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11. Rocks and Fossils

The Geological Record: Reading Rocks

Someone just found a fossil! Someone just found a fossil!

In many ways rocks and fossils are the most perfect form of scientific evidence. When rocks solidify, they lock in the record of their environmental conditions and they are able to preserve that geological information for millions and even billions of years. Likewise, all that we know about the life that has existed on our planet for hundreds of millions years comes from the fossils that they left behind. And what a story do they tell! Continents colliding into each other, strange plants and animals that lived in the seas, gigantic creatures that roamed the Earth while giant pterosaurs flew overhead. Best of all, almost any physically fit person can directly confirm most of these mind boggling facts by going on field trips in search of rocks and fossils.

Of the three basic types of rocks - igneous, sedimentary, and metamorphic rock – we are most interested in the sedimentary rocks. For a quick geology review, igneous rocks form from magma coming up from the Earth’s interior. When hot liquid material flows up from the Earth’s interior it solidifies as igneous rock. Volcanic magma is liquid material that is either ejected into the atmosphere or flows as lava from the volcanic opening. Often the rising magma falls short of reaching the Earth’s surface, and when this happens the slowly cooling magma allows minerals to crystallize as plutonic rocks such as granite. But no matter how these igneous rocks formed, once they are exposed at the surface erosion eventually wears them down and washes them away. These bits and pieces that are washed away later settle to form sedimentary rock layers. Another way the sedimentary rocks form is through chemical precipitation, salt deposits, and the accumulation of marine fossils. The third type of rock, metamorphic rocks, forms when igneous or sedimentary rocks are buried too deeply. Deeply buried rocks are subjected to the high temperature and pressure of the Earth’s interior and this changes their crystalline structure. The sedimentary rocks are the ones that give us a geological record of the Earth’s past and so for our purposes these rocks are the most important.

Pikes Peak Granite It's OK to take this rock for granite. It's Pikes Peak Granite.

Because rocks are so incredibly common there is a strong tendency to take them for granite, yet to a trained geologists there is a wealth of information that can be derived from common rocks. The type of rocks found at a site, how well they are sorted, and the wear marks they show provide us with information regarding a regional sedimentary deposit. Likewise, evidence of glaciers scraping, windblown deposits, along with mud cracks and ripple marks, tell us about the climate. Yet all of this information is of limited value if we don't know the age of the rocks. Fortunately we can determine the age of rocks by applying our understanding of how sedimentary layers form.

Two of the most fundamental ideas in geology are the law of original horizontality and the law of superposition. Together what these laws mean is that sedimentary rocks are initially laid down in horizontal layers, sort of like a stack of pancakes, and just like those pancakes, the youngest sedimentary rock layer has to be the last one placed on the stack. Thus we know that as we drill deeper we are going back in time as we pass through successively older rock layers. Almost always, the oldest rock layers are going to be the lowest rock layers.

Sedimentary Rock Layers Sedimentary Rock Layers

Just one more thing. On account of gravity, the material that is forming these sedimentary rock layers must come from a higher elevation. So while sedimentary rock layers are being created in low areas such as valleys, lakes, flood plains, river deltas, and sea beds, high elevation places such as hills and mountains are being eroded away. In addition, continents sometimes crash into each other to form mountain ranges and these continents even oscillate up and down with the coming and passing of ice age glaciers. Because of this kind of geological activity most locations will not show an unbroken sedimentary record going from youngest to oldest. Furthermore, even if there is a location where there is a continuous sedimentary rock record, eventually the lowest sedimentary rock layers would be buried so deeply that the heat and pressure would cause them to change to metamorphic rock and thus destroy most of the geological information. The upshot of all of this is that a geologist can not go to a single location and start digging or drilling down with the expectation of being able to read the Earth's history from the current time back to Earth's beginnings.

To give an analogy, think of the geological record as a collection of old tattered books that we read by starting at the end and then proceeding backwards. The layers of sedimentary rock are like the pages of each book. As we dig deeper we go progressively farther back in time. Erosion may have produced a nonconformity such that in our geological record some of the sedimentary layers may be missing. In terms of our book analogy we could say that some of the pages are missing. If the sedimentary rock was buried too deeply then it will have changed to metamorphic rock. These metamorphic rocks are not much help to us since they are like written pages whose writing has been smeared and no longer legible. As we go deeper we may eventually hit the igneous rock base. At this point we cannot read the Earth’s history anymore at that location.

There is no place on Earth where we can continuously read our geological book from the end of the story back to the beginning, of even come remotely close to completing this goal. Nevertheless we can still read extremely far back in time by piecing together the information we find at numerous locations. This is because fossils give geologists the ability to chronologically match up the sedimentary rock layers found at one location with the sedimentary layers found at another layer.

trilobites Trilobite Fossils

Fossils – the remnants of previous life and the impressions ancient life has left in the rocks – tell us even more about the Earth’s geological history than the rocks. From the smallest microscopic species to the largest organism, fossils are useful in determining everything from the local paleoclimate to finding valuable resources such as oil. Without fossils we would have no record of the dinosaurs and the numerous other fascinating species that previously thrived on this planet.

One of the most beneficial attributes of fossils is that they mark geological time. During the last half billion years there were numerous species that achieved worldwide distribution, thrived for a few million years, and then went extinct. During the time of their existence the relics of these species became embedded in the sedimentary rock as it was being laid down. Consequently the sedimentary layers at numerous locations around the world can be linked together as being laid down at the same times if they contain these same key index fossils.

Geologists have also discovered that there is an order to the placement of fossils such that the fossils of certain species were always higher or lower than other species. Since younger rock must be laid down on top of older rock, the vertical placement of the fossils is actually giving the chronological order. If ever a geologist comes across the same pattern of fossils or even just a single key fossil he or she immediately knows the chronological age of the rock layer being examined.

By comparing present day geological processes to the rock layers showing evidence of same processes occurring earlier, eighteenth and nineteenth century geologists developed an understanding that the Earth is extremely old. Yet for these geologists the actual age of the Earth was still a mystery. The fossil evidence by itself can only give the chronological order of geological events, not the actual age of the rocks. It was not until the twentieth century that radioactive dating methods enabled geologists to determine the absolute age of the rocks.

Radioactivity Time Decay Equations Radioactivity Half Life Equations

Radioactive material, in addition to the fossils, can sometimes be found embedded in the sedimentary rock layers, and like fossils the radioactive material is useful in determine the age of the rocks. Once created, the radioactive material of an element decays exponentially into a new material consisting of its daughter element. The decay constant λ is the exponent determining if this decay process requires less than a second or as long as billions of years. To get a better feel for the quickness of this transition, the decay constant of a radioactive material is often expressed as the half-life of the element. The half-life is the amount of time required for half of the radioactive material to decay to the daughter element. A technician can determine the age of a sample of radioactive material by knowing the half-life of a radioactive material and measuring the percentage of radioactive nuclei remaining in the given sample. By using radioactive dating methods to determine the age of a sedimentary rock layer the age of the fossils found in the rock layer is also known. Having repeated these analyses numerous times the absolute age of numerous key fossils have been determined. Knowing the absolute age of these key fossils, a geologist in the field needs only to find one or more of these key fossils in the rock layers to know both the chronological and the absolute age of the rock.

Periods of Geological Time Periods and Eras of the Phanerozoic Eon: the Most Recent 544 Million Years

About 544 million years ago single cell life evolved into a diversified set of organisms capable of leaving behind fossilized evidence of its existence. These fossils give this eon its name: Phanerozoic, means visible life. As described earlier, geologists are able to easily date the age of sedimentary rock by identifying the fossils embedded in the rock layers and so it is much easier for geologists to make sense of the sedimentary layers laid down during the Phanerozoic eon than the rock layers than came before the Phanerozoic eon. Thus geologists have a much better understanding of the past 544 million years of the Earth’s history than all of the history than came before this.

The Phanerozoic eon is divided up into eleven periods and three eras. The logic for the naming of the periods is based on the set of fossils found in each period. Each period represents a time when there are numerous fossils that are unchanging and unique to that period.

This chapter explores the connections between the environment, events, and the species that evolved on this planet. Specifically we want to make sense of the geological observations in the context of well established science principles. For instances, if new species evolve gradually as Darwin explains, then why is it that the fossil record often shows new species suddenly appear out of nowhere? Besides the Mesozoic era being the Age of the Dinosaurs, what other features set it apart? And finally, how can events occurring during the Phanerozoic eon determine the thickness of the Earth’s atmosphere, and conversely, how does the thickness of the Earth’s atmosphere explain the fossil evidence?

Evolution and the Fossil Record

In 1859, Charles Darwin put forth his Theory of Evolution when he published On the Origin of Species. The concept that new species evolve from preexisting species was, and still is, a major breakthrough in understanding biology. Initially Darwin got his ideas regarding evolution from the evidence he collected from the Galapagos Islands. Yet over time he also recognized how mankind had created many new species such as pigeons, dogs, and many crops and farm animals by breeding an originating species for desirable attributes. Furthermore, Darwin investigated embryo development and discovered that embryos show how widely separated species of animals can have a common ancestry: for example, the fact that a human embryo has a large tail and looks very similar to the embryos of turtles, birds, fish and other seemingly unrelated classes of animals. Since Darwin’s time DNA analysis has confirmed and occasionally further refined our understanding of the ancestral linkage between the species that have evolved over time.

Picture of Charles Darwin Charles Darwin

Darwin's Theory of Evolution is both an outstanding scientific theory and an irrefutable fact. It has to be ruled as an irrefutable fact because there are so many independent sets of evidence supporting the Theory of Evolution. The common biochemistry of cells, comparative anatomy, common biogeography, comparative embryology, molecular biology (DNA), genetic commonalities, direct observations, and verifying experimentation is still just a partial list of the independent arguments that can be made in support of the Theory of Evolution. Scientists no longer debate the validity of the Theory of Evolution for the same reason that they no longer argue over whether the Earth is round: there is overwhelming evidence supporting each of these facts. Yet despite the evidence, many people, particularly those holding conservative religious beliefs, still reject the Theory of Evolution.

In public debates the opposing sides have interpreted fossils as either supporting or conflicting with the Theory of Evolution. Creationists point out that the fossil record seems to contradict Darwin's ideas of gradual evolution because the fossil record often shows new species suddenly appearing and then remaining unchanged for millions of years before going extinct. Scientists countered by searching for and sometimes finding the transitional fossils showing the steps for how new species evolved from previous existing species. Nevertheless objective scientists should admit that there are far more unchanging fossils than transitional fossils and so without a new interpretation of how evolution works the fossil record appears to favor the perspective of the creationists. No worries, we will see how appearances can be deceiving.

It was not until 1972 that Niles Eldredge and Stephen Jay Gould published Punctuated Equilibrium, the landmark paper that reconciled the fossil evidence with scientific understanding. To be clear, Punctuated Equilibrium does not overthrow Darwin’s Theory of Evolution but rather it is an improvement towards better understanding how the evolutionary process works.

Difference Between Gradualism and Punctuated Evolution Graphical Illustration of the Difference Between Gradualism Evolution and Punctuated Equilibrium Evolution

According to Punctuated Equilibrium new species have the possibility of evolving when a small portion of an established species is isolated from its community. An example of this would be when a volcanic island emerges above the ocean waters and various birds, insects, and terrestrial plant seeds arrive on its shore. Without other biological completion, these early arrivers quickly evolve so as to fill whatever open environmental niches may be available. If the conditions are right, the evolution of a new species can be extremely fast relatively speaking. For example, from the arrival of just one pair of wayward birds several new species of birds may evolve in just a few dozen generations.

Such was the case for the finches that Darwin found on the Galapagos Islands. On the Galapagos Islands Darwin collected specimens representing fourteen species of birds, yet after arriving home from his voyage a colleague, John Gould, informed him that all of the specimens were finches. Apparently all of these species of birds had evolved from an extremely small number of early arriving finches: maybe a single pregnant female, a couple, or a small flock.

The time required for each reproductive cycle is a key factor in determining how fast a new species can evolve and grow. This time required for each generation varies greatly and it mostly depends on the size of the individuals. For example, the generation time of the largest animals, say humans or elephants, can be twenty years or more while at the other extreme the generation time for most bacteria is usually less than an hour. Yet even for the species that require twenty years to complete each generation, a thousand years is enough time to complete fifty generations and fifty generations is more than enough time for a new species to evolve in the way that Darwin described. Likewise fifty generations is more than enough time for the population of a new species to explode. For example, if we start with a single couple, and model this couple and all future couples as being able to raise six reproducing adults each generation, then after fifty generation we would have more than 100,000,000,000,000,000,000,000 individuals.

Once a new species evolves and has grown to being a large population it may stay in its original geographical region or it may expand into new regions. Like the initial evolution and population growth of a species, this expansion into a new and possibly much larger region can be rather fast relatively speaking. In all, a new species can easily evolve and obtain world wide distribution in no more than a few thousand years. Granted, from the perspective of our short lives a few thousand years may seem like a long time but from the prospective of geological time this is nothing. Keep in mind that the sedimentary deposits that record geological time moves at a pace that ticks away on a scale of millions of years, and so a change that occurs within only a few thousand years has the appearance of being instantaneous. Thus with this deeper understanding of how the evolution process plays out in nature it makes sense that the geological fossil record shows many new species that seem to appear almost out of nowhere.

Now that we have addressed this first geological mystery, we realize that the more challenging problem is to explain how it is that once a species evolves it usually remains mostly unchanged for millions of years. Frankly it appears that scientists have been so fixated on clarifying to the public that species evolve that it appears there is not nearly enough research investigating the factors that cause a new species to lock into its new form. While all species have the ability to evolve that does not mean that they will evolve; there needs to be vacancies in the environment to allow them to exercise that ability.

Two of Galapagos marine iguanas Marine Iguanas

To understand evolution better it first needs to be emphasized that there is no guidance to evolution. For example, just because there are obvious benefits for a species to evolve the ability to fly a species has absolutely no ability to make a choice to evolve in that directions. Instead the only way that a species can evolve in a specific direction is if this evolution can take place through numerous small steps where each of those small steps are beneficial to the species. Second, before it even gets starts evolving, a species must first overcome inherited resistance to change. For example, an individual may receive a beneficial trait and yet that individual will often fail to reproduce and propagate that trait because the individual's odd trait may be deemed as being socially unacceptable to the norms of the community.

While many biologists comment on how environmental pressures shape species, a slightly better way of stating this would be to focus on environmental opportunities. If a new ecological niche opens up then either existing species will fill this new ecological niche or new species will evolve to fill this niche. But what happens if there aren't any new ecological niches opening up? Without changes in the environment the evolutionary process stagnates. In a completely unchanging environment both the populations of the communities and the traits of the species can remain unchanged for millions of years. Conversely, the Galapagos and other volcanic islands are unique in that these newly created lands presented opportunities for the evolution of new species.

Besides geological changes creating new lands, another way that ecological niches can open up is through mass extinctions. Mass extinctions wipe the environment clean of the old species thus clearing the way for the evolution of new species.

Mass Extinctions, Evolution, and the Changing Environment

As stated earlier, the geological fossil record is divided into periods and these periods represent times of tranquility: during a period some new species will emerge while others die out, yet many if not most species remain unchanged throughout the period. Usually a mass extinction marks the end of one period and the beginning of the next.

What causes mass extinctions? This is a controversial topic. While asteroid impacts seem to get all the attention, geologists are coming around to recognizing that super volcanoes and even life itself are probably the leading causes of mass extinctions.

Super volcanoes - otherwise known as traps, flood basalts, or large igneous provinces - are not just slightly larger volcanoes but rather they are roughly a million times larger than the any of the largest volcanic eruptions recorded during human history. Possibly the largest of these super volcanoes, known as the Siberian Traps, was responsible for the mother of all mass extinctions that wiped out 85 to 95% of all the land and marine species at the end of the Permian period. Another famous mass extinction was the Cretaceous–Tertiary (K-T) extinction that wiped out the dinosaurs sixty-five million years ago. Initially there was a great deal of publicity given to the hypothesis that an meteor impact killed the dinosaurs, but decades later the evidence continues to grow in favor of the Decca Traps of India being responsible for this mass extinction.

Crowd of People in New Dehli Overpopulation

Currently the Earth is experiencing a mass extinction, and sadly we are the ones that are causing it. Human beings have become such a successful and dominating species that we now overpopulate the Earth. Currently we are rapidly using up many of the Earth's nonrenewable resources while simultaneously polluting the Earth's lands, seas, and atmosphere. The consequence of this is that we have decimated the populations of many species, including the populations of most of the other large non-domesticated vertebrates living on Earth.

While we are currently creating the most havoc, we are far from being the only species that has had a global impact on this planet's environment. From the moment life evolved, various species have had a dramatic impact on both the environment and other species on the Earth.

While the evolution of a species depends on the environment, the evolution of the environment is often depends on what has evolved: thus life and environment are interdependent. When a new species is evolving, it evolves in the direction that enables it to thrive in its environment. Yet once a species has captured its environmental niche it then has the potential to change its environment, usually without any intention of making the change. Most often this is simply because life makes use of available resources within its environment while discarding leftover substances. When the species first evolves this arrangement of using available resources while discarding waste is usually not a serious problem. But this can change once a species reaches global distribution and its population still continues to grow. If nothing changes then this highly successful species - such as human beings - can not only damage the environment and have a negative impact on other species, but it can be its own worse enemy in greatly reducing its own quality of life and even causing its own extinction.

But it does not always end this way. Sometimes a symbiotic relationship will develop where the pollution from one species or set of species may become the treasure of another. For example, initially photosynthesizing bacteria and then plants were using up the carbon dioxide while releasing oxygen. Yet soon oxygen respiring animals evolved that were using this oxygen while releasing carbon dioxide. By completing the cycle these two groups have created one of the greatest symbiotic relationships on Earth.

However, symbiotic relationships that produce no net changes to the environment tend to be less common than the species that change the environment either for better or for worse. The reason Earth's atmosphere is now so different from the primarily carbon dioxide atmospheres of its neighboring planets of Venus and Mars is because once life evolved on Earth it changed Earth's atmosphere.

Earth's atmosphere began its deviation from the path of its neighboring planets with the evolution of cyanobacteria along with all the other photosynthesizing plant life. At first it was the creation of oxygen rather than the removal of carbon dioxide and nitrogen that had the greatest impact. This oxygen was extremely important because once there was a significant amount of diatomic and triatomic oxygen in the upper atmosphere ultraviolet radiation was no longer able to reach the Earth's surface. This greatly diminished how quickly the water on Earth's surface was being destroyed. So in contrast to the dry planets of Venus and Mars, water continued to be abundant on Earth and so life on Earth continued to thrive.

The next major way that Earth distinguished itself from Venus and Mars was the removal of carbon dioxide from the atmosphere. As noted, plants remove carbon dioxide from the atmosphere as part of the photosynthesis process, and yet it is a mistake to believe that the photosynthesis process was the primary means that the Earth lost is carbon dioxide atmosphere.

Plants usually do not permanently remove carbon dioxide or, to be more precise, carbon from the atmosphere. This is because usually after plants die fungi, bacteria, and other detritivores break down and decompose the plant material. Then these plant nutrients such as carbon dioxide and nitrogen are returned to the environment. The exception to this is if the microbial decomposition process is slowed down such as when plant material falls into swamping anaerobic water. In this case, at least some of plant material converts to peat, that later, after burial and a great deal of compression hardens to become coal.

Coal is a fairly abundant resource and so this process must be responsible for some of the reduction of the Earth's carbon dioxide atmosphere. Nevertheless, the amount of carbon locked up in coal deposits is still hardly comparable to the amount of carbon that is locked away in carbonated rock.

Life is almost certain to be involved in the creation of the main types of carbonated rocks: limestone and dolomite. Life's role in the creation of limestone is obvious because of the numerous marine shell fossils that are usually found in limestone. Yet life's role in the creation of dolomite is not so obvious and in fact the process of how dolomite rock formed is still very controversial among geologists. Nevertheless, it does appear that bacteria played a role in the formation of dolomite. What forms of life created each of these main forms of carbonated rocks turns out to be important since it tells us something about how fast these sedimentary deposits were being created: it appears that limestone formations are laid down at a faster pace than dolomite formations.

But how do we know that limestone creation occurred at a faster pace than dolomite creation, and why is this important? While geologists find layers of dolomite going back nearly as far back as the evolution of life - about four billion years - most limestone is a byproduct from the creation of shells and so most limestone was laid down only during the half billion years. Yet within this shorter amount of time as much if not more limestone was created than all of the dolomite that was created. So we know from this that the creation of carbonated rock occurred at a faster pace during the Phanerozoic eon: the last 544 million years. This is important because it means that the removal of carbon dioxide from the atmosphere was occurring at much faster pace during the last billion years.

Because the atmosphere consisted mostly of carbon dioxide throughout the Phanerozoic eon, the thickness of the Earth's atmosphere fluctuated as a function of whether there was a net increase or decrease in the amount of carbon dioxide in the atmosphere. So far we recognize that the creation of marine shells speeded up the rate of carbon dioxide removal, yet this is only half the story since we still need to know something about the volcanic activity that was adding carbon dioxide into the atmosphere.

Release of Vocanic Gas Over Time Initially volcanoes on terrestrial planets released mostly hydrogen and helium. Then latter the heavier gases - water and CO2 - became the more dominate gasses being released by volcanoes.

It is extremely difficult to factually prove or to even estimate how much carbon dioxide was released from volcanoes over time. In the Earth chapter, the difficulties in estimating the current global output of volcanic gases were explained, yet imagine now the additional difficulties of trying to make the same types of estimates throughout geological time based on nothing more than the geological rock evidence. After we recognize that current estimates were off by a couple of magnitudes it leaves us a bit hesitant at even guessing at what the answers might be for the output of volcanic carbon dioxide over geological time. At best, we can argue that since the tidal heating causing the release of gas molecules from Earth's interior was consistent over time, that the volcanic output of carbon dioxide must have been more or less steady over time.

What does this all mean? If life never evolved on Earth then the atmosphere of the current Earth would be similar to, yet much thicker, than the primary carbon dioxide atmosphere of Venus. Yet because life did evolve on Earth and because various forms of life facilitated the removal of carbon dioxide from the atmosphere, today's atmosphere is much thinner and it is almost completely depleted of carbon dioxide. Furthermore, because of the occasional mass extinction diminishing life and thereby momentarily shutting down the carbon dioxide removal process, the progression from the Earth having an extremely thick carbon dioxide atmosphere to today's relatively thin atmosphere was not at all a steady process. This is seen in the geological record showing the Earth fluctuating between thick atmospheres producing warm uniform global climates and thin atmospheres producing ice age climates.

Correcting the Eras

The Phanerozoic eon begins about 544 mya (million years ago) with the Cambrian explosion of life and takes us up to the current time. Two of the greatest mass extinctions, the P-T mass extinction at about 245 mya and the K - T at 65 mya divided the Phanerozoic eon into three eras: the Paleozoic, Mesozoic, and Cenozoic eras. Considering the root meaning of these words Paleo - zoic means oldest - animals, Meso - zoic means middle - animals, and Ceno - zoic means new - animals. Most geologists would go farther to make a connection between each of these eras and the dominate group of animals living during that time. From this perspective the Paleozoic era is the age of fishes, the Mesozoic era is the age of dinosaurs, while the Cenozoic era is the age of mammals. Of course this is a simplistic perspective since actually fishes thrived throughout nearly all of the Phanerozoic eon while mammals and mammal like animals existed even before the Mesozoic era began. It was only the dinosaurs that were limited to just their era, the Mesozoic era.

Besides dividing the Phanerozoic eon into three eras based on two major mass extinctions and the major changes in the fauna that inhabited the Earth, we could also look at changes in the global environment as well.

Our current climate is actually not the norm for the Phanerozoic eon. We are currently in an interglacial moment between ice ages, a cycle of numerous ice ages that have been going on for the last few million years, if not much longer. Besides the present thin atmosphere ice age climate there are at least two other glacial times within the Phanerozoic eon. The first one occurred near the end of the Ordovician period and the other one occurred during the late Carboniferous and Permian periods. Other than these times, most of the Phanerozoic eon had a rather consistent mild climate. For most of the Phanerozoic eon there was only slight temperature variations between the equator and the polar regions, sea level and mountain summits, or even much of a temperature difference between day and night. For most of the Phanerozoic eon every day was simply balmy.

With few exceptions, these two distinct global climates match up well with the Phanerozoic, Mesozoic, and Cenozoic eras. As a first review, breaking the Phanerozoic eon into three parts does make sense since the entire Mesozoic era was a warm thick atmosphere climate. But if we desire a more accurate picture of Phanerozoic eon based on these dramatic climate changes then we need to make further adjustments. First we could insert a new small era to account for first thin atmosphere ice age in the Phanerozoic era that occurred between 450 and 420 mya during the late Ordovician / early Silurian periods. Next we need to insert another era at the end of the current existing Phanerozoic era to account for the late Carboniferous / Permian thin atmosphere ice age that occurred roughly between 300 and 252 mya. Then finally we might consider moving the division line for the last thin atmosphere ice age, the current ice age, to start at a later time than at the end of the Cretaceous period so that it more precisely marks the change in the global climate. However, this last move may stir some controversy. While it makes more sense to mark the eras according to the global climate, these changes in the global climate are often gradual changes occurring over tens of millions of years and they do not always match up with the dates of major mass extinctions that mark the end and beginnings of each period.

Idenification of New Era based on Climate Eras need to be matched up with the thick atmosphere or thin atmosphere global climates.

Features of the Thick and Thin Atmospheres

Within the Phanerozoic eon we can identify at least three thick atmosphere eras and three thin atmosphere eras. In addition to times when the atmosphere was extremely thick or relatively thin there are also the transitional periods between these extremes. Three times during the Phanerozoic eon, roughly during the Ordovician, Carboniferous and the Cretaceous / Paleogene periods, the atmosphere transitioned from being extremely thick to being relatively thin. With a massive amount of carbon dioxide being removed from the atmosphere we would expect to see large carbon deposits during these times and indeed that is the case. During the Ordovician period we find large limestone deposits in the eastern United States and elsewhere around the world. The Carboniferous period is named for the numerous coal and carbonated limestone deposits during this time. Likewise the Cretaceous is named for the chalk deposits which are formed from microscopic calcium carbonate skeletons and shells of marine animals. Going the other way, the atmosphere transitioned from being relatively thin to being extremely thick only during the times when shell creating marine animals were all but wiped out. With hardly any animals facilitating the removal of carbon dioxide from the atmosphere, the volcanoes continue to pump carbon dioxide into the atmosphere causing the atmosphere to transition from being thin to thick.

We can summarize our expectations for the two types of atmospheres and the transitional periods between the two as follows:

Characteristics of a Relatively Thin Atmosphere

Ice Age Landscape Ice Age Landscape

1. Carbon dioxide levels nearly depleted.

2. Because the atmosphere is thin, there are three atmospheric convection cells in each hemisphere instead of just one. This produces regional climates that are different from the much thicker atmosphere when there is just one convection cell in each hemisphere. Most noticeable differences are that there is ice at the polar regions and deserts between the twenty to thirty degree latitudes.

3. Since white snow reflects solar radiation the accumulation of some snow encourages the accumulation of more snow. This continues until the glaciers move all the way down to the middle latitudes where it can no longer sustain itself in the warmer latitudes and so then the glaciers finally retreat. Thus we see numerous back and forth glacier movements during these times.

4. Because of all the water that is stored in the glaciers, the back and forth movement of the glaciers has an up and down effect on the sea levels; the sea levels drop to their lowest point when the glaciers are extended the farthest down into the middle latitudes.

5. The up and down movement of the sea level produced numerous coal bearing cyclothems deposits.

6. Terrestrial vertebrates of the second thin atmosphere time - the late carboniferous and Permian periods - cannot be larger than the current terrestrial vertebrates.

7. Current flying vertebrates cannot grow nearly as large as the vertebrates of the Mesozoic era when the atmosphere was much thicker.

Characteristics of a Thick Atmosphere

1. Carbon dioxide is the most abundant gas in the atmosphere.

2. No ice at the poles or on mountain peaks.

3. Nearly the same temperature everywhere regardless of latitude.

4. The possibility of terrestrial vertebrates as large as whales.

5. Terrestrial vertebrates evolve a distinct shape of having a strong fish like tail and rear legs much larger and stronger and than their forward legs. The reason dinosaurs had these features is because they enhanced their likelihood of surviving by enabling them to move faster through the extremely thick atmosphere.

6. The much higher air density makes it much easier for vertebrates to achieve flight. This enables even low metabolism reptiles of the Mesozoic era to achieve flight and to even become the largest flying vertebrates that ever existed.

Characteristics of a Thin Atmosphere Transitioning to a Thick Atmosphere

1. Transition followed a major mass extinction that for several millions of years wiped out the animals that were removing carbon dioxide from the atmosphere.

2. No forming of limestone or coal deposits because most of the animal and plant species that usually facilitate the formation of these deposits are now dead.

3. As life recovers dolomite formation is the first to show up and then millions of years later limestone formations finally reappear. This is because the bacterium that facilitates the formation of dolomite is a much simpler life than the more advanced shell creating marine animals and so much more time was required for the limestone forming marine animals to recover.

Characteristics of a Thick Atmosphere Transitioning to a Thin Atmosphere

1. Massive carbonated rock deposits.

2. An atmosphere that is a quarter or half as thick as before is still more like a thick atmosphere than a thin atmosphere, so throughout most of the transition the climate tends to be more like a thick atmosphere climate.

3. As the transition approaches the thin atmosphere condition the size of terrestrial animals decreases.

4. An atmosphere that is only a few times thicker than the present is still a huge benefit in enabling larger animals to fly, and so almost right up to the end of the transition we are still able to find some exceptionally large flying vertebrates although these flyers are birds rather than the less capable flying reptiles.

5. As always, underwater volcanic activity is pumping massive amounts of carbon dioxide into the atmosphere and yet the abundance of marine life is removing the carbon dioxide at a much faster rate. Thus we see an abundance of carbonated rock deposits throughout the transition as the atmospheric carbon dioxide levels drop.

Timeline: Making Sense of the Geological Evidence

We can now look at a timeline for the major events of Phanerozoic eon based on our understanding of Earth having either a thick or a thin atmosphere:

Timeline for Events with Changing Atmosphere thickness The numbers 1 to 14 refer to key moments in the plot of the atmosphere's thickness as a function of time. Most of these fourteen events have puzzled paleontologists but now they make sense in the context of understanding if the Earth's atmosphere was thick or thin or transitioning from one of these global climates to the other.

1. At the start of the Phanerozoic eon the atmosphere is extremely thick.

2. Carboniferous period is named for the abundant coal deposits formed during this time.

3. The Pennsylvanian period is a thin atmosphere time.

4. Most of the Permian period is part of the late Carboniferous / Permian thin atmosphere era.

5. The volcanic emissions of the Siberian Traps that produced the P-T mass extinction actually started during the late Permian period and it went on for millions of years.

6. After the P-T mass extinction wiped out nearly all life on the planet there was no life to remove the carbon dioxide that was still being exhausted from the oceanic volcanoes. The atmosphere transitioned back to being a thick atmosphere as carbon dioxide filled the atmosphere.

7. In the oceans, bacterial life recovers and begins converting carbon dioxide to dolomite. This bacterial life creates the Dolomite of the Alps during the last half of the Triassic period.

8. Another major mass extinction occurred at the end of the Triassic causing the atmospheric carbon dioxide levels to rise even further during the early Jurassic period.

9. Dinosaurs that began evolving during the Triassic reach their greatest size during the late Jurassic period. This is also the same time that the thickness of Mesozoic atmosphere reached its peak.

Glaciation During Phanerozoic Eon Ice Ages occur when the atmosphere is thin because when the atmosphere is thin it is not nearly as effective at transferring heat from the equator to the polar regions.

10. During the Cretaceous period marine life is fully recovered, thus once again shell-creating marine animals start producing massive amounts of limestone, most notably example is the chalk that makes up the White Cliffs of Dover.

11. The Cretaceous period ends with the K-T mass extinction that eliminates the dinosaurs.

12. The atmosphere was still fairly thick since it was still only midway in its transition from thick to thin, and so this atmosphere was still able to reduce the effective weight of terrestrial animals. When new mammals evolved so as to replace the dinosaurs some of these new mammals grew to being nearly as large as the dinosaurs they replaced. Most notable is the hornless rhinoceros Paraceratherium, also known as the Indricotherium or Baluchitherium.

13. Near the end of the Tertiary period the atmosphere becomes thin enough that ice begins to form at the poles.

14. For the last 2.6 million years we have entered the present age of having a relatively thin atmosphere. This is an Ice Age. The Earth's atmosphere is relatively thin and so the Earth's climate oscillates roughly every hundred thousand years between glacial ice ages and warmer interglacial times such as what we are currently experiencing.

External Links / References

Types of Rocks

Sedimentary Rock Formations

Dating Rocks


Theory of Evolution Evidence

Punctuated Equilibrium

Mass Extinctions

Volcanoes Causing Mass Extinctions

Current Mass Extinction

Ice Ages

Atmosphere Thickness Timeline

Comments, Questions, and Answers

Selected comments and questions are given with the permission of the parties involved.

Wouldn't wing design be somewhat different in a thick atmosphere?

Scott McNay

January 2018

Hi Scott,

Yes! Many of the feathered dinosaurs were actually flying feathered dinosaurs. Certainly many more than what paleontologists currently believe. The fossil evidence is there, yet the paleontologists are struggling with the idea that such large animals could fly because they assume that the Mesozoic atmosphere had a relatively thin atmosphere similar to what it is today. Because the atmosphere was thicker the wings did not need to be nearly as large, and because the center of mass of these animals was closer to their rear legs most of these flying feathered dinosaurs had wing feathers on their legs in addition to the wing feathers on their forearms.

This will be explained in further detail in the next chapter: Dinosaurs and Dragonflies.

Please send an e-mail if you have questions or comments.

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