history of Gondwana? Let’s consider the range of scenarios that is consistent with the observed patterns. The presence of abelisaurs and other groups on multiple southern land-masses raises the possibility that their ancestors occurred throughout Gondwana prior to its fragmentation. According to this PAN-GONDWANA HYPOTHESIS, the presence of these animals on southern landmasses involved three events. First, abelisaur theropods (and other groups of organisms) evolved and spread throughout much of Gondwana by the Early Cretaceous, say, 140 million years ago. Second, Gondwana fragmented into smaller landmasses, stranding the life-forms on each. Third, the Mesozoic castaways on the daughter landmasses embarked on independent evolutionary trajectories, ultimately resulting, for example, in divergent groups of abelisaurs in South America, Madagascar, and the Indian subcontinent (and perhaps Africa) at the close of the Mesozoic.
The central problem with this hypothesis is timing. According to traditional reconstructions, most of the key landmasses were separate from one another by about 120 million years ago, fully 50 million years before Majungasaurus stalked titanosaurs on the island of Madagascar. This scenario is rendered suspect by the close evolutionary relationship between such forms as Majungasaurus and Carnotaurus. That is, the evidence suggests that the Malagasy and South American vertebrates present in the latest Cretaceous must have shared a much more recent common ancestor than permitted by generally accepted models of Mesozoic geography. To further complicate matters, we currently have no evidence of abelisaur theropods much older than about 100 million years ago.
Unbeknownst to us, while we were rethinking Gondwanan breakup based on fossil evidence, a group of geophysicists led by William Hay at the University of Colorado independently reached similar conclusions largely from geologic data. Their results indicated that Pangaea had fragmented into three continental blocks by the Early Cretaceous—the first consisted of North America and Eurasia, the second of Africa, and the third comprised South America, Antarctica, Australia, Madagascar, and the Indian subcontinent. They, too, postulated that South America and Antarctica retained terrestrial connections with Madagascar and the Indian subcontinent until sometime in the Late Cretaceous, about 80 million years ago. This lingering connection is thought to have been maintained in part by an isthmus-like land bridge between Indo-Madagascar and Antarctica.
FIGURE 4.4
Branching diagram (cladogram) showing skulls and possible evolutionary relationships of four abelisaur theropods from different Southern Hemisphere landmasses. See text for discussion.
Paleontologist Paul Sereno of the University of Chicago has attempted to salvage the traditional scenario with a modified pan-Gondwana hypothesis. Sereno has led several highly successful expeditions to northern Africa in search of Cretaceous dinosaurs. His discoveries include several abelisaur specimens, including Rugopsprimus, or “first wrinkle-face,” which comes from rocks dated to about 95 million years ago. This makes Rugops the oldest and most primitive known abelisaurid, more distantly related to Carnotaurus and Majungasaurus than the latter two are to each other. In Sereno’s view, brief connections between key southern landmasses were present about 95 million years ago because of three narrow, intermittent land bridges: one between Africa and South America, another between South America and Antarctica, and a third between Antarctica and Indo-Madagascar. Sereno argues that these connections enabled abelisaurs and other Cretaceous beasts to spread through much of Gondwana many millions of years later than previously supposed.
The alternative hypothesis put forward by our group, and supported by the paleogeo-graphic evidence of William Hay and his colleagues, postulates that, following the isolation of Africa, the rest of Gondwana remained connected until the Late Cretaceous, perhaps as recently as 80 million years ago. Connections could have been maintained by a pair of land bridges, one extending from Antarctica to Indo-Madagascar and the other from Antarctica to South America. These lingering links would have created a corridor for plants and animals to move between the eastern and western portions of Gondwana exclusive of Africa. Because this model suggests that Africa was the first landmass to separate from the Gondwanan supercontinent, it has been dubbed the A FRICA-FIRST HYPOTHESIS. Thus, according to us “Africa-firsters,” whereas abelisaurs spread over much of Gondwana prior to its fragmentation, specialized members of the group, such as Carnotaurus and Majungasaurus, evolved after the isolation of Africa, when ancestors of the latter theropods moved between South America and Indo-Madagascar via Antarctica.
The jury is still out on the pan-Gondwana and Africa-first hypotheses, largely because of uneven sampling of fossils in space and time. Africa has yielded plenty of fossils from the middle of the Cretaceous (about 110–90 million years ago), but it has remained virtually mute as to the animals present at the end of the Cretaceous (about 80–65 million years ago). Conversely, India, Madagascar, and South America have generated a bounty of terrestrial fossils from the latest Cretaceous, but, with a few Argentine exceptions, almost nothing is known of their middle Cretaceous faunas. More detailed studies of the evolutionary relationships of key groups may help distinguish between these two hypotheses by showing how the faunas from each of the landmasses are related to one another. In addition, we need more geophysical data to test, for example, whether postulated land bridges actually existed during the periods proposed by each hypothesis. The Gondwana story is a fine illustration of serendipity in science. We never expected to recover fossils in Madagascar that would bring into question accepted notions of super-continent break-up.
By now, it should be evident that the fragmentation, movement, and collision of Earth’s crustal plates had dramatic effects on dinosaurs and other organisms. By continually changing the size, position, and connections of landmasses and oceans, plate tectonic processes provided an ever-shifting stage for life’s drama, resulting in profound and irreversible consequences. Yet assessing the size and location of the stage through time is only the beginning. Equally critical in this drama is the climatic backdrop, which, it so happens, is also crucially intertwined with plate tectonics.
Thus far, this chapter has addressed one aspect of Earth’s physical processes—the lithosphere or, more generally, the GEOSPHERE. Two other physical elements, or “spheres,” deserve attention: air (ATMOSPHERE) and water (HYDROSPHERE). The systems of the geosphere, atmosphere, and hydrosphere are intimately interconnected and interwoven with a fourth partner, the biosphere. Together these interacting systems generate the complex world we see around us, as they did during the Mesozoic. Whereas the activities of the geosphere (and in particular the lithosphere) are driven by energy from deep within Earth, the remaining spheres feed off the sun.
Let’s begin with the atmosphere, that astonishingly thin, life-supporting veil of mixed gases that envelops our planet. Compared to the 13,000-kilometer (8,000-mile) diameter of Earth, the atmosphere is positively diaphanous, thinning to nothingness only 100 kilometers (62 miles) above the planet’s surface. If Earth were the size of a basket-ball, the atmosphere would be as thick as an enveloping layer of plastic wrap. Yet this veil of air performs several critical roles, mostly without our notice. The atmosphere circulates ingredients essential for life, such as the oxygen we breathe. Somewhat paradoxically, air is also a critical circulator of water, the universal medium for life on this planet. Through its protective layer of ozone, the atmosphere shields the planet from cosmic radiation. And most important, the atmosphere enabled life to take hold and flourish, occupying virtually every nook and cranny at or near the planet’s surface.
Life is sustained by the lowest level of the atmosphere, the troposphere, which extends an average of about 11 kilometers (7 miles) above the surface. The wonderful array of cloud forms attests that the air is continually stirred and churned, in large part by convection. Just as in the lithosphere, atmospheric convection depends on heating from below, which causes air to rise, cool, and mix. However, the driving energy comes not from Earth’s interior but from the sun, which generates temperature differences across land, sea, and air. Physical features such as mountain ranges and large bodies of water accentuate these differences. Life also plays a key role. In particular, the metabolic activities of trees release vast amounts of moisture, cooling the surrounding air. The single-greatest temperature difference at Earth’s surface, however, is between the equatorial and polar regions, the result of the planet’s orientation with regard to the sun.
Faced with such a complex abundance of temperature differences, nature does its level best to reduce them and achieve some sort of steady state or equilibrium. The resulting pattern of flow profoundly affects climate and local weather patterns. It also drives the wind, which generates mixing in both the oceans and the atmosphere. If circulation of the atmosphere resulted entirely from solar heating, energy flow would be relatively simple, involving gigantic convection cells extending from the equator to the poles. Heated air would rise at the equator, flow to the poles, cool and sink, and ultimately flow back along the surface toward the equator. Think of that lava lamp again. Winds in this system would flow only from the equator to the polar regions.
But Earth’s rotation complicates atmospheric flow. Spinning like a top, the planet deflects winds and divides air into six latitudinal zones, three per hemisphere. That is, between the equator and poles, the atmosphere flows in three separate convection loops or cells: tropical, temperate, and polar. Warm equatorial air, known as trade winds, is separated from the cooler air of temperate regions by faster winds, known as jet streams, generally flowing from west to east. This phenomenon is familiar to anyone who has taken a coast-to-coast flight in North America; an airplane trip from New York to San Francisco lasts more than five hours, whereas tailwinds shorten the return journey eastward by about an hour. Although atmospheric circulation must have varied somewhat during the Mesozoic because of changes in continental positions, we can be confident that the same general patterns and processes applied because Earth has maintained its orientation and direction of spin for over 4 billion years.
In contrast, patterns of water circulation within oceans have been anything but constant. Whereas air moves freely around the atmosphere, landmasses impede the flow of seawater in the hydrosphere; large-scale movements of water, then, depend on the sizes, locations, and interconnections of continents. Nevertheless, many of the same principles and patterns of atmospheric flow also apply to oceans.
Earth is a water planet. Today, about 75 percent of its surface is covered with water, and this figure has been higher many times during the past. So the hydrosphere has a tremendous influence on Earth systems, including those on land. Oceanic circulation is driven by the movement of heat from warmer to cooler areas. The less dense, heated water warmed by the equatorial sun rises to the ocean’s surface. It then migrates naturally toward the poles in an attempt to disperse its heat energy. As it progressively nears the poles, the water slowly dumps its heat into the air, warming the surrounding environment. The water, now cold and dense, sinks to a great depth and heads back toward the equator like a liquid conveyer belt completing its circuit.
One such cell incorporates the Gulf Stream, a vast river within the Atlantic Ocean that carries warm water from the Gulf of Mexico northward to Europe, where it has a major moderating effect on coastal climates. The city of London is close to the same latitude as Hudson’s Bay in Canada, but the Gulf Stream provides London and the United Kingdom with a much milder climate. Thanks to this warming influence, there are locations in northwestern Scotland where palm trees can grow. If the Gulf Stream conveyer belt were to cease circulating, the north Atlantic would stop receiving an influx of warm southern water, and, within a few years, air temperatures in northern Europe would likely drop by several degrees. (There is some evidence that global warming has already caused a slowing of the Gulf Stream, something that has Europeans understandably concerned.)
Because continental positions and connections differed during the Mesozoic, we can be confident that the oceanic circulation cells during the Age of Dinosaurs were significantly different from those active today. To give the most obvious example, the pole-to-equator circulation of ocean currents would have been very different during the time of Pangaea, when there was one giant “superocean” referred to as Panthalassa. More recently, Antarctica was attached to South America until about 50 million years ago, long after the major extinction of the dinosaurs. This configuration of continents impeded circulation of circumpolar currents, undoubtedly with major effects on global climates. The initial breakup of Pangaea resulted in formation of the Tethys Sea, but well-developed circulation cells for ocean currents in the Atlantic likely were not possible until the Late Cretaceous. So it appears that oceanic circulation shifted throughout the Mesozoic in response to changing continental configurations. These fundamental shifts in ocean flow would have had profound effects on climate and therefore on the terrestrial world of dinosaurs.
Just two parts hydrogen and one part oxygen, water has remarkable heat-dispersing properties. It requires ten times as much heat to raise the temperature of water 1°C as it does to do the same for iron, so water is an excellent heat sink. Because fluids are mobile, heat spreads out and is stored more easily than in solids. Nature tends to distribute heat energy as evenly as possible. When heated, the surface temperature of solids tends to increase quickly, often becoming hot to the touch; yet just below the surface the substance remains cool because solids are typically poor conductors. In contrast, the tendency for fluids to disperse heat and reduce temperature differences means that heat distribution is more uniform in water. In general, an entire body of water must be heated for it to feel warm. These basic properties of heat flow explain why the beach is often hot to walk on during the summer, whereas the ocean only a few steps away feels cool or even frigid. The key point is that continents and oceans have radically different responses to heat, with dramatic consequences for weather and climate. Because the expansive oceans are much slower to heat up and to cool down than landmasses, their temperature-stabilizing effects tend to mitigate the rapid thermal shifts characteristic of continents. This phenomenon applies particularly to coastal regions. Landlocked Wichita, Kansas, near the center of North America, experiences extreme seasonal swings in temperature, with the average daily temperatures in summer and winter varying by about 20°C (50°F). Conversely, coastal San Diego, California, enjoys minor fluctuations of about 7°C (15°F) in average temperatures.
Like everything else in the world of dinosaurs, we can’t observe Mesozoic climates directly. So we employ a variety of techniques to assess climates indirectly. Fortunately, climates modify sediments, leaving a variety of clues for paleosleuths. For example, we’re confident that Canada and the northern United States experienced an ice age in the not-too-distant past because we can see where glaciers scarred mountains, where vast lakes once filled continental basins, and where ancient soils preserve chemical signs of water saturation in areas that are dry today. The fossils tell the same story; remains of Pleistocene plants and animals (e.g., woolly mammoths) indicate adaptations to cold weather. Similar types of clues can be found for the Mesozoic, revealing how climates back then differed from those of today.
Let’s now consider the breakup of Pangaea from a hydrosphere perspective. Rather than focusing on landmasses, contemplate for a moment the changing sizes and positions of oceans. We’ll start in the Late Triassic, about 225 million years ago, near the time of the first dinosaurs. With all landmasses united in Pangaea, temperatures over land fluctuated between much wider temperature extremes than today. Lacking the moderating effects of oceans between continental chunks, Pangaea underwent rapid and severe temperature swings. In coastal regions, this pronounced seasonality included hot and dry periods followed by extremely wet, often monsoonal conditions. Temperatures may have oscillated more than at any other time in the last 500 million years. During the Northern Hemisphere summer, the northern part of Pangaea was sweltering hot, while the southern portion stayed cool. Moisture from the oceans, particularly the growing Tethys Sea, was pulled into gigantic low-pressure cells, producing extensive monsoon rains in the coastal regions. This pattern reversed during the Southern Hemisphere summer.
Jumping forward in time to the Late Jurassic, about 150 million years ago, we find that Pangaea has fragmented into smaller landmasses surrounded by oceans. The close proximity of large bodies of water moderated the vast land temperature swings of earlier times, an effect amplified by rising global sea levels. Relatively warm conditions initiated in the Triassic continued, so there was no sign of polar ice or glaciers. Global sea levels were much elevated relative to the present day, in part because there was no water tied up in massive ice sheets. The greatly increased volumes of seawater led to flooding of several continents, including North America, Europe, and North Africa. Shallow seas submerged low portions of these landmasses. High sea levels and inland waterways had a strong stabilizing influence on temperatures and decreased seasonality, so the Late Jurassic saw warm, equable climates.
Moving on to the Late Cretaceous, about 75 million years ago, we find strong evidence of exceptionally warm and mild climates, even at high latitudes. Equatorial temperatures were likely close to modern values at the equator, whereas temperatures at the poles were much balmier than those of today. Polar temperatures during the Cretaceous have been estimated at 0°–15°C (32°–59°F), with a temperature difference between the equator and the poles of about 17°–26°C (63°–79°F), as compared with 41°C (105°F) today. The major moderating effect during this interval was exceptionally high sea levels; just as in the preceding Jurassic, many continents were flooded. Europe became an island archipelago. In North America, the Western Interior Seaway flooded the central plains for about 30 million years, separating the continent into eastern and western landmasses. Had humans been present, Late Cretaceous real estate agents would have had a grand time selling spectacular, subtropical beachfront homes in Utah and Arizona (though they might have been hesitant to disclose the need for electrified fences to keep out the tyrannosaurs). One investigator aptly described the Earth of this time as “wall-to-wall Jamaica,” in stark contrast to today’s Midwest weather of smothering summers and snow-draped winters.
Clearly, then, the varying sizes of continents and oceans through time have had a tremendous effect on global climates. Yet climate at any given moment is a complex phenomenon, involving interactions among Earth (geosphere), air (atmosphere), water (hydrosphere), and life (biosphere). In particular, tectonic activity influences Earth systems in ways that go far beyond effects on landmasses. To give one pertinent example, global sea levels are closely tied to plate tectonic activity. During intervals of active tectonism—that is, periods of rapid seafloor spreading—the spreading centers at midocean ridges become elevated, pushed upward by the pressure of molten rock beneath. This elevation of the seafloor, in turn, shrinks the absolute sizes of ocean basins, causing sea levels to rise. In extreme instances, rising sea levels exceed the limits of marine basins, causing great volumes of water to spill out onto the continents as inland seas.
Periods of active seafloor spreading such as the Late Cretaceous are also associated with have an increased rate of subduction of oceanic crust along plate margins. More subduction results in additional volcanism, as oceanic crust is thrust down into the mantle for recycling. Volcanic eruptions dump vast quantities of carbon dioxide into the air, raising atmospheric levels of this greenhouse gas and reducing the amount of solar radiation that can bounce back into space from Earth’s surface. This captured radiation results in global warming, a phenomenon we are battling today for an entirely different reason (the release of greenhouse gases from human activities such as the burning of fossil fuels). During the Mesozoic, global warming melted the polar ice caps entirely, causing dramatic sea-level rises.2 In short, more tectonism means higher rates of seafloor spreading, which in turn translates into raised spreading ridges, smaller oceanic basins, higher sea levels, higher subduction rates, increased volcanic pumping of carbon dioxide into the atmosphere, and global warming. Talk about interconnections!
Under the influence of seafloor spreading, landmasses have fragmented, dispersed, and amalgamated through deep time, with profound effects on global climate change and the diversity of life. The dinosaurian odyssey closely overlapped the breakup of the supercontinent Pangaea, a fact reflected in the global distributions and evolutionary history of the group. We have seen that the remains of dinosaurs and other extinct organisms can even be used as independent tests of hypotheses of continental breakup and collision. We have also begun to explore Earth’s four major “spheres.” For eons, circulation within Earth, the oceans, and the atmosphere has created complex systems driven by the movement of heat. Because the primary sources of energy—radioactive decay in the center of Earth and the sun—have never been exhausted, the gradients have persisted, allowing the dynamic systems of the lithosphere, atmosphere, hydrosphere, and biosphere to persist as well. Importantly, these systems do not work in isolation but in unison. Here, then, are the beginnings of an integrated view of life with consequences for how we regard not only the world of dinosaurs but our own place in nature as well.
While one Tyrannosaurusclaims a Triceratops victim, others chase the remaining members of the horned dinosaur group across a clearing within a forest dominated by flowering plants.