Keywords

Although many aspects of modern ecological communities are not yet understood, describing the animals, plants, and climate of a particular point on the globe – at least in broad strokes – is relatively straightforward. For example, we have a pretty good idea of the number and types of birds, mammals, amphibians, and other animals that inhabit Great Smoky Mountains National Park in the eastern United States. We know that the area is covered by temperate deciduous forest, has a mean annual temperature of about 13°C, and generally receives 140–220 cm of precipitation per year. Similar data are readily available for such sites as the Iberá Wetlands in northeast Argentina, Krau Wildlife Reserve in Malaysia, and Kruger National Park in South Africa. As a result, much of the research that goes on in these areas focuses on clarifying details of these ecosystems, such as documenting inconspicuous and/or unrecognized species, unraveling the complex relationships among organisms and between them and their environment, and predicting how small and large-scale human activities many affect these areas in the future.

Understanding ecosystems of the ancient past requires a different strategy. Since it isn’t possible to directly observe extinct organisms or measure rainfall or ambient temperature in the past, such ecosystems must be reconstructed from the physical evidence and chemical signatures left in the geological record by their biotic (plants and animals) and abiotic (rain, temperature) components. Instead of starting from the big picture and drilling down to the details, a paleoecologist must start with the details – such as fossil teeth and bones, phytoliths, or soil carbonates – and work up. This bottom-up approach is also necessitated by the nature of the fossil record itself, which preserves only a subset of the plants and animals that comprised an ancient ecosystem. The many methods that are available for reconstructing particular aspects of ancient ecosystems are all based on certain assumptions and generate some degree of error. Therefore, maximal confidence comes from combining analyses of entire faunal and floral assemblages with geological, geochemical, and other types of contextual data. Integrated paleoecological studies like these provide a historical perspective on the evolution of Earth’s ecosystems that permits macroecological analyses of patterns of climate change, the evolution of lineage and guild-level adaptations, and large scale biogeographic patterns.

The goal of this volume is to review some of the most common techniques that are used to reconstruct ancient terrestrial environments and ecosystems in order to illustrate the myriad sources of data and types of analyses that can inform such an effort. The chapter authors are experts in fields that span chemistry, geology, and biology. They describe approaches that provide varying levels of resolution about the environmental conditions and the plants and animals that existed thousands to millions of years ago. They also provide glimpses into the future of paleoenvironmental reconstruction by discussing potential ways that these approaches may be refined.

While many of the techniques discussed in this volume can be applied to any part of the fossil record, we focus here on examples from Cenozoic terrestrial localities. Mammals are among the most conspicuous elements of modern ecological communities, and extinct mammals have long been a prime source of information about the ancient habitats in which they lived. This is primarily due to the fact that their teeth and skeletons withstand fossilization far better than those of most other organisms. Thus, most terrestrial paleontological sites from the past 66 million years yield mammal fossils, often in abundance. The first five chapters of this volume discuss techniques used to elucidate the paleoecology of particular mammal species or individuals by studying the structure or attributes of their remains. Although understanding the ecologies and adaptations of the mammals themselves is often the ultimate goal of such studies, these data can also be integrated into studies of the broader mammal community in order to characterize the paleoenvironment in which they lived, a topic addressed near the end of this volume.

Three attributes describe much of an animal’s ecological niche: body mass, locomotor style, and diet . The next four chapters address these topics in turn. Chapter 2, by Samantha Hopkins (2018), discusses how one goes about determining the size (body mass) of an extinct species through analyses of their skeleton. Body size is probably the most immediately recognizable aspect of any mammal, and it can easily be quantified in a living individual by measuring its body mass. Measuring body mass is much less straightforward when one is dealing with fossil remains – which are often fragmentary – particularly for mammals with a body form quite unlike any species alive today. Body mass estimates have traditionally relied on regression equations, and although the mathematics involved is relatively simple, many important considerations must be taken into account to obtain reasonably accurate estimates. More complex methods have been developed in recent years, but whether these are widely applicable remains to be seen.

The limbs are the load-bearing structures of most mammals, and thus there is a reasonably tight correlation between their structure and body mass. However, limb bone morphology is also tightly linked to function. Chapter 3, by Rachel Dunn (2018), describes the process whereby a functional morphologist deduces limb function in an extinct species by reconstructing the sizes and positions of muscles based on their bony attachments. This chapter provides a solid theoretical grounding for comparative anatomical studies, whether one uses traditional descriptive approaches or more computationally-intensive investigative techniques such as three-dimensional morphometrics . As noted in this chapter, a species’ evolutionary (phylogenetic) relationships can strongly influence many aspects of its anatomy and physiology, including its limb structure. For this reason, the influence of phylogeny in paleoecological studies is a recurring theme throughout this volume. It is likely that correcting for underlying phylogenetic biases in data will become increasingly important and feasible as the structure of the tree of life is ascertained with greater confidence and precision .

Chapters 4 and 5 both tackle the subject of determining what an extinct species ate, but they do so from different perspectives. Chapter 4, by Alistair Evans and Silvia Pineda-Munoz (2018), describes a mechanical and material properties approach to that question that strives to determine the types of foods a tooth (or dentition) is optimally structured to process. In other words, for what type of diet have the teeth been shaped by natural selection? This is a “long-view” strategy for determining the average diet to which a species is adapted that parallels methods used to reconstruct locomotor style, and certain aspects of tooth form, most notably hypsodonty , can be (and have been) applied quite broadly across space and time to deduce large-scale environmental and climatic patterns. But not all aspects of tooth form are dictated at birth. Teeth undergo wear through interactions with one another as well as with consumed foods. How one reads that wear record is described in Chap. 5 by Jeremy Green and Darin Croft (2018), who focus on mesowear and microwear analyses. Mesowear and microwear can be thought of a posteriori strategies, as they strive to correlate tooth wear (macroscopic and microscopic, respectively) with the foods an individual animal consumed prior to its death. Thus, they represent direct evidence of behavior, though they do so at different time scales. (The analogous approach for limb function might be analyzing the distribution of internal trabeculae in limb bones, but a standardized, broadly-applicable technique for interpreting such data has not yet been developed.) The techniques for inferring diet described in Chaps. 4 and 5 are complementary and, when combined, can assess diet variation among individuals and populations in addition to species.

Teeth and bones are affected by internal physiological processes as well as external ecological ones, and their microscopic growth proceeds like a biological metronome, evenly marking time in variable durations ranging from days to weeks to years. Chapter 6, by Russell Hogg (2018), details how such changes can be read in the teeth and bones of extant and extinct species and applied to a variety of paleobiological and paleoecological topics such as life history traits of particular species and seasonal environmental variation. The fields of odontochronology and skeletochronology are still in their relative infancy, at least in terms of their application to non-primates, and have the potential to provide fine-scale growth and development information about individuals and populations analogous to that provided by microwear and mesowear for diet .

Teeth and bones can also be analyzed to reconstruct diet on an even finer scale by documenting the relative compositions of the stable isotopes they contain. Such analyses most often focus on oxygen and carbon isotopes , and this topic is reviewed by Penny Higgins (2018) in Chap. 7. The theoretical basis behind stable isotope analysis is that the proportions of different isotopes vary depending on the environment in which an animal lived and the plants that it ate. These isotopes are incorporated into the animal’s tissues in similar proportions – though the values are altered to varying degrees by physiological processes – and can be preserved in the fossil record. Measuring these isotopes in fossil teeth and bones requires destructive sampling , but much less material is required now (typically 10+ mg for tooth or bone) than was necessary only a couple of decades ago. Moreover, newer microsampling techniques have enabled researchers to study changes in isotopic compositions within a single specimen, a strategy equivalent to sampling the composition of a mammal’s tissues at different points during its life. Isotopic sampling is usually performed on fragmentary specimens that have relatively little value as comparative specimens and, as a result, this technique has been applied to a wide variety of species and sites across the globe. This, in turn, has resulted in many new insights into past climate and habitat and the development of new hypotheses to be tested by other means.

Other micro-scale sources of information are organic molecules preserved in fossil-bearing sediments, which can provide clues about past moisture, temperature, and vegetation. These “chemical fossils” represent records of past life (biomarkers), often from organisms that are not themselves preserved in the fossil record of a particular area, such as bacteria and plants. Their study lies at the intersection of biology, geology, and chemistry and comprises a relatively young (less than a century old) field of scientific research known as organic geochemistry. In Chap. 8, Melissa Berke (2018) describes a portion of the wide diversity of organic biomarkers that have been discovered thus far, how they are collected and analyzed, and the types of information they can provide about ancient terrestrial environments.

Organic biomarkers, like fossils themselves, are commonly preserved in ancient (“fossilized”) soils, which are termed paleosols. As detailed by Emily Beverly and colleagues (2018) in Chap. 9, studying the macroscopic, microscopic, and chemical characteristics of these paleosols – the purview of paleopedology – can provide a wide range of complementary information about past landscapes and climatic conditions. Paleosols are the product of local conditions and thus act as archives of the paleoenvironment of a specific site over a relatively constrained period of time. They provide information about a site’s climate (mean annual temperature and precipitation), vegetation, sedimentary regime, and landscape stability and can be used to assess regional variation in these variables when studied throughout a basin.

Among the more commonly encountered fossils in paleosols are ichnofossils : traces of organisms and records of their behavior, often organisms not directly preserved in the fossil record. In Chap. 10, Daniel Hembree (2018) reviews various kinds of animal-soil interactions that can result in ichnofossils, such as nests, burrows , and trails, and describes the types of interpretations that can be gleaned from them regarding habitat and environmental conditions. One of the key challenges in continental (terrestrial) ichnology is correlating ichnofossils with the organisms that made them. In recent decades, researchers have begun to build an ichnological library based on laboratory studies of traces made by living organisms, a pursuit that will undoubtedly provide new information about otherwise unrecognized inhabitants of ancient ecosystems.

Plants are closely adapted to abiotic factors such as climate and soil composition, and any ecological interpretation of a terrestrial paleontological site is incomplete without considering its predominant plant types and vegetational structure. Unfortunately, the conditions that permit the preservation of plant macrofossils such as leaves are seldom the same ones that preserve vertebrate bones and teeth. However, many sites that preserve fossil vertebrates also preserve microscopic plant remains. Such remains include pollen and spores, reviewed by Luke Mander and Surangi Punyasena (2018) in Chap. 11, as well as phytoliths, which are reviewed by Caroline Strömberg and colleagues (2018) in Chap. 12. Pollen and spores have the advantage of being relatively widespread, but this can also be a liability when they record plants living far away from the site in question. Moreover, they are usually only found in relatively fine-grained sedimentary layers. Phytoliths, which are silica bodies that form in plant cell walls, are durable and can often be found alongside vertebrate fossils, but unlike pollen and spores, a single type of plant can contain many forms of phytoliths. Thus, a primary challenge of phytolith investigations is taxonomically identifying the types of plants these phytoliths represent. Studying phytoliths in deep time is a recent phenomenon, and much work remains to be done in documenting their diversity in modern taxa. Despite the challenges posed by both paleopalynology and phytolith-based paleoecology , these approaches are extremely useful for providing primary evidence about the plants that were living at a particular site in the absence of (or as a complement to) macrofossil evidence. It should come as no surprise that both techniques have effectively developed and tested a diversity of geohistorical and macroecological hypotheses.

In sites that preserve plant macrofossils, these specimens can be a source of detailed information about climate and habitat in addition to species diversity, as summarized by Daniel Peppe and colleagues (2018) in Chap. 13. The close linkages between plants and their environment, combined with the taxonomic information contained in fossil leaves, flowers, and other plant parts, means that associations of plant macrofossils are among the best methods of documenting environments of the past. Paleobotany is a comparatively old field, and much recent progress in this area has focused on quantifying fossil assemblages and creating more accurate algorithms for correlating assemblage characteristics with abiotic variables such as temperature and precipitation. The quest to refine paleoecological interpretations by quantifying form is a second conspicuous cross-cutting theme in this volume and a major focus of Chaps. 14–17.

Thanks to increases in computational power that continue to accelerate, it is now relatively easy to quantify form in ways that were scarcely dreamt about even 10 years ago. Although linear measurements remain more than satisfactory for describing many aspects of a fossil specimen, complex surfaces or morphologies can only be adequately described using more sophisticated methods. Chapter 14 by Sabrina Curran (2018) reviews techniques and applications of what is known as three-dimensional geometric morphometrics: describing morphology in three axes (or sometimes two) using devices such as 3D digitizers, laser surface scanners , and computed tomography (CT) scanners. There is no limit to the types of paleoecological studies that can incorporate such techniques, and the diversity of proven applications continues to expand as the devices used to acquire three-dimensional data become less expensive and more accessible.

The final chapters of this volume focus on broader themes of paleoecological reconstruction that build on the results of the types of studies reviewed in Chaps. 2–7. In Chap. 15, Andrew Barr (2018) describes how one aspect of form – such as a particular aspect of tooth or limb morphology – can be quantified across a broad range of extant species, correlated with an ecological variable such as diet or locomotor style, and used to interpret a paleoecological attribute. The shorthand name now used for such studies is ecomorphology. In our usage of the term, ecomorphological studies parallel paleobiological studies of diet and locomotion such as those described in Chaps. 3–5 but differ in their goal; rather than inferring the habits of a particular species, ecomorphological studies typically use form-function relationships to characterize an entire fauna or to compare associations within a particular group across broad spans of space or time.

Chapter 16, by Kris Kovarovic and colleagues (2018), describes a process whereby ecomorphological data from modern and fossil mammal communities are compared to one another with the goal of reconstructing past habitats and ecological associations, a process known as community structure analysis or ecological diversity analysis (EDA). A typical ecological diversity analysis examines the three primary determinants (or reflections) of an animal’s ecological niche mentioned earlier in this chapter – body mass, diet , and locomotor strategy – and uses the collective ecological niches of all members of a mammal community to infer the habitat in which they were living. Like many of the approaches described in this volume, this strategy for paleoecological interpretation relies on observed relationships between living mammals and their environment. Thus, a key consideration in EDA is selecting an appropriate dataset of modern comparative faunas, a task that is much simpler in principle than in practice. Some recent EDAs have also begun to suggest that certain mammalian communities in the past may have been structured differently from any that exist today, a finding that has important implications for understanding niche partitioning, ecosystem functions, and the diversity of mammalian ecological communities.

The final methods chapter in the volume, Chap. 17, by Wesley Vermillion and colleagues (2018), describes an approach to interpreting paleoclimate and paleovegetation known as ecometrics . Like ecomorphology, ecometrics strives to correlate particular traits with environmental characteristics, but it differs in its broader taxonomic scope (typically entire communities), more extensive geographic and temporal focus, and its use of mathematical modeling to understand long-term evolutionary and ecological processes.

Which of the many methods detailed in this volume can and should be applied to reconstructing ancient terrestrial ecosystems depends in large extent on the data that are available. But assuming the best-case scenario – a paleontological site with abundant and well-preserved fossils, a diversity of willing and able-bodied researchers, and generous financial and institutional support – how can results from an array of analyses be combined to create an integrated understanding of a particular place at a specific time in the past? What additional considerations should be taken into account, and what should one do in the case of conflicting results? The final chapter in this volume, by Denise Su and Darin Croft (2018), addresses these and other questions and attempts to outline a path forward so that knowledge of past ecosystems may one day begin to approach those of the present.