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.
One of the greater joys I have found as a geologist lies in the challenge of gleaning hidden information out of rock samples. A rock can speak if you know its language but it does not give up its secrets easily and since most of the answers to questions lie deep beneath the surface rather than right on top of it, a geologist often has to base theories on incomplete or cryptic information. Recall from earlier that geologic maps are generally based on a series of core samples that are drilled from time to time in an effort to determine subsurface features (especially when searching for rock or mineral resources). Using the principles set forth by early geologists like Steno and Hutton, it is possible to then link these individual core samples into one cohesive picture of exactly what lies beneath the surface without ever seeing it. When individual stratigraphic columns are linked in this manner, it is called a correlation.
Correlations are generally established based on two varieties of inferences: lithostratigraphic and biostratigraphic. Lithostratigraphy is generally the one used the most since it is based on similarities between rock units in different stratigraphic columns and can be used on any type of rock (igneous, sedimentary or metamorphic). Quite often, both physical and chemical properties of the rocks are used to determine matches between two or more columns. For instance, properties such as color, grain size, mineralogy, erosional surfaces and bedding characteristics are all used to identify related rock layers in correlations.
If the rock layers are fossiliferous, biostratigraphy correlations can also be made. These correlations are made by looking closely at fossil remains found in the rock and matching related animal and plant species from one stratigraphic column to the next. Of special importance here are index fossils. Index fossils are species that occur in high abundance but generally only exist for a short time in the stratigraphic record. Since they are common enough to be found frequently yet limited in their range of time period, the presence of such fossils is usually a clear sign the rock is of a certain age. The trilobite at right (Paradoxides) is just such an example of an organism that was very common around 435 million years ago but then disappeared from the fossil record shortly after.
Along the same lines as index fossils are non-biological events that can lead to establishing correlations between rock layers too. For instance, volcanic eruptions are geologic events that occur in the blink of an eye in geologic time but can still leave large enough deposits of ash and other debris behind that can be easily linked from one location to another. The tell tale signs of glaciation and the debris fields they usually leave behind when they retreat and melt can also be used to establish time lines from one place to another as well.
Correlations and Environmental Change
Since rock and mineral deposits are the direct result of environmental conditions, it should come as no surprise that these same rock layers can tell an astute geologist about the environmental changes that took place at a location over vast periods of time. Of special importance are series of certain rock and mineral types that are found in sequence with each other in a stratigraphic column. For instance, thick deposits of halite (rock salt) on top of limestone beds hints at the presence of an ancient shallow marine environment that eventually evaporated and left the salt. Recall that limestone beds are usually the remains of huge amounts of shelled marine organisms like clams, oysters or corals. In yet another example, consider the possible clues contained in a thick deposit of shale. Shale is composed of compacted clays and fine silts typically found underwater where the water movement is very low. Typically, these conditions are found in swamps but also in deep ocean settings. However, if a thick bed of coal is found in association with the shale, the deep ocean environment can be ruled out since coal is the result of decomposition of organic material now found in the ocean.
Correlations can also be made about environmental changes through time. In geology, a facies is a rock layer(s) that has noticeably different characteristics from those around it. Usually, this term is applied in sedimentary rock layers but it can also be used for other types of rock as well. In 1894, Johannes Walther made an important discovery in stratigraphy when he proposed the Law of Facies. Walther realized that vertical changes in rock facies could be translated as lateral changes in a depositional environment. This discovery was instrumental in understanding ideas like ancient sea level changes. Take a look at the example below.
This geologic cross section above is a view of part of the Michigan Basin, a huge bowl-shaped series of thick sedimentary rock deposits that is a very obvious feature on any geologic map of the region. As you can see, there are several thick deposits of igneous "basement" rock (red), considerable sandstones (yellow), limestone/dolomite (light blue) and even some shale (grey). The Precambrian deposits are at least 540 million years old (click here to bring up the Geologic Timescale again) while the others are younger (based on the Law of Superposition). The Principle of Original Horizontality also states that these layers (being largely sedimentary) were deposited horizontally. So what does the rock tell us about sea level changes? If you can remember the environments the sediments are deposited in, you're halfway to the answer already.
The sandstones found in the Cambrian Period are probably the work of either a desert or beach-type environment but which one is it? The limestone just above the sandstones gives it away. Since limestones are deposited in shallow marine environments, it stands to reason that the sandstones were deposited in a near shore zone. So, from this vertical change in the rock facies, it is logical to conclude that sea level changed laterally (i.e. moved landward) since what was once a beach became a shallow marine environment. This rise in sea level is known as a transgression.
Shortly thereafter (geologically speaking), another layer of sandstone suggests that the ocean once again receded (called a regression) before surging back in again to lay yet another layer of limestone. After that, the water became even deeper and the presence of the shale layer suggests that it was deposited in a deep marine environment before the ocean once again receded laterally and deposited more limestone. Since the uppermost layer of limestone is Silurian in age (~440 million years old) and the base of the Cambrian sandstones are ~540 million years old, we can identify at least two major changes in regional sea level over approximately 100 million years.
Gaps in the Rock Record
Not all stratigraphic columns offer up a continuous record of everything that has happened for millions of years at a given location. In fact, there are often gaps (sometimes referred to as a hiatus) where the given location was not a depositional environment. For instance, if a period of mountain building (known as an orogeny) occurred in a region, that area would experience erosion, not deposition so not only would new sedimentary levels not form there, older ones could be completely scoured away. However, the bright side to this missing information is that the gaps in the rock record can often be used to match up different stratigraphic columns in much the same way a deposit of volcanic ash can. Together, the various types of missing segments in the rock record are referred to as unconformities and come in several different forms. The first is perhaps the easiest to picture and is called a disconformity. A disconformity is an erosional surface between two successive rock strata. Disconformities may represent short periods of time but are usually at least tens of thousands of years (if not more) in duration. An angular unconformity is an erosional surface where the layers below the break have been tilted or folded but then buried by new horizontal layers on top. The photo on the Introduction page illustrates an angular unconformity that James Hutton used to explain his theories about a Dynamic Earth. Finally, a nonconformity is an erosional surface between igneous or metamorphic rock and younger sedimentary rock above it.
This cross section of the lower portion of the Grand Canyon (above) illustrates the three main types of unconformities. At the base of the Grand Canyon where the 1.7 billion year old Vishnu Schist lies is a distinct nonconformity where the overlying Unkar Group of rock layers meets it. Around one billion years ago, the entire region was tilted by tectonic forces and created mountain ranges that have long since disappeared but nonetheless created a hiatus of nearly 500 million years before the Tapeats Sandstone was deposited on top of the heavily eroded Unkar Group rocks around 525 million years ago creating an angular unconformity. From that time forward, no less than one dozen disconformities are recorded in the nearly horizontal layers of rock that overlay the basement rock of the canyon. Click here for a full view of the Grand Canyon strata.
Structural Geology Maps
The American Association of Petroleum Geologists is a professional organization whose mission is to "foster scientific research, to advance the science of geology, to promote technology, and to inspire high professional conduct". The AAPG is one of the leading geologic associations in America with over 40,000 members and it is often their members who are hired by the world’s major oil and gas companies to perform field surveys and resource prospecting missions. For this reason, it is their geologic maps that will form the basis of your introduction to geologic mapping in an upcoming lab. Called "geologic highway maps", AAPG combined some basic road map features with geologic ones so that any geologist driving through a region could quickly match up what they were seeing out the window with a road location on the map...quite a handy combination!
Reading the Maps
The AAPG maps follow a few basic conventions common to most USGS maps which you need to familiarize yourself with. First, they use standard rock type symbols in the stratigraphic columns. These symbols have been introduced to you already (at least in part) but will
now play a much more important role. Second, their stratigraphic columns are arranged according to the Law of Superposition with the oldest layers at the bottom. This means the geologic timescale is also in play. To help in standardize geologic maps, AAPG uses a pretty standard color scheme to show rock units of various ages on all of their maps. This color scheme is shown in the chart at right.
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.