Overview

Fossils have the potential to provide a wealth of information about the environment in which organisms were living. In this section, which is closely related to the preceding Paleoenvironmental Reconstruction section, we will take a closer look at the information harbored inside of the fossils themselves. The extraction and application of this information requires an understanding of biogeochemistry, which is the study of the integrated biological, geological, and chemical processes (e.g., water and carbon cycles) and reactions that dictate the dynamics of natural environments. As organisms live and grow, they incorporate isotopes—atoms containing a greater or lesser number of neutrons than typically found in an atom of the element—of oxygen, carbon, and nitrogen into their skeletal hard parts. There are many different isotopes and they fall into two general categories: radioactive and stable. Radioactive isotopes decay over time (e.g., uranium-238 eventually turns into lead-206) and are often used to find absolute ages of rocks, minerals, and fossils (An introduction to dating with isotopes can be found in the Geological Time chapter). In this section, we will look more closely at stable isotopes, which do not naturally change on their own as the radioactive isotopes do. Studies of stable isotopes can reveal detailed information about growth rates, life spans, seasonality, environmental conditions, and even evolution of the earliest life on earth.

Oxygen isotopes

Although isotopes of a variety of elements can be assessed, oxygen (O) and carbon (C) isotopes are among the most commonly studied. In large part, this is because they are common in the environments where organisms live and are readily incorporated into biological tissues. Two isotopes of oxygen, and the ratio between them, are commonly studied: ¹⁸O and ¹⁶O. Typically, an atom of oxygen contains eight neutrons in its nucleus along with eight protons; this is ¹⁶O. Occasionally, two extra neutrons are incorporated in the nucleus, creating ¹⁸O. A variety of other oxygen isotopes can occur; however, only ¹⁸O and ¹⁶O are commonly studied. As a consequence of the extra neutrons in its nucleus, ¹⁸O has a greater mass than ¹⁶O. When incorporated into a water molecule, this extra mass means that a water molecule with ¹⁸O is less likely to evaporate than a molecule composed of ¹⁶O, if all else is equal. Because of this, ¹⁸O tends to accumulate in oceans and ¹⁶O, as a part of a water molecule, is more likely to evaporate and return to Earth as freshwater precipitation. Thus, ocean water can be said to be “heavy” compared to freshwater.

There are several biologically important outcomes of this difference between isotopic water compositions. In general, the oxygen atoms making up molecules in an organism tend to reflect the types of isotopes in the organism’s environment. This means that by examining the proportion of ¹⁸O and ¹⁶O in a fossil (or living) organism, we can determine whether it lived in a freshwater or oceanic environment. The ratio of ¹⁸O:¹⁶O is typically converted to a δ¹⁸O value, which can be thought of as enrichment (more positive values) or depletion (more negative values) of ¹⁸O as compared to a standardized value. The standardized value, for example the average ¹⁸O:¹⁶O of seawater, allows scientists to compare different δ¹⁸O values to each other when studies take place in different locations and times.

In studies using oxygen isotopes it is important to understand other reasons why concentrations may vary, namely due to variations in temperature. Water molecules containing the lighter isotope, ¹⁶O, tend to evaporate more easily, meaning most precipitation is depleted in ¹⁸O. Consequently, ¹⁶O is more likely to turn into ice during periods when the Earth is cold and remain trapped in glaciers, ice sheets, and ice caps. During these colder times, organisms typically show higher ¹⁸O concentrations because the heavier isotope is more readily available in the environment. The opposite is also true. During warmer periods, there is less ice on Earth and therefore more ¹⁶O available to organisms for their use. Because the concentrations of ¹⁸O and ¹⁶O can change due to multiple factors (e.g., habitat, seasonality, climate change, organismal metabolism), it is important to have a some understanding of these potentially complicating factors prior to drawing conclusions based on isotopic data.

As is also the case with carbon, the manner in which plants use the different oxygen isotopes can influence the δ¹⁸O values found in animal fossils. Relating to oxygen, this difference is largely controlled by evapotranspiration—the sum of water vaper entering the atmosphere via evaporation and transpiration. When humidity is low and temperature is high, the rate of evapotranspiration will be relatively high (assuming all else is constant). Thinking about the mass of ¹⁸O and ¹⁶O atoms, how should the ratio of ¹⁸O:¹⁶O compare at high and low evapotranspiration?

Because ¹⁶O is lighter, it will compose a large percentage of the oxygen atoms lost through evapotranspiration and the plant would be “enriched” in ¹⁸O as a result. On a warm summer day, evapotranspiration will be high, as will the δ¹⁸O value of the plant. An herbivore eating the plant on the warm summer day will then also likely exhibit a high δ¹⁸O value. To the contrary, an herbivore consuming plants during a cold winter, at night, or in an aquatic habitat will have a relatively low δ¹⁸O value because little evapotranspiration occurs at these places and times. This relationship is useful because it can help assess herbivore behaviors in the past. For example, imagine two co-occurring species (as indicated by their presence in a fossil assemblage) that have exactly the same diet, but one has a significantly lower δ¹⁸O value than the other. What might explain this difference? In this case, the species with the lower ratio likely did its eating during the night, when evapotranspiration was low and plants retained more of their ¹⁶O. If we knew that the two species were both herbivores but did not have the same diet, different life habits, like consuming aquatic plants or consuming terrestrial plants, could also explain the difference.

The ratio of oxygen and other isotopes can vary significantly both between individuals in a community and within an individual. These sources of variability, which have been affirmed in studies of living organisms, can be beneficial depending on the goals of a study. For example, variability within an individual can be a very good thing if you are interested in studying growth rate or lifespan. In this case, it would be important to use a body part that continues to grow throughout an organism's lifespan, such as an elephant tusk or a clam shell, and therefore continuously records environmental conditions. (Bone is not useful for this, unfortunately, because it is continuosly reworked.) As we saw above, δ¹⁸O values are heavily influenced by temperature and thus can be used to assess seasonal patterns, with δ¹⁸O increasing during colder intervals (when proportionately more ¹⁶O is locked away in ice) and decreasing during warmer intervals. Looking at the image below, how old do you think this clam was when it died?

As indicated by the ten peaks and valleys, this clam was likely at least ten years old when it died. (Note, though, that these data do not tell us how old the fossil actually is, only how long it lived.) By examining data like you just did, paleoecologists can get a better sense of how life span changes over time, if organisms change their rate of growth, or even season of death. Although it can be a complicating factor at times, within-individual variation can be highly informative.

Variability between individuals of a species can provide a more refined perspective on what was “normal” for the species. As with any group of individuals, there are likely to be subtle differences between them due to their genetics and the environments in which they live. Consequently, it is often a good idea to sample as many individuals as possible, though the cost of sample analysis can be prohibitive. In many ways, analyzing isotopic samples from multiple individuals is like having insurance against abnormalities. Though it might be unlikely, there is always a chance that, in assessing a single individual, you have selected an individual that was in some way abnormal, or its remains were unexpectedly altered by taphonomic processes. Analysis of multiple individuals presents an average for the population and gives a sense of the natural range of variability that existed in the population. This consideration is not limited to analysis with oxygen isotopes and, more broadly, can be extended to many scientific analyses.

Carbon isotopes

Like oxygen, there are many isotopes of carbon, but only two are commonly studied: ¹³C and ¹²C. The typical carbon atom, ¹²C, contains six neutrons and six protons, whereas ¹³C contains a seventh neutron. This extra neutron makes ¹³C slightly harder to use from a biological perspective. As a result, biological life forms, including the fossils they left behind, tend to have lower ratios of ¹³C:¹²C than their surrounding environment. Like the oxygen isotopes, this ratio is standardized and expressed as a δ¹³C value, where more positive values indicate a greater amount of ¹³C relative to ¹²C.

The biological affinity for ¹²C rather than ¹³C has allowed scientists to identify some of the earliest life on Earth. As described in the video below, the earliest evidence of life on earth is in the form of low δ¹³C values, which we only find today in association with living organisms. Carbon with these ratios has been found in several places and indicates that life may have arisen on Earth 4.1 billion years ago.

Carbon isotopes can be used in a variety of ways, commonly including diet and paleoenvironmental reconstruction. Although organisms preferentially use ¹²C over ¹³C, not all organisms use them in the same ratio. For example, different groups of photosynthesizing plants (e.g., C3, C4 plants) use slightly different chemical pathways to convert carbon dioxide into sugars and oxygen, resulting in different ratios (watch the video below for an explanation of these differences). C3 plants, which includes most trees, shrubs, and cool climate grasses, produce lower δ¹³C values than C4 plants, which includes primarily the warm climate grasses. We can study living C3 and C4 plants and apply taxonomic uniformitarianism to translate this information in the past. Because the pathways used by C3 (Calvin cycle) and C4 (Hatch-Slack cycle) plants differs, when an animal consumes plants with different δ¹³C values, the animal maintains a similar ratio in their tissues (e.g., bone, tooth enamel). This means we can reasonably determine what types of plants the animal ate and, thereby, gain additional information about environment. Through analysis of the carbon isotopes and their preserved ratios in vertebrate fossils, as well as through comparisons with plants and animals living today, it’s possible to distinguish whether the community lived in a forest with a dense canopy or an open grassland.

These carbon ratios can further explain the manner in which similar species—at least through appearance of their fossils—coexisted in the same community. Often δ¹³C values from different species reveal subtle differences in their primary plant food, demonstrating that, although there was competition between herbivorous animals, it did not prevent them from coexisting. Comparison of δ¹³C values for diet reconstruction can also be extended to identify carnivorous species. In large part, this differentiation is possible because of how carbon is used by different components of the body. Collagen—a protein abundant in connective tissues of vertebrates—only retains the carbon that is derived from proteins in an animal’s diet. Hydroxyapatite—commonly included in vertebrate bone and teeth—retains the carbon from the entire diet, which includes the fats, carbohydrates, and proteins consumed by the animal. Conveniently, fats, carbohydrates, and proteins exhibit different δ¹³C values, with fats having the lowest, carbohydrates having intermediate, and proteins having the highest δ¹³C values. In essence, the different usage of carbon means that the difference in the δ¹³C values between collagen and hydroxyapatite in the same animal will be greater for an herbivore than for a carnivore. Thus, by comparing δ¹³C values in a fossil community, it becomes possible to reconstruct the food web describing the species-species interactions in that community.

Other useful isotopes

When reconstructing food webs and trophic levels—levels are associated with organisms with similar functions and dietary requirements—it is common to use nitrogen isotopes alongside, or in place of, carbon isotopes. As with oxygen and carbon, nitrogen can exist as several different isotopes but only two, ¹⁵N and ¹⁴N, are commonly studied. A typical nitrogen atom, ¹⁴N, contains seven neutrons and seven protons in its nucleus. By comparison, ¹⁵N contains an extra neutron. Conveniently, δ¹⁵N values tends to increase by approximately 2–4% for each increase in trophic level (e.g., coyotes are at a higher trophic level than squirrels). Once again preserved in collagen, these differences in δ¹⁵N values provide critical information for reconstructing diets and interactions in the past.

If attempting to reconstruction past habitat use of an organism or species, strontium isotopes are a good choice. Strontium, as ⁸⁷Sr and ⁸⁶Sr in this context, is ingested through the food and water an organism consumes and becomes a part of its tooth enamel. Unlike the isotopes covered so far, ⁸⁷Sr is a radiogenic isotope, meaning that it is formed through the radioactive decay of another isotope, rubidium (Rb) in this case. Over time, ⁸⁷Rb decays to ⁸⁷Sr, which is compared to ⁸⁶Sr. Differences in the strontium isotope ratio (⁸⁷Sr:⁸⁶Sr) are due to differences in the underlying geology in the region where the animal lived. This means that when an animal moves from one geologically distinct area to another it will incorporate a different ⁸⁷Sr:⁸⁶Sr ratio from each area. The ⁸⁷Sr:⁸⁶Sr ratio is highly dependent on the original type of rock in the area and also on how old the rocks are. Older rocks have a higher ⁸⁷Sr:⁸⁶Sr ratio because there has been more time for ⁸⁷Rb to turn into ⁸⁷Sr. Thus, when applying strontium isotopes to understand habitat use and movement (e.g., migration), it is helpful to understand the underlying geology of the region at the time the organisms were living to help interpret the data. This approach is commonly applied to the study of early hominins (our early evolutionary cousins) in addition to studying movement and habitat use patterns of other mammals.

Strontium is also useful for analysis of marine invertebrates. In this case, the utility is closely related to its size at the atomic level: it is almost exactly the same size as a calcium (Ca) atom. Additionally, both elements are alkaline Earth metals, which means they share many of the same properties, like tending to form ions with 2+ charges. In clams like Tridacna (see image above), the Sr:Ca ratio has been demonstrated to reliably capture daily cycles, providing highly detailed information on growth rate during the clams’ lives and their lifespans. In this case, the daily cycles in Sr:Ca are connected to the photosynthetic symbionts living in association with the giant clams because they photosynthesize and respire predictably based on exposure to sunlight and draw in different amounts of strontium and calcium during those processes. Thus, analysis of Sr:Ca ratios can be used to study the timing and dynamics of the coevolution between Tridacna and its algal symbiont.

Though there are still a great many other isotopes that can be applied in paleoecology, and others still that are being developed for new applications, those discussed here are representative of those commonly used. As we expand our knowledge of how organisms use different isotopes and how those isotopes are preserved in the fossil record, we will continue to learn new things about how organisms behaved and interacted in the past. The application of biogeochemical studies and techniques to paleoecology will continue to grow and produce exciting new information for many years to come.