Introduction

In paleoecological studies, it is always important to understand the environment in which organisms were living. To understand this, think about your own surroundings as you go through a week in your life.

Is it warm, or cold? Is the weather more often rainy, or sunny? Are there other people around you? How about other species, like dogs, cats, trees, or insects?

The answers to these questions all help to describe your environment. When a paleoecologist studies an organism or community from the fossil record, these are the same types of questions they try to answer. Answering these questions about the past environment can provide important clues for understanding what we find in the fossil record.

Attempts to describe past environments are often referred to as paleoenvironmental reconstruction. The focus of paleoenvironmental reconstruction is often on abiotic factors like temperature, water depth, precipitation, or water salinity, but can also include biotic factors like the proportions of herbivores and predators or the number of species in a fossil assemblage. To reconstruct a paleoenvironment, information is gathered from a variety of sources and put together like clues in an investigation to reveal the setting in which organisms from a fossil assemblage interacted. These clues might include the type of sediments (e.g., sand, clay) that are present and features preserved in them (e.g., ripples, cross-bedding); taphonomic factors like the orientation of particles (i.e., indicating flow directions) and the degree to which fossils are disarticulated (i.e., energy in the system); climate data from tree rings, ice cores, or another source; and also the organisms in the assemblage, plus what we know about their biology. Once again, these interpretations rely on the assumption that processes and organisms functioned in the past just like they do today.

In many cases, there are not direct measurements of factors like rainfall and temperature and so paleoecologists need a different way to assess these factors. Often, this involves the use of a proxy. A proxy gives information about a factor, even if it does not directly measure something about that factor. Imagine that you develop the ability to teleport, but you have not quite figured out how to control where you are going. On your first attempt, you open your eyes and see palm trees blowing in the wind. What type of place have you arrived to? What if the first thing you saw was a polar bear?

Just like these organisms are associated with particular environments today, so are many fossil organisms. By using what we know about modern organisms and extending that knowledge to their long-dead relatives, we can make similar guesses about the type of environment that once existed. This type of information is referred to as a proxy. You did not measure the temperature, but when you saw a palm tree, you were able to guess that you were in a relatively warm environment. If you found a fossilized branch from a palm tree, you could reasonably draw the same conclusion. This type of proxy gives a relatively coarse indication of the past environment. You know it was warm, but you do not know exactly how warm it was in the past. Other types of proxies can give us different types of information and, when viewed together, give us a sense of the environment and ecosystem that once existed. We will explore a few more of these proxies.

Tree rings

Tree rings, the circular bands a tree grows each year as it gets bigger and bigger, are another good source of information about the past environment. Most studies of tree rings—referred to as a field as dendrochronology (dendro = relating to a tree; chronology = study of historical records to establish dates of past events)—occur using living trees. Scientists use a special borer to extract a small section of the tree, which allows them to examine the tree rings and does not kill the tree. Records like these can be used to go back thousands of years in time. Although these are not fossils, cores from living trees allow a dendrochronologist to correlate the information they see in the tree rings with information about temperature and rain fall that was recorded with modern instrumentation. For example, trees grow thicker rings when the climate is warm and wet and only grow thin rings during dry, cold times.

This can happen seasonally (e.g., winter versus summer) or on longer timescales as climate changes over decades or centuries. These correlations allow dendrochronologists to be more confident in the conclusions they make using tree rings from times in the past when there were not measurements taken by instruments. The study of tree rings does not need to take place using living trees. Although it takes a little more work, scientists can use similar methods to study fossilized remains of trees to understand the temperature and rain fall occurring during the time those trees were alive. You can learn more about tree rings from NASA and NOAA.

Ice cores

Ice cores, as with tree rings, can provide a high-resolution record of the past climate but have a somewhat limited duration of applicability, at least geologically speaking. This limitation is because in order to collect ice core data, there must be a continuous (or at least correlated) accumulation of ice, and there must be ice in the first place. Very few places on earth meet these requirements over timescales of centuries and millennia. Glaciers, at high elevations and altitudes, are one possibility for data collection, but the best ice core data come from the polar ice sheets of Greenland and Antarctica. In these cold, polar environments, snow and ice have been accumulating for 130,000 and 800,000 years, respectively! The core extracted from Antarctica, which was over two miles long, provides the best record we have for climate during the last million years and, because it includes layers from more recent times when modern instruments were recording climate, we can have strong confidence in the evidence it gives us.

The high resolution of ice core data is due to the yearly accumulation of snow, ice, and gas bubbles trapped inside. It’s not always easy to see in the core, but if you look closely at the image above, you can clearly see some of these annual bands. Over time, the growing weight of accumulating ice compacts the layers below and preserves the climate information in a deep-freeze. Importantly, the ice and snow also collect things like dust, ash, pollen, salts, and anything else that was in the atmosphere when the layer formed. Analysis of these particles can provide a wealth of information about the past climate. Most notably, the Antarctica ice core allows us to reconstruct the amounts of greenhouse gases that were in the atmosphere during the last several hundred thousand years. These data show clear evidence of the climate change we are experiencing today—compare today’s carbon dioxide concentration of more than 410 ppm with concentrations from the past (blue lines). 

You can learn more about ice cores, including how they are sampled, stored, and analyzed from the National Science Foundation Ice Core Facility. The video below will give you a brief introduction to the work they do at the Ice Core Facility.

Sediments and their features

Ice cores and tree rings can provide high resolution, relatively recent data on the past environment. Particularly when studying life in the last thousands to million years, these data can be incredibly useful. When we go further back in time, however, other proxies such as sediment type, sediment features, taphonomic processes, and organismal biology play a larger role in reconstructing the paleoenvironment.

Each piece of information, like the degree of disarticulation, orientation of particles, or types of sediments, helps to develop a more complete picture of the past environment. Each piece on its own may not be sufficient to confidently characterize the past environment but, taken together as multiple lines of evidence, they can tell a compelling story.

Sediments

The sediments in which a fossil assemblage is preserved can provide information about the type of environment where the organisms lived, especially for marine ecosystems. Sediments (physical matter found in the natural world and subject to processes like weathering and erosion) come in all sorts of shapes and sizes, ranging from huge boulders to tiny flecks of clay. Based on how big or small these particles are, a paleoecologist can get a good sense of the amount of energy in the system and using this can pick up clues about the paleoenvironment.

To understand this concept, imagine that you need to pick up two pieces of sediment, a boulder with a diameter of one foot and a single grain of sand. Which of these could you carry for a longer distance? No matter who you are and how strong you might be, you can undoubtedly carry the sand grain over a longer distance than the boulder. You probably carry around a bunch of sand grains in your shoes every day! Just like carrying the boulder requires more energy for you to lift and carry than a sand grain, the geological processes—often involving the flow of water—that move these sediments also “get tired” and drop the sediments depending on how much energy is required to move the sediment. More energy is required to move sediments like boulders and cobbles than sands and clays. Consequently, large sediments can only be moved by fast moving streams and in places where gravity can act strongly, like in the mountains. To the contrary, even the slowest moving stream can usually move sand and clay particles. Most clay particles only settle out of water when it reaches a standstill, like in deep lakes or the ocean. By understanding this relationship between sediment size and the energy required to move sediments of different sizes, we can reasonably guess the environment based on sediment size.

Sedimentary features

Sediments are doubly informative about paleoecology because sedimentary features are also commonly preserved and these give additional clues about where and how the sediments were deposited. For example, mudcracks form in an environment where sediments are wet part of the time but also dry out completely at other times, leaving the cracking features we can see in image below. Hummocks and swales are additional features commonly found and form due to waves washed sediments back-and-forth in shallow marine settings. Another common feature, ripples, indicate a nearshore, shallow environment right at the interface of marine and terrestrial ecosystems. Features like these provide additional clues about physical aspects of the environment where they formed.

Fossils as sedimentary particles

From a sedimentological standpoint, fossils themselves can be considered particles just like pieces of sand and clay. As such, the orientation and disarticulation of fossils can provide yet more information about the environment of a fossil assemblage. Many fossils, and parts of fossils, are elongate (one physical dimension is considerably longer than the other two) and this makes them behave predictably in environments where there is flowing water. Examine the image below. What direction do you think water was flowing in this environment?

In this case, water was flowing approximately left to right, or right to left. We know this because elongate particles tend to orient perpendicular to the direction of flow. Using other information, like changes in sediments or habitat transitions, can help determine which way was shoreward and seaward. Other types of fossil particles, like clam shells, can provide additional information. Clams shells, which we will consider to be shaped like cups, can either have their opening fall upwards or downwards. When examining a fossil assemblage that includes clams, like in the image below, we can look at the proportion of upwards and downward shells to understand how much the assemblage was moved around before it was ultimately deposited and became a part of the fossil record. This can give us clues about how much time-averaging or habitat mixing might have occurred and gives us a sense of how intense waves and storm events were in the environment.

Disarticulation of fossils

The final way we’ll consider fossils as sedimentary particles is through the process of disarticulation. Disarticulation describes the extent to which the skeletal components of an organism have remained intact as a fossil relative to how the components were oriented in the living organism. Different organisms can exhibit different degrees of disarticulation. For example, a snail really only has one preservable hard part, its whorled shell—the operculum, which the snail uses to cover the opening in its shell, is found occasionally but not very often—whereas a clam has two valves. Thus, it is not common to talk about the articulation of a snail, but when clams are found with both valves as they were when the clam was living, it tells us that the clam likely was buried quickly after it died, before taphonomic processes could separate the valves.

Many organisms have more than two components. For instance, most vertebrates, like fish, horses, and us, have dozens if not hundreds of bones that can potentially be separated from each other through taphonomic processes. Just like the clam, when we find a vertebrate skeleton that is articulated, meaning it has all of its bones in a life-like orientation, it tells us that the skeleton was rapidly buried and undisturbed after death. To the contrary, a jumble of bones indicates a greater role of taphonomy. What types of processes do you think could lead to the disarticulation of a vertebrate skeleton? In marine settings, wave action can break apart skeletons and so too can other organisms through scavenging. Similarly, on land, scavengers will often move and destroy skeletal remains. In both settings, though somewhat more commonly on land, organismal remains may simply erode away into dust, leaving only partial bones or no fossils at all.