Author's note: This E-unit was constructed from excerpts of Kings of the Mesozoic, an undergraduate research project written by Dr. Schmidt in 1992.
Dinosaurs. Creatures of myth and legend. Responsible for the superstitious myths of huge flying monsters able to breathe fire and blot out the sun with their massive leathern wings, the fossilized remains of these magnificent beasts have inspired both awe and unbridled fear among Man since the Dark Ages. Even today, dinosaurs have managed to retain much of their original mystique and occupy a permanent niche in the domain of fantasy literature as well as in the folklore of many European, Oriental and South American cultures. Although modern science has managed to dispel most of the mythical traits of the dinosaurs, they remain one of the most fascinating subjects for scientists and civilians alike. In fact, few scientific topics are as familiar to the public as the dinosaurs and even children would not be hard pressed to describe a Tyrannosaurus rex or a Stegosaurus. However, while it is true that some paleontologists do study dinosaurs, there is much more to the field than just the "terrible lizards." In general, a paleontologist is an individual concerned with the life and evolution of organisms throughout the Earth's 4.5 billion year existence. These organisms can then, in turn, reveal significant details about the planet's geologic, ecologic and climatologic past. Unfortunately, since most of the organisms studied by paleontologists are now extinct, they must often rely upon incomplete evidence obtained in fossils to create their theories and models of past environments.
Retired dinosaur paleontologist Jack Horner formerly of the Museum of the Rockies in Bozeman, MT. Horner is perhaps best known in the scientific community for locating the first clear evidence that at least some dinosaur species cared for their young. He was also the technical advisor to Steven Spielberg for Jurassic Park and was the inspiration for Dr. Alan Grant, the movie's dinosaur paleontologist.
In spite of the fact that nature does not generally specialize in "missing links" and prefers a much slower style of evolution, we must still consider a few of the scientific fields mentioned above if a full understanding of the dinosaurs (or any other group of extinct animals) is to be acquired. This unit is organized around text written by R. Schmidt while still an undergraduate geology student at Penn State in 1992. At that time, his focus was primarily on the Mesozoic era and dinosaur paleontology but the intent of his work at that time was to someday use it teaching high school geosciences. Nineteen years later, it will be used for the first time in this unit. Throughout the geology portion of AGS 381, various topics already covered in the course (like plate tectonics) are included again as a review but also provide for a much more in-depth analysis of the Mesozoic. Since his original focus of study was dinosaurs, the majority of the text is geared towards vertebrate evolution with only passing mention of the various invertebrates although students will have an opportunity to work with those in the lab. Within this section will also be an introduction to the forms of life that preceded the dinosaurs with special focus placed upon the organisms that eventually led to the rise of the dinosaurs themselves. Next, we will assume the role of geologists and unlock the theories of continental drift and plate tectonics in order to understand the impact of the Earth's geography on the diversity of the dinosaurs between 225 and 65 million years ago.
Once we have examined the Earth through geologic and ecologic means, the dinosaurs themselves will be inserted into the environment and will constitute the direct focus of the remainder of the unit. However, since the dinosaurs and their relatives represent such a complex and diverse group of animals, the events leading up to their appearance and eventual domination will also be covered. First, a section on the field of taxonomy will be used to help standardize our approach to studying the dinosaurs as biological organisms. Second, the evolution of the animals directly related to the rise of the dinosaurs will be introduced so that the development of these reptiles can be compared against the evolution of other plant and animal found throughout the latter part of the Earth's history. Following these sections, the dinosaurs and their nearest relatives will be presented according to their taxonomic classification.
Again, the organization of this unit is intended to be modular in design. Since the topics discussed here range over a wide spectrum of scientific fields, it is possible to present each section as a separate entity with the actual study of the dinosaurs representing the culminating activity. Also, although the amount of information provided is intended to form a comprehensive high school level historical geology unit, many areas can be simplified to accommodate younger students while still maintaining continuity.
Constraints on Research and New Findings
While our knowledge about the dinosaurs is extensive, it is hardly complete. Consequently, many of the ideas and theories first presented in this unit by some of the world's leading scientists represent, at best, educated guesses since the one key link to answering all of our questions, a living dinosaur itself, has vanished from the face of the Earth. Nonetheless, the amount of fossil evidence obtained in the past 150 years alone has led to the discovery of many exciting aspects of the dinosaurs' evolution as well as their day-to-day lives. Many of these discoveries are discussed within the text to follow and serve not only to raise our awareness of what the dinosaurs truly were (and were not), but also to encourage the formulation of new questions and theories that as of yet have not been answered. As a result, many of the included theories that have been proposed by various scientists over the years have been left "unanswered." In other words, evidence that supports and refutes each hypothesis is discussed in a neutral tone thereby allowing the reader to come to his or her own conclusions based on what they have learned. Since the original writing of this work 30 years ago, even more exciting discoveries including the growing relationship between dinosaurs and birds and the introduction of fields like cladistics serves to show how our understanding of these magnificent beasts is even now evolving.
The Geologic Timescale
Since the dawn of science, the relative age of the Earth has been debated and adjusted many times over. First thought to be only a few thousand years old by some of the earliest scientists of the ancient world, modern science has shown that the Earth and most of the solar system is actually around 4.6 billion years old. Though it has taken Man only a fraction of that time to discover this fact, it is also interesting to note that modern Man as a species has only inhabited the Earth for about the last 200,000 years. Where did life on Earth come from? When did it first appear? How is it that so many types of creatures and plant forms came to evolve on the Earth?
To fully understand the answers to these questions, years of study and research would be needed. However, before one can truly appreciate the dinosaurs, one of the most dramatic and spectacular forms of animals ever to inhabit the Earth, something of the Earth's history and the evolution of life must be understood. After all, to even consider the possibility that the Earth was devoid of life on one day and suddenly stocked with herds of 75 feet long, 30 ton reptiles 24 hours later would be preposterous! Therefore, two important concepts will be introduced here, the notion of geologic time and how this system of describing Earth's history corresponds with the evolution of life.
To the normal human, the concept of time is determined by a small device attached to the wrist or on the wall. These amazing devices determine when that individual eats, sleeps and even when they can go home from work at the end of the day. However, to that small group of humans who are geologists, these simple time pieces are virtually useless. The reason for this is due to the fact that, to a geologist, time is not a concept that can be measured in hours and minutes, but in millions of years. In fact, a minute (geologically speaking) would represent less than the blink of an eye. To appreciate the breadth and scope of geologic time, normal methods of comprehending time must be abandoned.
Earlier, it was mentioned that the age of the Earth has been estimated at approximately 4.6 billion years old. If this amount of time was put in the perspective of the human life span, those of us who are living today would represent approximately the 65,714,286th generation of humans if our ancestors appeared on Earth the first day of its existence and each member of each generation lived for 70 years. Fortunately for the novice scientist, man did not arrive on Earth the first day of its existence and geologists rarely deal in exact numbers. However, though humans are among the newest inventions of nature, the first traces of life on Earth did appear around 3.5 billion years ago as microscopic blue-green algae and bacteria. Does this mean that humans are descendants from pond scum? Well, not exactly, although scientists are confident that these microscopic creatures were among the first inhabitants of Earth. However, many billions of years still had to pass before the first recognizable organisms that we normally associate with "life" evolved.
Earth's primordial atmosphere was much different from what it is today. Perhaps the most important aspect of this early atmosphere was the lack of any oxygen and although many of the first algae species probably produced oxygen as modern plants do, the gas would have rapidly disappeared as it combined with other elements in the Earth's developing crust. By most estimates, it seems that oxygen did not appear as a consistent part of the atmosphere until approximately 1.8 billion years ago. Around the same time, the first microscopic animals, the protozoans, evolved and became a part of Earth's earliest inhabitants.
Since all of the earliest forms of life on Earth were single celled animals and plants, the next logical step for evolution was the appearance of multicellular life and although these creatures left no direct fossil evidence due to their soft bodies, their burrows and traces have been found in rock layers over 1.0 billion years old at a few rare sites around the globe. It is also important to keep in mind that these billion years old soft-bodied marine animals represented the most advanced type of life the Earth had to offer at that time. It was not until almost 400 million years later that animals possessing the first skeletons (like the Cambrian trilobite at right) developed and began to flourish in what is known to geologists as the Cambrian Period.
The Geologic Timescale
Now that the first major segment of the geologic time scale has been introduced, it is necessary at this point to fully explain the method behind the system by which geologists compartmentalize the history of the Earth since it is not long after the appearance of the first marine invertebrates that the myriad of names and dates found in the timescale can become overwhelming to the unprepared.
Devised in the nineteenth century, the geologic timescale was initially based on the succession of different fossil assemblages found in various parts of the world. So, although it has always been called the geologic timescale, it was biological evidence that set up its structure. As new and different types of fossils were discovered, this first geologic calendar grew and became more complicated. However, because no scientific means to accurately date the rock layers existed, the first timescale was entirely based on relative time periods. In other words, even though early geologists could determine that one type of fossil animal developed before another based on their positions in the rock record, the actual age of these organisms could not be ascertained. It was not until nearly 100 years later when British geologist Arthur Holmes conducted the first radioactivity decay experiment on rocks that the geologic timescale took on its more modern form and importance.
Once a reliable method of establishing the age of rocks first became available to the scientific community during the 1950s, Earth history was charted precisely for the first time. What was known as the stratigraphic column and consisted of the sequences of rock layers deposited on the Earth's crust throughout its history suddenly became much more than a simple record of the succession of different fossil types. By using techniques that measured amounts of various radioisotopes in the rock layer, the approximate number of years when each type of life form existed and when specific mountains and continents were formed could be determined. In the space of only twenty years, historical geology was transformed from an almost ridiculously inexact science into one of the most important branches of the earth sciences.
One of the most interesting facts that emerged from the establishment of the modern geologic timescale was that the Earth itself was much older than the well-identified fossils found in the rock record. When the geologists first divided the Earth's history into different periods according to the presence of certain fossils, the oldest period they could recognize; the one with the simplest fossils of marine creatures, was named the Cambrian. Not surprisingly, these scientists might have thought that these "first" animals arose only slightly after the Earth formed. To account for the time before the fossil animals evolved, a Precambrian period was inserted in the stratigraphic column to take care of whatever time elapsed before animals who could leave prominent fossils developed. Once radioisotopic ages were available, it turned out that the Precambrian occupied almost 90 percent of Earth's history! The entire succession of periods that the early European scientists argued about encompassed only the last 13 percent of the Earth's history.
Geologic Timescale Increments
Now that the history behind the geologic time scale has been discussed, the actual divisions of the scale can be explained. This explanation is important because throughout this unit, many of the specific names found in the geologic timescale used to describe the history of the Earth will be employed on a regular basis.
First, the largest practical divisions of the geologic timescale is known as an era. Note that there are larger time increments (like eons) but they are not generally used in usual discussions of geologic time. For the first 4.4 billion years of the Earth's history, this region of time was known as the Precambrian. Although today the Precambrian is actually referred to as a supereon, it is still generically used to describe all of Earth history prior to the beginning of the Paleozoic era ~540 MYBP (Million Years Before Present). Since then, there have been three more eras: the Paleozoic meaning "early life," the Mesozoic or "middle life" which is the most important era for purposes of this unit since this represented the time of the dinosaurs, and the Cenozoic or "new life." This last era, the Cenozoic, includes the time period (the Quaternary) we live in today.
Although the various eras are the largest practical geologic time units, they are not the only ones. Within each era, a number of shorter periods are found, each lasting several million years. For instance, the Mesozoic Era, also known as the "Age of Reptiles," contains three periods, the Triassic, the Jurassic and the Cretaceous. These periods are also divided into lower (early), and upper (late) sections that correspond to certain rock
Snowball Earth, a condition that likely existed near the very end of the Precambrian. Argued for over a century, the idea gained considerable support in the 1990s when credible evidence was discovered. The end of this global event may have contributed to a massive jump start in biodiversity that marked the end of the Precambrian and the beginning of the Paleozoic. The event is popularly known today as the "Cambrian Explosion."
layers or biological assemblages. In addition, some periods, like the Triassic and Jurassic, also contain a middle section. Even smaller divisions known as epochs also exist in the geologic timescale although for purposes of this unit, the period represents the smallest segment of time that will routinely be used.
For those who lack a background in biology, the myriad of names and terms presentedhere will probably cause much frustration and confusion before their connotations are fully understood. Therefore, the short section included here will deal exclusively with the processes used by scientists to classify all known organisms into an internationally accepted and manageable system. Since this system must account for millions of different known organisms as well as new species constantly being discovered, the study of characterizing and naming species has been given its own name, taxonomy. Closely related to taxonomy is the field of systematics. This branch of science studies the evolutionary relationships between life forms and creates family trees based upon the affiliations outlined by the work of taxonomists.
For thousands of years, man has been devising various systems to organize living organisms.
However, not until 1758 when the Swedish botanist Carolus Linnaeus (1717-1778), at right, first introduced his taxonomic system did the dawn of modern classification begin. Today, Linnaeus's system is still used to classify all known living organisms and has earned him the title "father of modern taxonomy." To understand Linnaeus' cataloging procedures, we will next apply the system to classify the domestic dog as well as perhaps the most infamous of the dinosaurs, Tyrannosaurus rex.
The Modern Taxonomic System
Originally, the modern taxonomic system was invented so that scientists around the world "spoke the same language" and classified organisms the same way. Even today, this system remains a very convenient method for arranging life on Earth. However, as more species of organisms were integrated into the system, taxonomic classification also became a method for identifying an evolutionary "tree of life" that could diagram everything from the organism's most ancient and primitive relatives to its most recent form. For this reason, taxonomy now provides both the anatomical as well as the evolutionary relationships among organisms.
In its simplest form, modern taxonomic processes involve classifying an organism into seven different levels creating a pyramid-like structure where the base of the pyramid and therefore the largest of the seven tiers is known as a kingdom. At the top of the pyramid is the species, the most specialized of the seven tiers. To date, 6 kingdoms are recognized depending upon the source of information and at least 1,500,000 species have been identified using Linnaeus's system. To illustrate the complexity of identifying each of these species, it is important to understand that taxonomic classification is not simply based on physical appearance. Although similar structures like bones and organs are important, modern taxonomists must also consider many other aspects of the organism before it is categorized. Among these characteristics are habitat, ecological adaptations, behavior, development, life cycle, physiology and even chromosomal and biochemical traits. Typically, the more information that is available, the better the taxonomic interpretation of an organism's background.
Clasifying an Organism
The easiest way to learn the use of the taxonomic system is to actually apply it and follow the step-by-step process with a known organism. For purposes of discussion, the domestic dog will be used to illustrate the taxonomic procedure.
According to its scientific name, the common dog is known as Canis familiaris. Although to most of us this name reveals nothing about the animal itself, it does reflect both the genus and species of a dog. The name of the organism itself is derived from Latin since this was the language of scholars during the 1700s and, along with Greek, is the standard tongue still used to name all organisms. Since the modern taxonomic system includes two scientific names for all organisms, plants and animals are identified according to what is known as "binomial nomenclature." For example, since dogs, wolves and jackals are so closely related to each other, they are placed in the same genus, Canis. When referring to only the domestic dog, however, the species (specific) name familiaris is added to distinguish the dog from its closest relatives. In the same context, the scientific name for the grey (timber) wolf is Canis lupus while the red wolf is known as Canis niger.
In essence, the modern taxonomic system functions much like human first and last names, except in reverse. Once organisms are categorized by their two word scientific name, they are then grouped into higher taxonomic categories. In ascending order, genera (the plural form of genus) become part of a family. Families are then arranged according to order followed by class, phylum (sometimes referred to as division in botany) and ultimately kingdom, the highest standard taxonomic level. Within each of these levels except species, it is also possible for any category to be composed of one or several examples of the level below. In other words, a given class can be composed of a single order or several closely related orders. In general, the higher the taxonomic
category, the fewer shared traits there are among its members. Because of this, organisms within the same genus will share many common traits while two organisms within the same kingdom may not. In fact, the only similarities between some organisms of the same kingdom may be at the cellular level. Concerning evolution, the higher the taxonomic category, the more time that has usually passed since the organisms shared a common ancestry. While members of the same genus will share a relatively recent ancestry, two organisms of the same kingdom will probably share a much more distant one.
Now that the upper levels of the taxonomic system have been explained, we will return to the categorization of the dog and follow its further ancestry to the kingdom level. Although dogs, wolves and jackals are classified in the same genus, what about the classification of foxes? Originally, these very dog-like animals were considered part of the genus Canis but were more recently separated into their own genus (Vulpes) since their skull structure differed somewhat from dogs. However, foxes are similar enough to dogs and wolves to be included in the same family, Canidae. As the name implies, members of Family Canidae all possess elongated canine teeth as well other common anatomical structures. More distantly, the canids are related to cats, bears, raccoons and other meat-eaters. Together, these animals are brought together to form order Carnivora based on the particular eating habits shared by its members. These animals (including the dog) are next classified within class Mammalia, where they share similarities with other animals who nurse their live-born young, are warm-blooded and possess fur (hair). Further up the taxonomic hierarchy, mammals are related to birds, reptiles, amphibians and fish since all of these animals are characterized by an internal skeletons and dorsal nerve chords. Collectively, these animals are known as the chordates, a name derived from phylum Chordata which is in turn a major sub-category of kingdom Animalia, the animals.
Above all, it is important to remember that taxonomy is an ever-changing science. Each day, research and scientific study has the potential to unlock new information. Just as it is important for an animal to "classify" its surroundings to survive, it is equally important for humans to achieve a better understanding of the relationships among the life forms with which we share the Earth.
One reason the number of species included in the modern taxonomic system is so large is due to the fact the system is not restricted to the classification of extant (still living) species although that was originally its purpose. Therefore, dinosaurs as well as all other species of extinct organisms are also named using binomial nomenclature, as in Tyrannosaurus rex. However, the single greatest difference between classifying extant and extinct organisms is that no living examples of extinct species are available to base classification on. As a result, the classification of extinct organisms is often based on fragmentary evidence or incomplete fossils that sometimes lead to taxonomic mistakes. Thus,while our knowledge of dinosaur taxonomy is extensive, it is incomplete and undoubtedly contains errors. Finally, since the evidence used to classify dinosaurs is limited and new clues constantly make themselves available, it is possible that, with any given fossil, the
whole taxonomic tree of dinosaurs could be toppled in favor of a new one. To help alleviate this potential problem, most of the dinosaurs are classified a bit more meticulously than other organisms although the terms about to be introduced are occasionally used with extant species also.
Since the dinosaurs are classified in the same manner as other organisms, they occupy the same seven tiered structure used by the modern taxonomic system. However, this system can be further subdivided into "super," "sub" and "infra" groups. For example, the next normal taxonomic level above an order is a class. Similarly, the level immediately below an order is a family. However, between an order and a class, a specialized group known as a superorder can exist. Below an order, a suborder can exist between the order and a family. Finally, an even more specialized taxonomic group known as an infraorder can occur between a suborder and a family. Though at first this can sound quite confusing, the rules for classifying organisms into super, sub or infragroups are exactly the same as the rules for the normal seven taxonomic levels. For example, all of the members of infraorder Carnosauria, a specific group of bipedal, meat-eating dinosaurs, will automatically belong to suborder Theropoda, the taxonomic group that includes all of the carnivorous dinosaurs. The characteristic hip structure of these dinosaurs is subsequently responsible for the name order Saurischia, the "lizard-hipped" dinosaurs. The only other order of dinosaurs is order Ornithischia, the "bird-hipped" dinosaurs. To illustrate the use of some of the specialized taxonomic groupings found in the classification of certain organisms and most of the dinosaurs, the taxonomy of Tyrannosaurus rex has been provided above (as of 2009). As a side note to ongoing reclassifications, the dinosaurs were included in superorder Archosauria (not superorder Dinosauria) when the author originally wrote his research study on the subject in 1992. Since then, Archosauria has been renamed as infraclass Archosauromorpha in light of more recent findings.
Tectonics and the Dinosaurs
When scientists set out in search of the keys needed to unlock the mysteries surrounding a certain theory or phenomena, their investigations often lead them into the realm of many different yet interdependent disciplines of science to find the answers they seek. This simple fact is especially true about studying paleontology. Like a modern police detective, paleontologists must examine much more than the obvious evidence that appears in front of them before they can possibly expect to unravel a mystery. For instance, dinosaur bones have been examined and catalogued since the early 1800s. Despite the number of fossilized bones that were examined, however, the information needed to create a realistic picture of the lives, environment, habits and eventual extinction of these creatures simply could not be obtained. No dinosaurs existed in the present and only limited information could be extracted from bones.
As time passed, other fossils recovered from the same rock layers as dinosaur bones did provide additional information that the bones themselves could not. Perhaps the most significant of these fossil types came in the form of plants. To a paleontologist, the plant fossils eluded to possible dinosaurian climates very different from those of the present. In addition, plant fossils often represent the types of vegetation eaten by herbivorous animals. Nonetheless, even with the discovery of fossilized Mesozoic plants, scientists still possessed only disjointed pieces of the overall dinosaur puzzle. Not until the 1950s would the remainder of these missing pieces fall into place.
The Theory of Continental Drift
Ever since Man first attempted to explain the nature of the universe, the Earth had always been viewed as an unchanging celestial body where the various features of the planet's surface were considered permanent and unchanging. In 1912, however, climatologist Alfred Wegener first introduced a new theory that would eventually revolutionize how we looked at the planet. Known as the theory of continental drift, Wegener insisted that the Earth's continental blocks had moved at some point in the planet's history and might still be moving even at the present. Although he did not know how this could occur himself, Wegener also proposed that since the physical borders of the continents seemed to fit snugly together much like puzzle pieces, they must have been joined as a single landmass sometime in the Earth's past. Wegener named this ancient supercontinent Pangaea. We now know that, while impressive, this supercontinent also contributed to the largest extinction event in the planet's history. Dubbed the "Great Dying," it saw the disappearance of 90% of all living species on Earth but also provided the opportunistic survivors (including dinosaur ancestors) with a chance to take over.
Rocks and Rock Units
It is important to remember that geologic maps usually illustrate subsurface geologic features as much as surface ones through the use of core samples. These core samples are then turned into stratigraphic columns which are then connected into geologic cross sections. And, just like in biology, systems of classification exist for the identification of various types of related rock units.
As you begin to evaluate stratigraphic columns and cross sections from geologic maps, it will become useful to identify certain features by their names. In geology, scientists typically name distinct rock layers according to the names of towns, geographic features, etc. found in that area. The Wissahickon schist is an example of a metamorphic rock you have already been introduced to at Valley Green. These named rock layers are known as formations. If there are multiple formations that seem to be related (but are still distinct), they are grouped into a group. Formations can also occasionally be subdivided into members while they are in turn divided into beds. A bed is the smallest stratigraphic unit used in geology and may be only a few inches thick if it is significant
In spite of Wegener's evidence, however, the general scientific community was not as enthusiastic about their colleague's theory of a moving continents and dismissed the notion as preposterous. Nevertheless, a few scientists, notably South African geologist Alexander du Toit, firmly believed in Wegener's ideas and set out to further substantiate his heretical theory. What du Toit found consisted of even more evidence suggesting that the continents had indeed wandered across the face of the Earth.
In 1937, du Toit published "Our Wandering Continents," a book in which he theorized that glacial deposits from a huge continent composed of all of the present day southern hemisphere landmasses and India were geographically linked to coal deposits discovered in a similar megacontinent composed of North America, Greenland, Europe and Asia. By examining all of the evidence he collected, du Toit was convinced that the Earth's continents were once joined in two huge masses. In addition, du Toit also insisted that these two continental blocks, Gondwana in the southern hemisphere and Laurasia in the northern hemisphere, collided with each other and formed Wegener's supercontinent, Pangaea.
What made du Toit's testimony even more credible was that his evidence was derived from many different fields of science and not strictly geology. Paleontological evidence consisting of identical fossil reptiles and plant species found in both South Africa and eastern South America, geological indicators such as similar rock strata that spanned five continents now separated by vast oceans and climatological data that included coal beds in now frozen environments were all included in du Toit's report. Still, the world's scientists refused to accept the continental drift theory. Although Wegener's and du Toit's evidence seemed convincing enough, there was still no proof of a force strong enough to move the Earth's continents.
Seafloor Spreading and Plate Tectonics
Though continental drift was argued many times over throughout the early twentieth century, it was not until after the end of the Second World War that definitive proof supporting Wegener's ideas was finally discovered. Who would have guessed that the very instrument eventually used to prove continental drift would be the same device employed by the Allies to detect German U-boats?
Once World War II ended, many technological advances developed exclusively for the war effort were turned over to civilian enterprises. Perhaps the most important of these inventions to the field of geology was sonar. Due to the manner in which this device functioned, it was not long until scientists realized its potential in charting the geography of the ocean bottom. As a result, the field of oceanography was born and the first complete diagram of the Atlantic Ocean seafloor was created. What oceanographers did not realize immediately, however, was that they had also stumbled upon the very evidence needed to prove Wegener's continental drift theory.
The Atlantic Ocean sonar maps revealed a seafloor that was not a single, vast plain of rock as previously thought. Instead, these maps charted a highly contoured surface composed of mountains, valleys and plains that were typically much larger than any comparable features found on land. The most stunning of these features consisted of a single, continuous double ridge of mountains between which emerged molten rock. Since this molten material did not come from the rock composing the crust, scientists correctly surmised that the magma had originated in the mantle of the Earth and had welled up through these deep sea ridges. Surveys of the Pacific Ocean revealed additional features where crustal rock was consumed in incredibly deep oceanic trenches and subsequently "recycled" in the mantle creating a crustal conveyor belt where new rock was formed at the mid-ocean ridges and eventually swallowed up in the trenches to be remelted within the mantle.
Not long after the discovery of the complex global ridge and trench systems that had remained hidden from modern science for over a century, geologist Harry Hess of Princeton University (right) proposed the Theory of Seafloor Spreading in 1962. In this theory, Hess concluded that the crust of the Earth was not a single, cohesive shell but instead a set of constantly moving plates whose boundaries were marked by the oceanic ridges and deep ocean trenches. Driven by convective currents occurring deep within the mantle, Hess also proposed that the continents as well as the sea floor moved together (this movement is known as plate tectonics), since some of the Earth's crustal plates consisted of both continental blocks and oceanic crust. With the
overwhelming evidence presented by Hess and his associates, the newly coined Theory of Plate Tectonics (the combination of continental drift and seafloor spreading) was finally accepted. Most important, the field of geology at last had acquired the unifying theory it needed to explain many of its most perplexing ideas and phenomena including the world of the dinosaurs.
Whether by chance or design, many successes of the dinosaurs are owed largely to the existence of Alfred Wegener's supercontinent, Pangaea. Completely assembled sometime during the Permian period, Pangaea represented a spectacular collection of landmasses comprising all of the present day continents and occupied one-quarter of the Earth's surface although it probably also had a hand in the largest extinction event in the history of the planet as well. Pangaea subsequently created a geographic picture very different from the Earth of the present. Surrounding Pangaea was Panthalassa, a single global ocean that occupied a full 300 deg. of longitude. Spanning from pole to pole for over 50 million years, the existence of Pangaea explains many of the mysterious aspects of the dinosaurs and how they came to occupy every continent on Earth including Antarctica. Now, the Theory of Continental Drift and the breakup of Pangaea will be applied to examine how these two factors not only affected the lives of the dinosaurs, but ultimately their diversity and possible extinction.
The Paleogeography of the Early Mesozoic
Since the Mesozoic era was exemplified by the presence of the dinosaurs, it is only natural that an examination of the global geographic effects on the "terrible lizards" begins during the Triassic period. However, before the dinosaurs even appeared sometime during the Triassic, Pangaea had a definite impact upon the dinosaurs' immediate ancestors, the thecodonts, who ranged over much of the Earth's surface earlier in the period.
Throughout most of the Triassic, Pangaea was the dominant force behind not only the earliest Mesozoic climates (discussed later) but also the distribution of early Mesozoic reptiles. Supporting evidence for this theory is derived primarily from the mammal-like reptile Lystrosaurus (right), whose remains were discovered in Antarctica during the 1960s. What made this find important to paleontologists, however, was that Lystrosaurus fossils had also
been unearthed in Australia, China, South Africa and India thereby validating the proposed existence of Gondwana since it would have been impossible for the terrestrial animal to spread throughout the Southern Hemisphere if the continents had been separated by oceans. The fossils of other extinct animals and plants also bolstered the theorized existence of Laurasia and ultimately Pangaea. Since the thecodonts inhabited many of the same regions as Lystrosaurus, they too could take advantage of the supercontinent's lack of physical barriers and expanded their geographic range accordingly. As a result, the first true dinosaurs, since they were descended from very similar reptiles, resembled each other very closely in appearance and physical structure. By the end of the Triassic, however, Pangaea began to show signs of crumbling as newly developed convection currents in the Earth's mantle made their presence known. As for the first dinosaurs, the separation of Pangaea by inland seas and eventually oceans meant that some populations would become isolated and, accordingly to Charles Darwin, evolve in a fashion and at a rate independent of other dinosaur populations. According to all fossil evidence, this is exactly what happened.
The Fragmentation of Pangaea
Although the exact sequence in which Pangaea broke apart to form Earth's present continents is still debated, certain general phases can be discussed in order to reflect their effects on the increasing diversity of the dinosaurs throughout the Mesozoic. It is important to remember, however, that the time frames and sequences described here can be considered, at best, educated guesses. Fortunately, the first phase of Pangaea's destruction is the most agreed upon by scientists and therefore provides a firm base from which to proceed further.
This first phase, which began approximately 210 million years ago near the end of the Triassic, involved the separation of the northern and southern continents, Laurasia and Gondwanaland (Gondwana) respectively. Soon after, a separate rift began to emerge between the North American and European sections of Laurasia though it was not until sometime in the Cretaceous that this fracture became very active. What this separation caused among the Earth's dinosaur populations was a blossoming of new and diverse species as the two continental blocks moved apart and the Tethys Sea flooded the area between. As a result, two unique dinosaur communities began to arise, one that populated Gondwana and a second that dominated Laurasia with both evolving independently of the other.
Though Pangaea's destruction demonstrated the immense power of a dynamic Earth, it is important to keep in mind that these changes did not occur overnight. For instance, the distance traveled by today's crustal plates is measured in fractions of an inch per year, not feet or miles. Therefore, the varieties of dinosaurs that began to emerge on Gondwana and Laurasia also took millions of years to emerge as geographically isolated populations. In fact, it is estimated that the complete halving of Pangaea took 40 to 60 million years to accomplish. However, definite fossil indicators of this partitioning have been recovered. For example, although North American fossils of the Jurassic sauropod Brachiosaurus are identical to those found in South Africa, the plated dinosaur Stegosaurus (below left), whose bones have also been found in large numbers throughout North America, appears very different from its African relative, Kentrosaurus (below right). Also, it appears that China became increasingly isolated as the Mesozoic progressed due to the very strange assemblage of dinosaurs found there and nowhere else in the world.
The second major phase of Pangaea's disintegration occurred during the Jurassic and was confined almost exclusively to Gondwana. Most important, this phase did not depict a single massive rift between two landmasses. Instead, the complete dismemberment of the huge southern continent occurred. By the end of the Jurassic Period 144 million years ago and into the early Cretaceous, geologic evidence supports the apparent separation of South America from Africa, Africa from the combined land mass of Antarctica, Australia and India, and finally the separation of India from Australia and Antarctica (the actual sequence of these multiple separations remains highly debated). In a global sense, the rending of Gondwana left very significant impacts on the new southern continents.
First, South America was left a completely isolated island until a mere 20 million years ago when it joined North America near the Isthmus of Panama. Second, Africa was now separated in the same manner as South America until its northward course met Europe and formed the Mediterranean Sea 170 million years later (3-5 million years ago). Also because of Gondwana's demise, India was "tossed" into the primordial Indian Ocean where it would meander northeast until it eventually collided with Asia and formed the Himalaya mountains 55 million years ago. Finally, the sutured combination of Antarctica and Australia headed off together until their eventual splitting sometime during the late Cretaceous period or early Cenozoic era.
Since the Gondwana continental separations were complete, the ramifications of these splits on dinosaur diversity become quite evident. Unlike the two branching populations of the earlier Jurassic, the end of this period began to show signs of an increasingly diverse group of reptiles with many distinct populations that corresponded to several discrete continents. Today, the same effects that the breakup of Pangaea had on the dinosaurs can also be seen concerning the mammals. By sheer coincidence, the first mammals had already appeared and spread throughout the world before most of the Pangaea rifting took place. Once Pangaea began to separate, however, the numbers of different species began to increase for the same reasons as the dinosaurs. Nowhere is this evolutionary isolation more prevalent than in Australia. Separated early from the rest of the Earth's continents, Australia remains an evolutionary island even to this day. As a result, the most primitive types of mammals, the monotremes (egg-laying mammals) and the marsupials were able to survive even though their relatives from the other continents have long since become extinct due to the evolution of other, better adapted mammals like the placentals.
The third and last phase of Pangaea's breakup to occur during the Mesozoic involved further separation of the southern continents and the gradual splitting of Laurasia during the Cretaceous period. By the end of this period, which also marked the end of the dinosaurs, the Earth's global geography finally began to resemble today's as most of the modern land masses began to approach their modern locations. Most obvious among these features was the completion of a northerly clockwise turn by Africa to effectively close off the Mediterranean Sea from the Atlantic and Indian oceans. Also by the end of the Cretaceous, India had reached the equator on its northeasterly trek toward Asia while in the north, the eastern coast of Greenland had begun to tear away from Europe. This separation continued well into the Cenozoic era before North America, Greenland and Europe finally bid farewell.
As a side note, it is also interesting to note one additional effect the disintegration of Pangaea had on the Mesozoic Earth besides its obvious impact upon dinosaur evolution and global climate patterns. According to all available evidence, it is now likely that the appearance of the mid-ocean ridges and the general rifting of the continents during the Cretaceous caused a severe rise in global sea level.
According to certain estimates, many scientists believe that as much as one-third of all the Earth's dry land became submerged in epeiric (shallow) seas during this period. In the case of North America, a great inland sea known as the Cretaceous (or Interior) Interior Seaway was created and effectively partitioned North America down the middle. Of the entire continent, only the Rocky mountain belts, Great Lakes and Appalachian regions remained above water.
Earlier, it was mentioned that paleontologists are much like detectives concerning the methods they use to solve their mysteries. Unfortunately, the secrets surrounding the dinosaurs still remain locked behind the portals of time although one door, marked "geology," now stands open to our probing investigation. Now, we must ask ourselves about what the dinosaurs might have eaten and what their habitats looked like. Were the features of the Earth the same as they are now but in different parts of the world? If not, how did the dinosaurs see their surroundings? The answers to these exact questions will be explored next.
Snapshot of Paleozoic Life
We just discussed that, at the end of the Precambrian, life on Earth reached a point where some of its more advanced forms evolved hard exoskeletons that led to the first prominent fossil evidence found in the rock record. The appearance of this fossil evidence is what led geologists to mark the event as an entirely new era of geologic time. The event itself has now been dubbed the Cambrian explosion after the sudden appearance of a multitude of hard bodied organisms that represented almost every group of invertebrates found on Earth today including the most successful phylum of animals ever, the arthropods. In fact, 75% of
all fossil evidence obtained from the Cambrian period consisted particularly of a well known arthropod group known as trilobites. Also appearing during this period are the ancestors of the echinoderms (a group of animals that includes starfish and sea urchins), the porifers (sponges), the molluscs (represented in the Cambrian by squid ancestors) and the first examples of an extremely successful group of predominantly Paleozoic marine animals known as brachiopods which resembled clams (in appearance only). The cnidarians also blossomed as hard bodied corals began to form reefs that would someday lead to the massive beds of limestone that litter the globe today. The image at right shows what a typical Cambrian marine ecosystem might have looked like with a variety of arthropods, sponges and marine algae occupying center stage. The large, free swimming arthropod Anomalocaris might have been one of the planet's first apex predators.
Following the Cambrian, the Ordovician period began in earnest around 505 million years ago. During this period, the brachiopods diversified into many new varieties while the trilobites refined their already "advanced" structures. In addition, other major groups of creatures to appear during this period included the first primitive snails and bivalves, more corals, bryozoans and two interesting groups of sedentary filter feeders, the crinoids (a type of echinoderm) and the graptolites. Finally, Ordovician rock strata have yielded the first fossils of jawless fish, the first known vertebrates.
The Silurian period that followed the Ordovician ushered in even more new forms of marine life. Among these were new types of brachiopods, crinoids and graptolites. Also abundant during this period were particular forms of corals known as chain corals due to their peculiar growth pattern. Within this period, the first jawed fishes began to populate the Earth's marine environments in increasing numbers. Perhaps the most fascinating new marine animals of this period, however, were the eurypterids, also known as "sea scorpions." Found in large numbers throughout Silurian rock layers in the Great Lakes region of the United State and Canada, these ferocious looking arthropod predators possessed two large pincers in addition to the usual four pairs of walking legs. Inhabiting both fresh and salt water, the eurypterids, who reached lengths of 9 feet, are still the largest arthropods ever found though these animals and all of their relatives became extinct by the Permian period. Finally, the Silurian period has also been recognized as the geologic period when the first primitive land plants began to inhabit Earth's continental masses.
Beginning approximately 395 million years ago, the next period of geologic time, dubbed the "Age of the Fishes," was the Devonian. At no other time in the planet's history was the diversity of fish so high and the fossil record of fish evolution is exceptional. At first, the early jawed fish experimented with a heavily armored body that did not have true teeth. Instead, the "teeth" of the fish were actually extensions of the skull plating. These fish, the placoderms (at right), were impressive to behold and were likely top predators in early Devonian waters but their success was short-lived since they rapidly died out by the end of the period for reasons still unknown. However, prior to their disappearance, they gave rise to both the two main groups of fishes still found on the planet today; the bony fishes and the cartilaginous fishes. In fact, some of these animals, especially the early sharks, were so well adapted to survive
in their marine environments that, even today, they have undergone very little evolutionary changes from their oldest ancestors and remain, "living fossils." From other species of fish very different from the sharks, the first amphibians also arose late in the Devonian. Later, these amphibians will be discussed in much greater detail as their importance in the evolution of the reptiles is fully realized.
While the fishes were flourishing in almost all of the aquatic environments of Earth, further developments were occurring among the early land plants. At about the same time, one of the most unloved groups of creatures, the arachnids, also appeared among the ancient seed ferns and mosses. By the end of this period, some of these plants had even evolved into the first trees that sometimes grew to heights of 80 feet.
The next period, the Carboniferous (below), allows us to finally shift our scientific curiosity from the seas to the land and is perhaps best known for its huge swamps and tropical climate. Today, these same swamps are responsible for most of the world's petroleum and coal resources. To the evolving animal species of the Carboniferous, however, the coal swamps did not represent a source of future monetary value but a new source of food and a habitable environment away from aquatic predators. As a result, the earliest species of truly land-dwelling animals began to appear as some of the amphibians evolved into the first reptiles who feasted on the huge populations of insects that had also inhabited the huge swamps by this time. During the early part of the Carboniferous, the first conifers also appeared among the seed ferns and mosses.
Meanwhile, the fishes continued to evolve and diversify in their marine habitats. Among the invertebrates, the trilobites had virtually disappeared from the ocean bottom while the brachiopods, mollusks, corals and bryozoans continued to evolve into new, and sometimes very strange species.
The last period of the Paleozoic era, the Permian, began 280 million years ago and ended 55 million years later when the Earth suffered the greatest mass extinction in its history in which nearly 90% of all the living organisms on the planet disappeared forever. Included among the doomed marine animals of the Permian mass extinction (often referred to as the "Great Dying") were the last of the trilobites and eurypterids, most of the early types of corals and bryozoans, the archaic species of jawed fishes, almost all of the crinoids, the blastoids and almost all of the species of brachiopods. On land, many of the amphibians and earliest species of reptiles also became extinct as more advanced varieties out-competed these animals for food and overall dominance of their environment. Although the causes of this great extinction are still hotly debated, massive volcanism at what are known as the Siberian Traps and the formation of Pangaea seem to be at the top of the list. What we do know is that the event (or successive events) began in the oceans and devastated many of the major fossil groups of the Paleozoic. On land, biodiversity did not fare much better as most of the planet dried out and gave rise to some of the largest deserts in the planet's history. Even advanced early Permian "winners" like the pelycosaurs (including Dimetrodon pictured below) were ultimately to succumb to the environmental chaos that ended Earth's second geologic era.
However, looking only at the list of animals that disappeared by the end of the Permian does not create an accurate description of the period. It is also necessary to consider the species of plants and animals that survived the Great Dying and lived on into the Mesozoic Era to continue the chain of life. From within the now firmly entrenched groups of land-based plants, at least two completely new phyla arose between the Permian and the Triassic periods. The first of these plant forms is known as ginkgoes. Occasionally referred to as maidenhair trees, this ancient group of plants possessed tall, thick trunks and unique fan-shaped leaves. Today, although few people know of this plant's far reaching origins, the last remnants of this tree group are found in small numbers in China and are often grown as ornamental shade trees. The second phylum of new trees, represented
today by a single remaining family of individuals, resembled modern palm trees and are called cycads. Throughout the animal world, the Permian extinction had caused many new types of animals to rise up and fill the suddenly empty biological niches in their environments. In fact, the reptiles alone produced at least three different major groups of new animals. Also, the reptiles expanded their control to include the seas and the air as well as the land by the end of the Triassic. In view of this, it is no surprise that the Mesozoic era was considered the "Age of Reptiles" and although the first mammals were known to be skittering about by the middle of the Triassic, man's earliest mammal ancestors would be forced to wait for over 160 million years before they were given the opportunity to rise up and conquer their surroundings at the beginning of the Cenozoic era, a mere 65 million years before the present.
In retrospect, it should now seem obvious why a calendar such as the geologic time scale is necessary in order for a geologist to explain the entire history of the Earth in a logical and meaningful fashion, even to someone who knows nothing about it. Without this chronological table, the numerous and sometimes intimidating scientific terms associated with the Earth's history would probably discourage many inquisitive minds away from paleontology.
To learn about any given organism and its interaction with its environment, it is not enough to simply look at just that plant or animal at face value. For example, where did the animal live? What did it eat? How did it reproduce? What other organisms influenced the way in which the animal in question behaved? Before a composite picture of the life and evolution of this animal can be determined, all of the questions listed above must be answered. Unfortunately, a paleontologist is often plagued by the inability to accomplish these tasks. To overcome the many obstacles associated with examining only select pieces of a complete puzzle, a paleontologist must become a "jack-of-all-trades" concerning the disciplines of science he or she employs to piece together the scant segments of the mystery. At any one point in time, a paleontologist could actually be using techniques common to a geologist, a botanist, an ecologist, a biologist or even a climatologist to interpret the secrets held in a single piece of cryptic evidence that could become one of the most important discoveries in science. Unfortunately for the ambitious paleontologist, however, such scientific finds are extremely rare and almost never lead to fame and fortune.