The World of the Dinosaurs

The Middle and Late Jurassic Epochs

To understand the past requires imagination as much as it requires facts. One place where people learn about the past is school. The history of human civilization can be laid out on a timeline to show the events that shaped the development of humankind. An understanding of the peoples and cultures of the past requires imagination, but such an understanding is readily attainable because the people of the past were driven by much the same motivations—whether emotional, political, or survival—that drive the people of the present. The civilized people of the past were much like the people of today. The layout of the continents, the climate, and the topography of the Earth have varied little, except for human-drawn political borders, during the development of civilization. A person transported back in time 2,000 or 3,000 years would find a planet that, geographically at least, resembled the Earth of today in most ways.

To understand life before humans, especially the deep past of many millions of years ago, presents more of a challenge. During the days of the dinosaurs, the Earth was not the same as the Earth of today. A person flung back in time some 150 million years would not recognize the shapes of the continents or the plants, animals, and climate zones that held sway during that time. It was very much a foreign world compared to that which we know today.

Paleontologists grapple with geologic and fossil evidence to paint a picture of the world of the dinosaurs. The accuracy of this picture depends on the availability of geologic and fossil strata from spans of the Mesozoic Era. This chapter examines evidence

EVOLUTIONARY MILESTONES OF THE MESOZOIC ERA

Period

Epoch

Span (millions of years ago)

Duration (millions of years)

Organismal Milestones

Triassic

Early Triassic

251-245

6

Diversification and distribution of amniotes, particularly synapsid and diapsid reptiles

Middle Triassic

245-228

17

Euryapsid marine reptiles

Late Triassic

228-200

28

Early turtles, dinosaurs, crocodylomorphs, pterosaurs, and mammals

Mass extinction

Casualties: dicynodonts, carnivorous cynodonts, phytosaurs, placodonts, nothosaurs

Jurassic

Early Jurassic

200-175

25

Radiation of carnivorous and herbivorous dinosaurs, first crocodyliforms

Middle Jurassic

175-161

14

Rise of armored and plated dinosaurs, rise of sauropods

Late Jurassic

161-145

16

Diversification of sauropods, theropods, the first birds

Cretaceous

Early Cretaceous

145-100

45

Continued diversification of dinosaurs, birds, marine reptiles, and pterosaurs

Late Cretaceous

100-65.5

35

Rise of large theropods, horned dinosaurs, hadrosaurs

Mass extinction

Casualties: Dinosaurs, marine reptiles, pterosaurs

for the geologic and climatic conditions that influenced the evolution of the flora and fauna of the Middle and Late Jurassic Epochs.

SHAPING THE WORLD OF THE DINOSAURS

The Mesozoic Era stretched from 65.5 million to 251 million years ago and thus had a total span of 186 million years. The Mesozoic is divided into three periods of time, beginning with the Triassic Period, moving to the middle or Jurassic Period, and concluding with the Cretaceous. Each of these periods is subdivided into smaller units (early, middle and late, except the Cretaceous which does not have an official "middle"), called epochs. The Early and Middle Triassic Epochs witnessed dramatic changes to the ecology of the Earth and the extinction of older forms of vertebrates carried over from the Paleozoic Era. Among them were several prominent lines of carnivorous and herbivorous reptiles. From those reptilian roots rose the first dinosaurs in the Late Triassic, about 228 millions years ago. Dinosaurs became the story of the Mesozoic, dominating life for 162.5 million years until the dramatic extinction of the last of their kind 65.5 million years ago.

The Late Triassic and Early Jurassic Epochs, a span of 53 million years, were a time of evolutionary innovation for dinosaurs. It was during that time that the two fundamental lineages of dinosaurs—the Saurischia and the Ornithischia—first took shape. In those earliest of dinosaurs can be seen clues to the great clades of dinosaur descendants to follow. Small theropods such as Eorap--tor (Late Triassic, Argentina) and Coelophysis (Late Triassic, New Mexico) set the mold for the body plan of all theropods to follow. The early sauropod Vulcanodon (Early Jurassic, Zimbabwe), with its long neck and tail, small head, bulky body, and four-legged posture, presaged the giant, long-necked herbivores to follow. Even though the first ornithischians are very poorly known, examples such as Lesothosaurus (Early Jurassic, Lesotho) provide a preview of the sophisticated plant-eating jaws and teeth that became the hallmark of most of the ornithischians that followed. The evolution of these earliest dinosaurs took place at the dawn of the dinosaur age, at a time before the span covered in this book.

The next important stage of dinosaur evolution occurred during the Middle and Late Jurassic Epochs, the span that is the subject of Time of the Giants. The Jurassic Period marks the middle span of the Age of Dinosaurs.

Many gaps exist in the fossil record of dinosaurs. For example, little is known from the Middle and Late Jurassic of eastern North America. Fossils of Late Jurassic age are also scarce in South

America and Australia. What was happening to dinosaurs, plants, and other organisms from those time periods and places is a matter of speculation; paleontologists do their best to make educated guesses about the geologic and climatic conditions of those areas based on knowledge from other parts of the world.

Dinosaurs of the Middle Jurassic are poorly known except for two regions of the world, Europe—most significantly, England and Scotland and, to a lesser degree, France, Germany, and eastern Russia—and China, especially the province of Sichuan. Remains of Middle Jurassic dinosaurs from North America consist primarily of fossil footprints. Evidence of dinosaurs from this time period in the Southern Hemisphere (South America, Africa, Antarctica, and Australia) is very scrappy. The total of undisputed fossil sites for Middle Jurassic dinosaurs numbers only 69, 20 percent of which lack skeletal material and include only trace fossils such as footprints.

Dinosaurs of the Late Jurassic are much better known. They are found to some extent on all continents except Australia and Antarctica, and fossil riches from the Late Jurassic are found in abundance in North America, Europe, Asia, South America, and Africa. Unlike most Middle Jurassic fossil sites, with their spotty remains, some of the best known localities for Late Jurassic dinosaurs have yielded whole and partial skeletons of more than 35 individual species of dinosaurs, both large and small. Such tremendous stores of fossils and associated evidence for plants and climate conditions have allowed paleontologists to reconstruct a vivid picture of life during the Middle and Late Jurassic.

From the standpoint of continental shifts, the Mesozoic Era was notable for slow but dramatic changes to the sizes and shapes of the world's terrestrial and oceanic bodies. The Mesozoic was not without its geologic catastrophes; for the most part, however, the shifts in climates and landmasses and associated changes in the herbivorous food supply took place at a rate with which vertebrate evolution could keep pace. Dinosaurs and their archosaurian kin ruled for nearly 163 million years, until the end of the Cretaceous Period, when a combination of massive volcanic activity in Asia, climate changes, and asteroid hits shook the Earth and sent these creatures into oblivion.

The dominant geologic event of the Mesozoic was the breakup of the supercontinent Pangaea. All of the currently known continents were joined as one enormous landmass at the beginning of the Mesozoic Era. By the end of the Mesozoic, however, the terrestrial bodies of the world had separated into forms that approximate those seen today. Pangaea began to split apart in the Early Jurassic, first dividing into two landmasses. The northern landmass, which geologists call Laurasia, included areas that became North America, Europe, and Asia. The southern landmass, known as Gondwana, included the regions that would eventually become today's South America, Africa, India, Australia, and Antarctica.

Before dividing up, Pangaea was bounded on the west by the Panthalassic Ocean and on the east by the Tethys Ocean. The Atlantic Ocean began to appear in the middle of Pangaea as tectonic plates separated and continental landmasses radiated outward, moving to the north and south from the equator. Water displaced by the rise of massive mid-ocean ridges was pushed onto the lower elevations of terrestrial habitats. Continental areas that today are associated with North and South America, Europe, central Asia, and northern Africa were the sites of extensive inland seas. During the Cretaceous Period, North America had a shallow inland sea that stretched down the middle of the landmass from what is now Alaska to the Gulf of Mexico.

Geologically, the Jurassic was perhaps the quietest period during the Mesozoic. It was a time of limited mountain building and volcanic activity. Instead, there is much evidence worldwide that the Jurassic was a time during which many great highlands became eroded, thereby creating large areas of low-lying land. Despite the low profile of most continental interiors, during the Jurassic the surrounding oceans did not encroach on the land, with the

Pangaea during the Triassic Period

Pangaea during the Triassic Period exception of some continental margins. This stable period in the history of the continents encouraged the formation of freshwater lakes, rivers, and floodplains. One extraordinarily large freshwater lake formed in Australia. The lake covered nearly 300,000 square miles (777,000 square kilometers)—about the size of the state of Texas. Sediments left behind by that lake are an astounding 7,000 feet (2,134 meters) thick. Some parts of western North America were also subject to massive land deposits, especially on what is today the

Colorado Plateau, where some sedimentary deposits of sandstone reach thicknesses of hundreds of feet.

Further evidence in North America for erosional deposits is the widespread Morrison Formation along the western interior. The formation is an extensive area of alluvial deposits covering 750,000 square miles (1,942,000 square kilometers). Known as an abundant source of Late Jurassic dinosaur fossils, the Morrison Formation was a broad, sandy, fan-shaped, or alluvial, lowland formed at the juncture of smaller streams with ravines and larger streams.

Most of central Asia and Asia were above sea level during the Jurassic; the same is assumed for Africa, although only a few Jurassic--age deposits are found there. The gap now formed by the Atlantic Ocean between North America and Europe was much narrower during the Jurassic Period. The similarity of some fossils from the Gulf of Mexico region and Europe suggests that a temporary, sediment based land bridge may have once spanned the Atlantic gap as late as the Late Jurassic and Early Cretaceous. Likewise, Africa and South America were still linked by a solid land bridge through the Jurassic Period and into the earliest Cre--taceous. Proof of this comes in part from the similarity of dinosaur and other taxa found on both continents in rocks dating from those times.

CLIMATES AND HABITATS

The Middle and Late Jurassic Epochs were evenly warm and temperate across the globe; there is no evidence of polar ice caps at that time. Widespread warmth enveloped the Earth and allowed dinosaurs to roam to every habitable corner of the planet. Today, widespread coal deposits and fossil evidence of marine coral and reefs located 2,000 miles (3,218 kilometers) north of where they exist today all provide supporting evidence that the Jurassic world was warm. In lands to the north that now are extremely cold, such as Greenland and parts of northern Europe, there grew such mild--climate plants as ginkgo trees and certain conifers. None of these plants could have survived subfreezing temperatures on a regular basis.

The evidence for great sandy deposits in Australia and North America suggests that some parts of the planet were drier and more arid than others. Coal deposits, on the other hand, indicate a more humid habitat; such deposits are found in places as widespread as British Columbia and Vancouver Island—both in western Canada— and in Mexico.

An innovative study published in 2000 by paleontologists Peter McA. Rees, Alfred M. Ziegler, and Paul J. Valdes used the morphology (shape) of Late Jurassic fossil leaves to create a map of likely worldwide climate zones. Leaves are particularly excellent gauges of climate because they represent a direct means by which plants interact with the environment. The shape and biology of a leaf is adapted by evolution to optimize prevailing climatic conditions. Jurassic plants were particularly diverse and were distributed in zones across terrestrial ecosystems. The research team found that mid-latitude plants were the most diverse; flora in those latitudes included forests mixed with ferns, cycads, horsetails, seed ferns, and conifers. Patchy forests of small-leafed cycads and conifers were present at lower latitudes. Polar vegetation consisted mainly of large-leafed conifers and ginkgo trees. Based on their analysis of the climate-related characteristics of such plants, the team was able to identify several predominant climate zones across the planet. These included zones of tropical, desert, and warm temperate conditions over much of Earth's land area.

Analysis of the oxygen isotope content of Mesozoic marine fossils provides additional support for a temperate world climate. Samples taken from locations in North America, Europe, and Russia show that the temperature of shallow marine environments ranged from between 60°F and 75°F (15°C and 24°C), making for a relatively warm day at or near the beach most of the time.

A key reason for the moderating of global temperature during the Jurassic Period was the breakup of Pangaea. The division of the

Zdenek Burian Sponge

Jurassic landscape

Jurassic landscape giant landmass into smaller continents made all land more susceptible to temperature changes moderated by ocean currents. During the Mesozoic, the lack of ice caps meant that the Earth was covered by more water than during the latter stages of the Paleozoic. Because water is a tremendous sponge for solar radiation, the oceans became warm; ocean currents that now could flow around the equator, where the most intense and directly sunlight is received, collected and distributed heat to the north and south, providing a more evenly temperate world climate.

(continues on page 31)

THINK ABOUT IT

Mesozoic Plants and the Evolution of Dinosaur Herbivory

When two species of organisms influence each other's evolution, coevo-lution is taking place. Coevolution is a change, through natural selection, in the genetic makeup of one species in response to a genetic change in another. Some well-documented examples occur between birds and plants. In the lowland forests of Central and South America, for example, hermit hummingbirds have evolved a long, curved beak especially suited for feeding from the curved flowers of the Heliconia plant. Another example comes from the Galapagos Islands—the famous setting for many of Charles Darwin's formative observations about natural selection. Different species of Galapagos finches have adapted beak sizes suited for eating seeds of particular sizes.

There appears to be a coevolutionary relationship between herbivorous dinosaurs and the development of Mesozoic plants. It was probably not a coincidence that the first large, l ong-necked herbivorous dinosaurs—the "prosauropods"—appeared alongside conifer trees that were growing taller and taller. What is not fully understood is whether the conifers were growing taller to get out of reach of low-browsing "prosauropods" or whether "prosauropods" were standing upright to reach taller plants that were out of reach of shorter browsing or grazing animals. In either case, a symbiotic relationship between the height of plants and the size of dinosaurs appeared to have begun by the Late Triassic.

Several researchers have linked the jaw mechanisms and sizes of herbivorous dinosaurs with the kinds of plants that were prevalent in their habitats. Paleontologists David Fastovsky, Joshua B. Smith, and others have made a convincing case for associating various kinds of plant-eating dinosaurs with tiering—different levels at which herbivorous dinosaurs ate according to their height and the size of plants in the habitat. The accompanying table summarizes this tiering phenomenon for different groups of herbivores.

COEVOLUTION OF HERBIVOROUS DINOSAURS AND MESOZOIC PLANTS

Dinosaur

Plant-Eating

Taxon

Adaptation

Tier

Likely Diet

"Prosauropods"

Plucking teeth;

High browsing

Conifers

gut processing

(3 to 10 feet;

(evergreens);

with gastroliths

1 to 3 m)

cycads; ginkgo trees

Sauropods—

Cropping teeth

Low to intermediate

Conifers

Diplodocids

browsing (3 to 24 feet;

(evergreens);

1 to 7 m) and some

cycads; ginkgos

grazing (less than

3 feet; less than 1 m)

Sauropods—

Cropping teeth

Medium to very

Conifers

Camarasaurids,

high browsing

(evergreens);

Brachiosaurids,

(3 to 33 feet;

ginkgo trees

Titanosaurs,

1 to 10 m)

others

Ankylosaurs

Cropping beak;

Low browsing

Seed ferns; ferns;

puncture

(3 feet; 1 m)

club mosses;

chewing

cycads; horsetails

Stegosaurs

Cropping beak;

Low browsing

Seed ferns; ferns;

puncture

(3 feet; 1 m)

club mosses;

chewing

cycads; horsetails

Heterodonto-

Cropping beak;

Low browsing

Seed ferns; ferns;

saurids

puncture

(3 feet; 1 m)

club mosses;

chewing; gut

cycads; horsetails

Iguanodontids processing with gastroliths Cropping beak; shearing and low--grade chewing

Low to intermediate browsing (3 to 10 feet; 1 to 3 m)

Seed ferns; ferns; club mosses; cycads; horsetails; conifers (evergreens); ginkgo trees

(continues)

(continued)

Dinosaur

Plant-Eating

Taxon

Adaptation

Tier

Likely Diet

Hadrosaurids

Cropping beak;

Low to intermediate

Seed ferns; ferns;

shearing and

browsing (3 to 13 feet;

club mosses; cycads;

high--grade

1 to 4 m)

horsetails; conifers

grinding with

(evergreens); ginkgo

teeth

trees; flowering plants

Ceratopsids

Cropping beak;

Low to intermediate

Seed ferns; ferns;

high--grade

browsing

club mosses; cycads;

slicing and

(3 to 7 feet; 1 to 2 m)

horsetails;

grinding with

flowering plants

teeth

Data based on Fastovsky and Smith, 2004, from The Dinosauria.

Data based on Fastovsky and Smith, 2004, from The Dinosauria.

The crowning achievement of sophisticated plant-eating jaw morphology developed late in the history of dinosaurs. The ornithischians of the Late Cretaceous—particularly the later horned dinosaurs and hadrosaurids—had dental batteries so well suited for chewing plants that they rival those seen in later mammals. This burst of innovation in the chewing mechanisms of later dinosaurs occurred at the same time as the rise of angiosperms—the flowering plants. Angiosperms are more nutritious than gymnosperms and reproduce and grow more quickly; therefore, they can recover better from having parts removed. Because of this, some researchers maintain that the Late Cretaceous burst of ornithischian dinosaur evolution was literally fueled by the flowering plants, a new and better source of nutrition for dinosaurs. There is one conundrum in this scenario, though. Evidence for the gut contents of Late Cretaceous ornithischians shows that they were still eating primarily gymnosperms—conifers, evergreens and the like. Still, the possible coevolutionary relationship between the rise of flowering plants and the last of the great plant-eating dinosaurs is too suggestive to be ignored, so perhaps it is merely a matter of time and better fossil evidence before the picture becomes entirely clear.

(continued from page 27)

BIOGEOGRAPHY AND THE EVOLUTION OF THE DINOSAURS

The evolution of new species is mitigated by a number of factors, including the genetic makeup of an organism and environmental influences on natural selection. The place or habitat where an animal lives has much to do with its ability to find a mate, reproduce, and continue the species. Genetics aside, the evolution of new species is often affected by geographic and climatic influences on a species.

The goal of the science of paleobiogeography is to explain the distribution of extinct plants and animals. Paleontologist Ralph Molnar has taken a close look at how geography plays an important role in the continuance and evolution of dinosaur species. Molnar believes that paleobiogeography reveals much more than simply which dinosaurs inhabited which landmasses. "It also illuminates," he explains, "dinosaurian evolution and features of tetrapod evolution in general. The existence of dinosaurs in the tropics, near the poles, and in deserts shows that climate was not a limiting factor in dinosaurian distribution and evolution."

The study of Mesozoic geography and continental plate movements brings into play two geologically based concepts related to the distribution of dinosaurs. One theory, called vicariance bio-geography, depicts distributions taking place by the movement of continents. As the continents moved, so too, did all of the plants and animals riding on their surfaces. The history and movement of organisms, then, are considered as part of the history of continental drift, changes in ocean configurations, and the isolation and recombination of landmasses and the organisms that lived on them. Because continental drift is a slow process, vicariance is also considered to exert a long-term influence on animal distribution. For example, if whole populations of dinosaurs remained on a given landmass, their distributions would eventually, after millions of years, be influenced by the shifting and colliding of continents and all effects that would have on climate and habitat.

Another core concept behind paleobiogeography is that of dis-persalist biogeography. According to this theory, moving animals, rather than moving continents, are most significant. Given the existence of land bridges—even temporary ones—between continents, animals can move from one geographic location to another much more quickly than the time it takes continents to drift. Such widespread movement, perhaps over several generations of a species, could eventually place its members in a habitat that was significantly different than that of its ancestors. The resulting evolutionary adaptations to habitat and climate could result, over time, in a new species.

The current view of paleobiogeography synthesizes vicariance and dispersalist theories, assuming that each plays a role in shaping the course of evolution. In either case, the development of a new species seems to occur most frequently when two or more parts of the same population become separated by some impassable geologic barrier. Mountains, deserts, and bodies of water are examples of geologic barriers. If the barrier persists long enough—for the time it takes for many generations to reproduce—the separate populations may accumulate enough genetic changes to make reproduction between them impossible if ever they meet again: They would be two separate and distinct species. Just how this might happen is not too difficult to imagine.

Land bridges are a means for animals to disperse from one continent to another. The appearance of a land bridge can facilitate the connection of two populations that were previously separated. During Mesozoic time, as Pangaea (and later Laurasia and Gondwana) split apart, the appearance or disappearance of land bridges was a significant factor in the ability of dinosaurs and other animals to disperse across continents.

The disappearance of a land bridge leads to geographic isolation, another key influence on the evolution of species. This may occur as continents break apart, as sea levels rise to form islands, or by means of mountain building and other naturally occurring geologic events that separate once-joined populations. The separate parts of a population that have become isolated from each other by the geologic barrier eventually diverge genetically and form news species with unique traits. Island populations provide a dramatic example of geographic isolation. On the grandest of scales, the directions taken by dinosaur evolution in South America, Australia, and Madagascar during the Late Cretaceous—when these landmasses were essentially large islands, unconnected to other landmasses—provide examples of species formation that diverged significantly from their distant ancestors that lived when these same continents were joined.

SUMMARY

This chapter examined evidence for the geologic and climatic conditions that influenced the evolution of the flora and fauna of the Middle and Late Jurassic Epochs.

1. The middle span of the "Age of Dinosaurs" is represented by the Middle and Late Jurassic epochs, a 30 million-year period during which dinosaurs greatly diversified and often grew to huge sizes.

2. The fossil record of Middle Jurassic dinosaurs is poor, primarily consisting of specimens from Great Britain and China. Dinosaurs of the Late Jurassic are much better known, with excellent fossil specimens having been found on all continents except Australia and Antarctica.

3. Geologically, the Mesozoic Era is noted for the gradual breakup of the supercontinent Pangaea into the pieces that would become today's continents.

4. The climate of the Mesozoic Era was evenly temperate over most of the globe.

5. Paleobiogeography is the study of the geographic distribution of extinct organisms.

6. The evolution of new species is influenced by geographic features that affect the distribution of animals. Land bridges can broaden the distribution of a species and lead to new species. So, too, can geographic isolation be caused by impassable geographic barriers such as bodies of water, mountains, and deserts.

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