Badlands Fossil Blog

Evolution

2009-10-09T06:36:00.000-07:00

If there is any topic that can beat climate change as a controversial dinner topic, it’s evolution. I’ve spent 8 years of my life, in one way or another, devoted to the study of evolutionary biology. It was in my undergraduate geology classes that I had my first real discussions about evolution. I didn’t realize then what a huge topic it was. I wonder if that’s because it wasn’t such a hot topic then, or if it just wasn’t talked about in my high school in TN. But it seems that it is discussed everywhere now, and some of the people talking are giving bad information.

Evolution is a huge topic. So huge that there is no way to cover its many dimensions without writing a book. So I want to talk about why I accept the scientific theory of evolution. I want to explain what evolution is, but perhaps more importantly, what it isn’t. There are several misconceptions about evolution, so it is important to recognize and understand why they are wrong. I’ve heard them numerous times from numerous sources.

The best online resource that I have seen explaining evolution is the “Understanding Evolution” website created by the University of California Museum of Paleontology. I can’t stress enough what a wonderful website this is! One of its many resources is a list of the most common misconceptions about the theory of evolution and the way it works. My list below is inspired by the most common misconceptions from their list that I have encountered in my years of study and defending the theory of evolution.

Misconception: Evolution is a theory about the origin of life
The theory of evolution involves the study of how life has changed after its origin, not an explanation of the origin of life itself. Other areas of science do make attempts to explain the origin of the universe or of life (Big Bang theory, experiments creating proteins from inorganic compounds, etc.), but these are not addressed by the theory of evolution. Evolution studies not the origin, but the branching off of life after its start.

(Image from Wikipedia: Famous Miller-Urey experiment simulating conditions of an early Earth, resulting in the creation of organic compounds from inorganic compounds)

Misconception: Evolution involves the progression of organisms into better, more perfected life forms
This misconception springs from a misunderstanding of the role natural selection plays in evolution. During the process of natural selection, some individuals have genetic traits that improve their chances to survive and reproduce, during which these genes are also passed down to their offspring. As these traits continue to be passed down among offspring, they become more common, weeding out the less advantageous traits.

But no organism is perfect. Various organisms (some fungi, mosses, crayfish, and sharks) have remained relatively the same over a very long period of time. They are not marching up a ladder of progress, as they are fit enough to survive and reproduce without further adaptations. Other organisms have changed and diversified a great deal, but this still doesn’t necessarily make them better. Organisms are adapted to the environment that they live in, but if the environment changes these adaptations may not prove to be as useful as they once were.

(Image modified from http://www.billbrouard.com/darwin.htm)

Misconception: Evolution occurs solely as the result of change due to random chance
Random mutation is the primary source of genetic variation, so there is some truth in saying that life changed “by chance.” But, it’s only a very small part of the evolution story. Natural selection drives evolution, and that process is not random. Natural selection is the result of the complex interplay of living organisms trying to survive in the environment they have been born into. For example, the different beak shapes of the Galapagos Finches often are used to illustrate differences in species, and how the beaks relate to the finches’ environments and food sources. Chance mutations give them the traits or skills, and then it’s up to the organism to survive. If they are well adapted to their environment, they will pass on their traits. If not, they face extinction. (Image modified from http://www.news.harvard.edu/gazette/2006/08.24/31-finches.html)

So what is evolution?
The short answer: biological evolution is descent with modification. Evolution is sometimes described as change over time, which is in part true. But lots of things change that are not examples of biological evolution. The distinction must be made that evolution is change caused by the inheritance of genetic traits. Natural selection then determines which traits are fit for the environment at the time. The central idea to biological evolution is that the diversity of life on earth has changed over time and that different species share common ancestors. The relationships between organisms can be shown on evolutionary trees. These trees are used to classify and understand the organisms.

To explain the lines of evidence for evolution is out of the scope of this post. I accept the theory of evolution because I have read the scientific articles, been in the field to understand geology, rock sequences, and collect fossils. And everything makes logical sense to me. I understand the dating techniques that give the earth an age of 4.6 billion yeas old. If changes in genetic variation can be seen in as short as a few hundred years, think of what can happen in millions of years, with the changes in climate and environment that the earth is known have undergone.

The group that I work on, the Leptomeryx, can offer a great example of this. Their lineage can be traced through time, with the primitive species changing the morphology and size of both their teeth and bodies, potentially in response to a changing environment. Researchers Timothy Heaton and Robert Emry proposed an evolutionary history where the species Leptomeryx yoderi probably gave rise to all later species. A speciation event gave rise to Leptomeryx speciosus and Leptomeryx mammifer. Leptomeryx speciosus then appears to have undergone gradual changes resulting from mutations such that the ancestor population went extinct, with a new species, Leptomeryx evansi, taking its place. This hypothesis is consistent with the fossils found in a specific sequence in the rocks, as well as looking at the different patterns on the teeth. Though no cladistic analysis has been done for the Leptomeryx, their evolutionary tree might look something like the image I have drawn, shown below.

Numerous other examples exist, some with support from many different scientific methods, be it paleontology, genetics, geology, or chemistry. The most important thing to remember when discussing evolution, in my opinion, is to not take what you are told at face value (even what I say!). Research the material yourself to gain an understanding of the arguments, keep an open mind, and decide for yourself what your opinion will be.

What is a Species?

2009-10-06T12:38:00.000-07:00

Speciation and adaptation are two concepts in evolutionary biology that are often confused and misunderstood. One, speciation, explains the how new species arise. The other, adaptation, refers to the acquisition of new structural, functional, and behavioral traits that allow the organisms to be more reproductively successful. They are related and work together, but are two different concepts. Understanding these concepts can help us to understand the changes our planet and the life on it has seen these past years.

Last year I took a class that might have been the hardest one I have taken in graduate school to date. It was called “Systematics in the Fossil Record.” The entire purpose of the class was to discuss, in a nutshell, how paleontologists classify and group organisms based on their evolutionary relationship to other organisms. Broadly speaking, this is known to paleontologists and biologists as “phylogenetics.” Phylogenetic systematics studies the diversification of life, both past and present, and the relationships among living things through time. These relationships are then shown on evolutionary or phylogenetic trees.

One day in class we had a discussion about species. Interestingly, this topic was one of the most difficult to grasp, the one that made me realize something. When asked what a species was, I gave the answer I had always heard growing up. Species are groups of organisms that share similar features and produce viable offspring. Well, turns out it’s a lot more complicated than that. There are several different concepts of what defines a species. Some are based exclusively on physical appearance. Others are based on reproduction. With all of these different concepts, how do you answer the question: What is a species?

For most purposes, the “Biological Species Concept” is used to define the word. According to the biological species concept, species are groups of interbreeding natural populations that are reproductively isolated from other such groups. It’s this isolation that is the key to this concept. Today, certain hybrids between animals exist, such as the “Liger”-a hybrid between a tiger and a lion. These animals only exist in captivity because in nature, these animals are from different regions. Under natural circumstances, these animals do not breed. So, according to the biological species concept, lions and tigers are two different species. Ligers themselves do not have a scientific species name because they are considered to have human assisted ancestry, and ligers are generally infertile.

Isolation can be either geographic in nature, such as mountain or an ocean, or could be simply because the animals develop different mating habits, or are not anatomically compatible, and therefore do not mate and reproduce offspring with similar groups. Either way, the populations’ hereditable traits, or genes, are isolated from similar groups.

Each group will experience different environments, and will adapt to them in their own, unique ways. Adaptations are features that are common in a population because it provides some improved function. Because adaptations improve the quality of living, the organisms with these adaptations have a better chance of surviving. For instance, at some point in their evolution, horses with higher crowned teeth were better suited for their environment, and therefore survived to pass on their genes. High-crowned teeth are an adaptation that allowed horses to eat grasses. The Leptomeryx that I study have crenulations and extra enamel that I believe are an adaptation to eating tougher plants.

Given enough time and enough variation of adaptations among the groups, new species may arise. This process is known as speciation.

Speciation is the biological process by which new species arise. These speciation events require specific THINGS to happen:

• First, you have a single species, a population of naturally interbreeding organisms.

• Next, some genetic mutation must occur and spread through part of the species, with the bearers of this mutation only breeding with bearers of the same mutation; the two gene pools are isolated from one another.

• The initial population is now split into two separate, though related species. The two new, isolated interbreeding populations experience different conditions and different random events, and evolve different adaptations as a result.

• Given enough time, the gene pool of each becomes distinct from its ancestor, and the two populations would not be able to reproduce with each other even if they were reintroduced.

It’s sometimes hard as a paleontologist to rely on the Biological Species Concept, because you can’t really know if species are interbreeding or not. This is when phylogenetic systematics, or cladistics as it is sometimes called, comes into play. Cladistics is a method of hypothesizing relationships among organisms by constructing evolutionary trees.

To do this, a paleontologist will base their hypothesis on a set of traits or characters possessed by the organisms. These characters could be anatomical and physiological characteristics, behaviors, or genetic sequences. In cladistics, the more characters two organisms share, the more closely they are related. The result of a cladistic analysis is a tree, which represents a supported hypothesis about the relationships among the organisms.

For example, in the picture below, 7 groups are analyzed based on 6 traits. The tree shows that amphibians are equally related to both primates and dinosaurs. The tree also shows that primates and dinosaurs share a more recent common ancestor than either does to amphibians. The tree shows both the relatedness and splitting of lineages of the organisms involved.

(modified from http://evolution.berkeley.edu/evolibrary/article/0_0_0/phylogenetics_07)

In the class I mentioned above, I preformed a cladistic analysis on 7 known Leptomeryx species. In general, cladistic analysis involves examining hundreds, if not thousands of teeth to identify what species they belong to and what traits they have. Character lists can be extremely long, especially if you are looking at numerous species. My analysis was a very simple experiment, using characters other researchers had identified. And it still took days, weeks, months.

Understanding speciation events and the adaptations that new species evolve can make the study of diversity and the relationship of organisms an easier task. The methods scientists use are very thorough, and well supported by fossil evidence. Discussing evolution is a tricky topic, but I think it’s important to know some of the key concepts behind it in order to make educated opinions on the matter.

Fossil Teeth and Diet

2009-10-02T07:52:00.000-07:00

Have you ever wondered how paleontologists are able to say what animals in the past were eating? Well, there are many different methods they use. Sometimes they observe the plant fossils that are found in association with the animals (or, what other animals were around to be prey for the meat-eating folks), and make assumptions based on the availability of food sources. Other times, they make inferences based on the teeth of the animals. “The present is the key to the past.” That is one of the first things I was taught in geology. Observing the feeding behaviors in modern mammals and comparing them to tooth morphology, or shape, is one of the best ways to determine the relationship between teeth and diet.

Animals are often categorized based on food preference. For instance, carnivores eat meat, while herbivores eat plants. Herbivorous mammals can be often classified based on which type of plants they prefer to eat. Plants are variable in their shape and chemical composition, leading to different adaptations in the anatomy and behavior of herbivores. Herbivorous mammals are generally lumped into one of two categories: those that eat primarily grasses (Grazers), and those that eat primarily leaves, shrubs, and flowering plants (Browsers). Each plant type has its own positive and negative aspects as a food source, and the herbivores that eat these sources are marked with distinguishing characteristics as a result.

A grazer’s diet consists of as much as 90% grasses. Grasses tend to have thick cell walls, which are tough and require a large amount of energy to break down. These cell walls contain plant fibers known as cellulose, which digest slowly. Grasses also have chemicals that act as a defense for the plant. They have a high concentration of silica, or glass, which increases the wear on herbivores’ teeth. Grasses are a relatively consistent food source, with the leave, stem, and fruit of the plant being indistinguishable to the grazing animal.

A browser’s diet is primarily made of leaves, succulent plants, and fruits, collectively known as browses. Browses have thin cell walls which contain the indigestible fiber known as lignin. But within the cells are compounds like sugars and proteins that are completely digestible. Browses, like grasses, have defensive chemicals. These chemicals reduce protein and dry matter digestibility. Unlike grasses, browses have a variety of parts that are easy for the browser to distinguish. These different plant parts (young buds, mature leaves, stems, and fruits) each have different nutritional values.

Modern grazers and browsers have been studied in detail, especially the hoofed mammals, or ungulates. Key traits have been identified that can be used to determine which of the food sources make up the animals primary diet. These observable characteristics from modern mammals can then be examined on fossils to try and determine diet. These types of studies are great for determining past ecosystems and the relationships between animals and plants in our Earth’s history.

Shape/Height of teeth
One of the most recognizable differences between grazers and browsers is in the structure of their molars. Grazers tend to have higher crowned teeth (more tooth above the gum, with a short root), allowing for longer wear. This adaptation allows them to eat the extremely abrasive grass throughout their lives. A grazers tooth will often be modified with complex ridges designed for grinding down grasses. Browser teeth in contrast generally have less complex, short crowned teeth (less of the tooth above the gum with a longer root). This is because the leaves and shrubs are softer, easier to break down, and do not cause as much wear on their teeth.

Microwear patterns
Microwear analysis is used to identify wear features on enamel surfaces by the way they reflect light when viewed under high powered microscopes. These microscopic wear features will be different depending on what types of plants the herbivore eats. Wear features are commonly classified as scratches, pits, or gouges. In general, grazers are characterized with a higher concentration of scratches, caused by the highly abrasive grasses they are eating. Browsers, on the other hand, have fewer scratches but might have more pits and gouges from eating seeds and stems.

Carbon isotope signature
The element carbon, like oxygen, has different isotopes (C-12 and C-13). Different carbon isotopes are taken in by plants during photosynthesis, the process by which plants convert carbon dioxide into a food source. Browses use a different type of photosynthesis than grasses, leading to a difference in the carbon isotope signature between the two types of plants. Trees and shrubs generally have a lower carbon isotope signature than grasses. When an herbivore eats these plants, the carbon isotope composition is recorded in their teeth. Therefore, fossil teeth of a browser eating trees and shrubs will have a lower carbon isotope value (around negative13) than a grazer eating grasses (around positive 1).

For example:

During the course of their evolution, the teeth of horses increased in crown height. Early horses were small, with simple, short-crowned teeth, indicative of browsing. Modern horses have much higher-crowned teeth, and are known to be grazers. Recent isotopic work by Bruce MacFadden has shown this change in diet. The earliest horses have carbon isotope values indicating browsing feeding behaviors. Fossil teeth from later horses have intermediate carbon isotope values, suggesting that they were mixed feeders, eating both grasses and browses. The more advanced horses, including the modern species, have carbon isotope values reflecting their almost exclusive diet of grasses. Microwear analysis also reflects these changes.

People have been collecting fossils for thousands of years. They've been used by ancient cultures as jewelry and adornment, tools, and possibly even as a form of currency. Kids of all ages enjoy looking at their collections and imagining what the animal would have looked like when it was alive, how it might have acted. I was one such kid! One thing that graduate school has made me realize is that fossils are most exciting when they can be used to learn something about ancient environments and ecosystems. These methods are just the tip of the iceberg of what paleontologists can do!

Scientific Articles:

MacFadden, B.J. 2005. Fossil Horses-Evidence for Evolution. Science 307, p. 1728-1730.

Solounias, N. and Semprebon, G.M., 2002. Advances in the reconstruction of ungulate ecomorphology with application to early fossil equids. American Museum Novitates 3366, p. 1–49.

My love, the Leptomeryx

2009-09-29T08:08:00.000-07:00

When I started graduate school, I felt pretty intimidated. Why? Well, I was a geologist. I knew rocks. I knew a bit about crinoids, commonly known as “sea lilies,” since that was the organism I did my senior research project in. I knew I wanted to be a vertebrate paleontologist, and study animals with bones (nothing at all against invertebrate paleontologists! I just like the fuzzy, cute variety of animals!). But I didn’t really know anything about vertebrates.

One of the first paleontology classes I took, thankfully, was simply titled “Vertebrate Paleontology.” For a class project, we were given the freedom to pick an area of interest and investigate a question related to a particular animal group, or subject, pretty much whatever we wanted. As mentioned in a previous post, I discussed with my advisor potential projects, and started work on a project involving the genus Leptomeryx. The project didn’t quite work out by the end of the semester, but showed a lot of potential to answer some interesting questions about climate change, mammalian evolution, and the relationships between animal diet and tooth morphology, or shape. I decided to keep working on it, and eventually the Leptomeryx project evolved into my Master’s thesis.

Unfortunately, it’s hard to talk about exactly what a Leptomeryx is. The group has been described as “deer-like,” but they are not actually related to deer at all. Some scientists believe that they are part of a larger group that includes the modern day “mouse deer”, making these very small mammals’ cousins to Leptomeryx. The hornless mouse deer are also not true deer, the males of which grow and shed new antlers each year.

(Image from http://latimesblogs.latimes.com/unleashed/2009/03/lesser-mousedee.html)

In my mind, whenever I think about Leptomeryx, I envision Bambi. Only, Bambi grew up, and got bigger. The Leptomeryx never got past “Bambi size.” Most species averaged about 17 pounds, though one species, aptly named Leptomeryx mammifer, might have been around 50 pounds. In relation to animals we are familiar with today, Leptomeryx were probably somewhere between the size of a rabbit and a medium-sized dog.

Leptomeryx first appeared in the middle Eocene, around 41 million years ago, and became extinct in the early Miocene, around 18 million years ago. This means that this group existed both before and after the Eocene-Oligocene Transition that occurred about 30 million years ago. Researchers have shown that the climate change actually had little effect on most of the mammal groups at the time. The Leptomeryx were one of the three groups that did show significant changes over the boundary. That makes these little guys special.

(Leptomeryx evansi, the dominant species after the transition. Image from http://www.flickr.com/photos/48435373@N00/2222859479/)

What the Leptomeryx did that is so different than other mammals is gradually change the morphology, or shape, of their teeth. As one of my old professors once said, being a vegetarian is tough to do. There are many specializations an animal must be equipped with to deal with eating plants, which are tougher on their teeth than meat would be. For instance, grass actually contains strands of silica, or glass. This wears down the teeth of plant-eaters over the course of their life. Plant-eaters, or herbivores, have come up with some interesting ways of dealing with this. Horses, for example, evolved higher crowned teeth. These longer teeth are better suited to eating grasses. Because grasslands were starting to spread, horses with these types of teeth were better adapted to their environment, and these were the horses that survived to give rise to our modern horses.

Leptomeryx did something a little different than the horses. Around the same time as the Eocene-Oligocene Transition, the enamel of Leptomeryx molars started to become increasingly complex. Early, primitive species of Leptomeryx had very smooth enamel, while in the later, more advanced species, the enamel became more wrinkled. An extra fold of enamel, known as the Palaeomeryx fold, is also present on later species.

Like the changes in the teeth of horses, these changes in Leptomeryx are also correlated with a change in environment. In the areas where Leptomeryx fossils are found, a change in the ecosystem is seen in the plant fossils. Areas that were once moist forests gave way to dry woodlands and finally open grasslands. These more complex patterns seen on the later Leptomeryx were perhaps a response to eating the tougher vegetation that resulted from the climate change.

There has been a great deal of research on the links between diet and tooth morphology, between what animals eat and what their teeth look like. By studying these relationships in the present, paleontologists are able to make hypotheses about the ecosystems of the past. This is one of my favorite aspects of paleontology, and one you'll probably hear more about later!

Proxies

2009-09-26T08:14:00.001-07:00

In my last blog I talked about climate change, and used one instant in history (the Eocene-Oligocene Transition) to illustrate some of the dramatic effects climate change can have on our planet. Today, I wanted talk about some of the evidence for that particular climate change and how scientists look at certain things, referred to as proxies, to help study and understand climate change.

So, proxies... In climate research, proxies are measurable attributes used to infer the value of another attribute of interest. A non-climate related example would be using tree-rings to infer the age of a tree. The tree rings are not in-and-of themselves the age, but by counting them, you can come up with a very good approximation of the age of the tree. The rings therefore, are a proxy for age.

In climate studies, ocean drill core proxies are usually the most direct way of making inferences into past climate. A core sample is obtained by drilling into rock or sediment with a hollow steel tube. Thousands of cores have been drilled from all of Earth’s oceans, as well as many bodies of water on land. The core is removed from the tube in the laboratory, and inspected and analyzed by different techniques and equipment depending on what question is being asked. In a core, one can see changes in the rock or sediment composition, evidence of plants or pollen, ash layers from volcanic eruptions, or different species of very small micro-organisms. In my opinion, one of the most important proxies for temperature reconstructions is the ratio of oxygen isotopes found in drill cores.

So what exactly are isotopes, and why are they used as a proxy for temperature? All of Earth is made up of atoms of different elements (oxygen, carbon, nitrogen, etc.). Every atoms are made up of three sub-particles: protons, electrons, and neutrons. Isotopes are different atoms of the same element, each having a different number of neutrons. The difference in the number of neutrons will slightly alter the weight of each isotope relative to each other. For example…..

Most elements exist primarily in one state: 99.8% of all the oxygen on Earth is found in the form of O-16. This means that this isotope has 8 protons and 8 neutrons. However, a very small percentage of Oxygen exists as O-18, with 2 extra neutrons. These extra neutrons make the O-18 isotope heavier relative to O-16 atom. This becomes important when looking at temperature.

The way to use oxygen as a proxy for temperature is simply by examining and slightly expanding on the concept of the water cycle. Water is in the ocean (or a lake or stream), it evaporates and forms clouds, and eventually it rains down to rejoin bodies of water. Sometimes the precipitation falls as snow, to accumulate as glaciers or ice sheets. It is easier to evaporate lighter isotopes, so the oxygen in water vapor is mostly O-16. This leaves the water in the ocean with a higher concentration of O-18 compared to O-16. When the Earth is particularly cold, the lighter oxygen (O-16) gets locked in ice sheets, leaving the ocean even more enriched in O-18 relative to O-16.

The shells of tiny animals and corals are typically made of calcium carbonate (CaCO3). They use the oxygen in the water to form their shells. When they die, their shells fall to the bottom of the ocean, and can be recovered from drill cores. Because these organisms record the relative abundance of oxygen isotopes in their shells, the isotopic signature can be used as a proxy for temperature.

Oxygen isotope curves show the abundance of O-18 relative to O-16 (designated δ18O). So, whenever the earth is colder (an icehouse earth), an abundance of O-16 will be locked in ice sheets, and the ocean will have a higher ratio of O-18 to O-16. The organisms’ shells, and therefore the isotope record, will have higher δ18O values. Conversely, when it is warmer (a greenhouse world), the organisms’ shells and isotope record will have lower δ18O values. This is because the lighter isotopes are not frozen and return to the oceans.

Make sense? Yeah, it confuses me a lot too. But, the simple answer is this: Increase the temperature, decrease δ18O values. Decrease the temperature, increase δ18O values.

The isotope record presented by James Zachos and his colleagues (pictured above) shows the dramatic climate shift over the past 65 million years brilliantly. This figure from their 2001 paper may seem a little complex, but on the left you can see how the record shifts from lower values (around 0 or 1) to higher values (around 4 or 5). I’ve circled the Eocene-Oligocene transition to demonstrate just how dramatic of a change it really was. On the right, in the red circle I’ve pointed out some of the effects this change had on the organisms alive at the time, as I talked about in the last blog entry. As you can see, the climate is constantly shifting, and organisms are either forced to adapt or face extinction.

So what about fossils from land? While alive, animals drink water. Their teeth record the isotopic signature of their drinking water much in the same way that the tiny organisms in the ocean record the ocean’s oxygen isotope ratio in their shells. This isotopic signature can be used to determine temperature as well. Alessandro Zanazzi and his colleagues collected hundreds of teeth from four of the most abundant White River mammals from Nebraska, South Dakota, and Wyoming: Mesohippus (an ancestral horse), Merycoidodon (a sheep-sized artiodactyl related to ancestral camels), Leptomeryx (a small deer-like artiodactyl) and Subhyracodon (an ancestral rhino). Using these fossil teeth, the scientists looked at their oxygen isotope composition to come up with an oxygen isotope curve. What is special about this curve is that it addresses the changes occurring on land, versus the ocean. They were able to determine that the temperature dropped approximately 15°F in central North America during the Eocene-Oligocene Transition.

Isotopes aren’t the only evidence we rely on when making these paleoclimate reconstruction. Fossil plants also play a big role, since the relationship between temperature and plant life is fairly well understood. Even looking at the proportions of reptile and amphibian species can give us clues, as the regions these animal live in is temperature dependent as well.

I know this post has been quite heavy, but I wanted to explain why (and how) scientists are able to make statements about climate change in the past, or the present. If you have questions, please ask! I certainly did while writing this, and consulted with friends who work with isotopes on a day to day basis, unlike myself. If I can’t answer your question, I’m sure they can!

And I promise next post I will talk about something fun and easier to explain!

Scientific Articles:

Zanazzi A, and Kohn MJ (2008) Ecology and physiology of White River mammals based on stable isotope ratios of teeth. Palaeogeography, Palaeoclimatology, Palaeoecology 257:22--37.

Zachos J, Pagani M, Sloan L, Thomas E, Billups K (2001) Trends, rhythms, and abberations in global climate 65 Ma to present. Science 292:686--693.

The Eocene-Oligocene Transition

2009-09-22T18:05:00.000-07:00

Climate change is a hot topic right now (no pun intended). Everyone is talking about global warming, the effects of climate change on the environment, “going green,” etc. You can’t turn on the TV without hearing about it from someone. It is a very heated topic, having broad implications. Growing up, one of the sayings I remember various people saying was “don’t discuss religion or politics at your dinner party,” because those were the things that ended up as debates, as not so fun dinner conversations. I almost feel like climate change has reached that status.

But then, climate change is somewhat political now. I don’t confess to understand all of the political implications of the debate, but I do understand some of the science behind climate change. One of my favorite classes I have taken as a graduate student was “Global Climate Change.” In the class, we started in the past and worked our way forward, looking at all the major climate changes the Earth has experienced in the past hundreds of millions of years. We looked at different hypotheses for what might have caused each of these changes, and how these changes affected the ecosystems on Earth. If anything, the class served to prove just how complicated a system the Earth’s climate really is, and that it’s not easy to make black-and-white statements about climate change.

BUT:

We can talk about the evidence. We can make logical suggestions based on what the evidence shows us. As a student of paleontology, I began working on a project studying small, deer-like mammals known as Leptomeryx. What makes Leptomeryx so interesting is that they survived and adapted to changing environmental conditions that are correlated with a major climate change, known as the Eocene-Oligocene Transition.

Today, I would like to talk about some of the characteristics of the Eocene-Oligocene Transition, which occurred approximately 33 million years ago, and how the Earth changed as a result. In my next blog entry, I’ll talk more about the methods scientists use to reach these conclusions.

The Eocene-Oligocene Transition isn’t one of those major events in Earth’s history that gets a lot of attention. It’s not like the mass extinction at the end of the Cretaceous period (approximately 65 million years ago) that wiped out all the non-avian dinosaurs, as well as numerous other creatures. Or, the mass extinction at the Permian-Triassic boundary (approximately 250 million years ago) with estimates of up to 95% of marine and 70% of terrestrial vertebrate (animals with a backbone) species becoming extinct.

During the Eocene (approximately 59 to 33 million years ago), the world was much different than today. Fossil plants from areas as far north as Washington, Oregon and North Dakota demonstrate that these regions once were more similar in appearance to the rain forests of Central America than what they are today. Average annual temperatures in these areas have been estimated by several different methods to be approximately 65-75° Fahrenheit during the early Eocene. Today, the average annual temperature of North Dakota is about 41° Fahrenheit. Visualize the badlands then, being covered by a dense tropical rainforest, teeming with wildlife as varied as crocodiles and turtles, large carnivores such as the creodonts, small ancestors to our modern horse, primitive primates distantly related to living lemurs, as well as my favorite, the Leptomeryx, just to name a few. (image from http://anthropology.si.edu/humanorigins/faq/gt/cenozoic/eocene.htm)

Oh, and there was the “terror crane,” the Diatryma. You can call me crazy if you’d like, but I think seeing one of these guys around today would be AWESOME!

By the end of the Oligocene (approximately 33 to 23 million years ago) the world had changed dramatically. The tropical forests of the Pacific Northwest were replaced by plants such as oaks, ash, sycamores, elms, braken ferns and horsetails, similar to modern redwood forests. The average annual temperature of the Oligocene has been estimated to have dropped by as much as 10-15° Fahrenheit compared to the Eocene. Along with the change in the types of trees making up the forests, shrub and grasslands began to spread. There is evidence of a steady drying trend, as once moist tolerant plants and animals were replaced by animals and plants more adapted to arid, dry conditions. These changing environmental conditions were particularly hard on the reptiles, amphibians, and some invertebrates such as land snails, but very little change was seen among most mammal groups. Some more primitive creatures, such as the relatives to camels and the large brontotheres, died out. Most mammals however were able to adapt to their new surroundings. I will discuss some of these adaptations later. (image from http://anthropology.si.edu/humanorigins/faq/gt/cenozoic/oligocene.htm)

So, though the climatic event known as the Eocene-Oligocene Transition does not have a major, mass extinction associated with it, it is special because of its magnitude. In a study using fossil tooth enamel, Alessandro Zanazzi and colleagues found evidence of a decrease in average global temperatures of approximately 15° Fahrenheit. This 15° drop in temperature is estimated to have occurred in as short a time scale as 400 thousand years: this seems like a long time to us humans, but in geologic terms, its like the blink of an eye! This dramatic shift has been demonstrated to be one of the most pronounced climate events of the Cenozoic Era, that is, the geologic era we now currently live in, starting 65 million years ago. The Eocene-Oligocene Transition marks the time when the Earth shifted from a “greenhouse” to an “icehouse” world, with rapid ice growth occurring on the Antarctic, only to be followed millions of years later by growth on the arctic pole.

The Eocene-Oligocene Transition set the stage for the world that we live in today.

A brief look at Badlands Fossils

2009-09-18T17:21:00.000-07:00

I say “brief” because it almost has to be. The fossil content of the badlands is so rich that you could write a book on the subject. In fact, many people have. The White River Badlands in particular is well known for its spectacular fossil finds. Dr. Cleophas C. O’Harra, in his 1920 book The White River Badlands, described it as “the most important badland area of the world.” The White River Badlands derives its name from the river that provides most of its drainage, and are considered to lie primarily in southwestern South Dakota as well as in northwestern Nebraska and eastern Wyoming.

Within the rocks of the White River Badlands, fossils have been found representing many different environments, from marine, to tropical forest, to woodland forests, to grasslands. Turtles and crocodiles, rodents and insectivores (small mammals like hedgehogs and shrews), horses, camels and rhinos, all have been found. The vast majority of fossils found are poorly preserved, isolated skeletal fragments. In my experience collecting in northwest Nebraska, fossils were most often seen eroding out of the ground, exposed to the elements that quickly broke and wore them down, sometimes making identification almost impossible.

Probably by far the most common fossils I encountered while doing my field work were blown out turtles. This occurs when a turtle shell is exposed at the surface, only to break apart into the many smaller bone fragments that make up its carapace (upper, curved shell) or plastron (lower, flat shell). These scattered pieces often littered the ground I was walking upon.

Though most fossils found exposed at the surface are often fragmentary, there are plenty of fossils that can be correctly identified and used for scientific studies. Various mammal jaws, though often isolated, are quite common in the White River Badlands as well. And there’s always the chance of coming upon some spectacular finds, such as the dueling mammoths previously discussed, or the fossil trackways that Toadstool Geologic Park is known for.

In order to keep this brief, I’ll concentrate on discussing some of the most common fossils found in the White River Badlands.

My experience with the extraordinary number of turtle shell fragments is an indication to the great abundance of fossil turtles found in the badlands. Specimens can be found ranging from a few inches to a few feet. The most common species is the land tortoise Stylemys nebrascensis. I never found a complete tortoise myself, but I did take this picture while touring Reptile Gardens, South Dakota. It’s a very poor picture, but much better quality ones can be found using any online search engine. Fossil crocodiles have also been described from the White River Badlands, but as far as I know, their record is relatively poor. Dr. O’Harra refers to only two species, Crocodilus prenasalis and Caimanoidea visheri.

Obviously, I’m biased. I study fossil mammals, so that is the group I know the most about. Luckily, this is the group of animals that have probably the most abundant fossil record in the White River Badlands. Again, I do not wish to bore you with long, textbook like descriptions of all the different mammals that can be found. So I will just highlight some of the most common fossils of the badlands that I have either personally found during my field work, or that are favorites of mine I have learned about through my studies.

Ungulates, or hoofed mammals, are by far the most abundant fossil mammals I encountered while collecting in badlands. Ungulates can be divided into two groups: those with an odd number of toes (perissodactyls) such as horses and rhinoceroses, and those with an even number of toes (artiodactyls) such as pigs, deer, sheep, and cattle, to name a few.

Among the perissodactyls, the Mesohippus and Brontops are my favorites. Mesohippus was an ancestral horse about 60 cm tall. It first appeared about 40 million years ago, and went extinct by 28 million years ago. Unlike modern horses, these animals had lower crowned teeth and three toes. Most reconstructions of their environment show these animals having come from forests near flowing water, or occasionally in swamp areas. I was very fortunate to find what potentially might be a skull of such an animal, but more work will have to be done in the lab before I can positively identify the fossil.

The elephant-sized Brontops belong to an informal group known as the brontotheres, or “thunder beasts.” These mammals are known from their prominent hornlike features on the front of their skull. The Brontops have low-crowned, crescent-shaped teeth used for eating soft vegetation. The brontotheres are interesting because, unlike other mammals, this group went completely extinct by the end of the Eocene (approximately 33 million years ago). This time period, at the transition from the Eocene to the Oligocene age, was marked by many changes in climate and environment, so the impact it had upon the mammals is of particular interest to me.

Among the artiodactyls, or even-toed ungulates, Merycoidodon is the most common fossil found in the White River Group. These animals belong to a group commonly known as oreodonts, thought to be related to ancestral camels. Their size has been estimated to be around 100 lbs, and they have been suggested to have inhabited environments as varied as evergreen forests to savanna grasslands. While in the field, my most spectacular fossil find has been identified as potentially belonging to this group. It was late in the day, my field helper and I were tired and hot, and we were headed out. My friend spotted something in a ravine below, and thought it might be a jaw. It turned out to be a skull, which was later identified as possibly being an oreodont (again, I need more time studying it in the lab to be sure!). We stayed an extra 2 hours or so to make sure there wasn’t any extra material, and to carefully get it out of the ground. This is my reminder to myself that you always have to keep your eyes peeled, even when it’s quitting time!

My favorite group of all, the Leptomeryx, is the second most common fossil artiodactyl in the White River Group. This is the group that two years ago I began studying, and have come to love almost as much as I love my puppy. I guess I would love them just as much if I had one in real life, and it proved to be as cuddly. But, though they are just bones and therefore not so cuddly, I still hold a special place in my heart for these small, deer-like animals. They were about the size of a rabbit, though one species was a bit larger than this, maybe closer to a medium sized dog. They first appeared about 40 million years ago, and went extinct approximately 18 million years ago. Because this is the group I conduct my research on, I have a great deal to say about them. So I think I’ll wait until later to talk more about these fascinating animals and what they can tell us about the earth in the past!

As for other common mammal groups, rodents are among the other most common jaws that I personally found. Though most material I found were very small rabbit jaw fragments, ancestral squirrels, beavers, and rats are also known from the White River Badlands. Paleocastor was an ancient beaver known for digging its characteristic burrow, the Daemonelix or “Devil's Corkscrew.” While I was in Nebraska, I saw many, many of these astonishing structures along the road. The Trailside Museum has a great example of one such structure, with the animal preserved inside.

Carnivores have a strong fossil representation in the White River Badlands. Sadly, I never personally found any while I did my own collecting. But I do love the fierce beasts, so will mention a few of the more common groups found fossilized in the badlands.

Primitive carnivores known as Creodonts are known from only one family here, the Hyaenodons. Though this group is not found abundantly, enough is known to indicate that these animals would have been wolf-like in appearance, and approached the size of a modern black bear (photograph from BBC: http://www.bbc.co.uk/nature/wildfacts/factfiles/443.shtml ).

More than 20 species of canids are known from the White River Badlands, a group which today includes wolves, coyotes, foxes, jackals, and the domestic dog. Though a great number of species are described, only a few are known from complete skeletons. The most abundant of these was Cynodictis gregarious, smaller than a common red fox. The group Daphoenus is a member of a larger group known as the “bear dogs,” has been proposed to represent the ancestral stage of the present day wolf.

The cat family, or felidae, is also well represented in the fossil record of the badlands, though less so than the canids. The most well known are from one of two groups, Hoplophoneus and Dinictis, early forms of the saber-tooth cats or tigers. Not as large as later great cats, these groups are still characterized by powerful canine teeth, strong bodies, and strong claws. They doubtless made life much more interesting for the numerous herbivorous mammals of the time!

Recommended reading:

My knowledge is far from complete, and I urge anyone who is interested to look into the various books and papers on the matter. All information in this blog comes from the following three sources, as well as my own personal experiences.

O'Harra, C. C, 1920. The White River Badlands, South Dakota School of Mines Bulletin No. 13, Rapid City, South Dakota.

Zanazzi A, and Kohn MJ (2008) Ecology and physiology of White River mammals based on stable isotope ratios of teeth. Palaeogeography, Palaeoclimatology, Palaeoecology 257:22--37.

The Paleontology Database: an EXCELLENT source on all things ancient and modern:
http://paleodb.org/cgi-bin/bridge.pl?user=Guest&action=displayHomePage

Geology and the Badlands

2009-09-15T09:34:00.000-07:00

I remember the exact moment when I wanted to become a geologist. I was a sophomore at the University of Tennessee, sitting in my undergraduate Earth, Life, and Time class. My professor, whom I would later do my undergraduate research and thesis with, picked up a container that was left behind in the classroom, and started examining its contents with a magnifying lens. “Pure gold,” he says. “About 10 dollars worth.” WOW. How cool, I remember thinking, to be able to know something like that, to be able to look at something and determine what it is based on its physical characteristics. It seems like such a minor thing, but something about having the ability to possess that kind of knowledge became my ultimate goal. The next day, he came in with a small lizard skeleton. BAM. I wanted to be a paleontologist.

I graduated with a degree in Geology in the Fall of 2006. The summer before that, I went to Geology field camp, required by some departments to teach their students about working out in the field. I should also say that this was another reason being a geologist or paleontologist was so attractive to me. You mean I get to work outside? Hike around as a job? SOLD! During field camp, we traveled all over northern North

America. We studied glacial deposits which looked like fields dotted with rocks, some bigger than houses. It’s a testimony to the power of nature that these rocks were carried so far from their places of origin, only to be dropped off as the glaciers melted and receded back into the mountains. We spent the night at Devils Tower National Monument in Wyoming. I was amazed looking up at this giant volcanic rock, which is almost 900 feet from its base to the summit. If you’re ever nearby, I highly suggest you stop by.

But by far, my FAVORITE place we went to was Badlands National Park, South Dakota. All you could see for miles were hauntingly beautiful rock formations, heavily eroded into buttes, pinnacles, and spires. The vegetation was very sparse, so the colors and shapes of the rocks take center stage and command your attention. From the moment we arrived, I was in love. Because of my interest in paleontology, I was very excited to get to see the “Big Pig Dig” they were running at the time, where scientists and volunteers have excavated numerous fossil mammals. The site was to be opening the day after we left. Needless to say, I was upset.

I began graduate school at the University of Florida and started discussing potential projects with my advisor. He mentioned the possibility of working in the badlands of Nebraska, and I jumped at the opportunity. The badlands are renowned for their rich fossil collecting history, where fossils are both numerous and in many cases almost perfectly preserved. I began working on a project centered around a deer-like mammal known as Leptomeryx (don’t worry, you’ll hear more about this wily guy later!). After a year of working with the fossils in the collections at the Florida Museum of Natural History, it was decided that I would spend the summer in Crawford, Nebraska, to experience the geology and collect the fossils firsthand.

I lived at Fort Robertson State Park, where my day job was working at the Trailside Museum of Natural History. Again, if you’re ever in the area and want to see one of the most spectacular fossils ever found, go check out the dueling mammoths they have on display there. Two nearly complete

Columbian Mammoths were found with their tusks locked around each other, and their bones have been arranged to recreate the original excavation. It’s spectacular.

On my days off, I collected fossils near Toadstool Geologic Park and surrounding government lands. I did this under a permit issued to my advisor, as fossil collection is prohibited within the park areas. Collecting on private ranches nearby is solely up to the owners-some let you collect, some don’t. I was very fortunate to have the man that redefined Badlands geology, Dr. Hannan LaGarry, working nearby. One day, I met him at Toadstool Park and he walked me through Badlands Geology. He told me the story of how the original scientists that worked there defined the differences between the rock formations, collectively known as the White River Group, using different colored layers known as “purple-white layers.” They are neither purple nor white, but were named so, according to legend, because of the color they appeared due to the sunglasses the men were wearing. Funny how things like that make a difference.

Dr. LaGarry and his collegues have since defined the units based on their lithology, or basic, collective physical characteristics. The rocks in the area I would work in basically can be placed into one of two formations: the Chadron Formation, roughly Eocene in age (approximately 59 to 33 million years ago), and the Brule Formation, roughly Oligocene in age (approximately 33 to 23 million years ago). Rocks of the Chadron Formation are bluish green mudstones with a popcorn-like texture to them. At the contact between the two formations, the rocks change to more tan and brown siltstones and sandstones, with the occasional sandstone channel running through. The boundary between the Eocene and the Oligocene was once considered to be at the uppermost purple white layer, or UPW, but is now thought the be coincident with this change in lithology. Each formation has its own distinct fossil content within, and tells the story of two vastly different environments…but more on that later!