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STRATIGRAPHIC PALEOBIOLOGYUNDERSTANDING THE DISTRIBUTION OF FOSSIL TAXA IN TIME AND SPACE
By MARK E. PATZKOWSKY STEVEN M. HOLLAND
The University of Chicago PressCopyright © 2012 The University of Chicago
All right reserved.
CAN THE FOSSIL RECORD BE READ AT FACE VALUE?
In 1980 many paleontologists met with skepticism the claim that the dinosaurs and a majority of species on Earth died off suddenly as a result of Earth colliding with a 10 km bolide (Alvarez et al. 1980). After all, most paleontologists thought that the fossil record indicated that dinosaur diversity decreased gradually up to the Cretaceous-Paleogene (K-Pg) boundary. In the marine record, extinction patterns near the K-Pg boundary looked stepwise but certainly not abrupt. The controversy inspired paleontologists to think more critically about how to read the fossil record of species' last occurrences. It was soon realized that abundance patterns and sampling intensity distort the stratigraphic ranges of fossil species, such that the last occurrence of most species predates their actual time of extinction. As a result, a biologically abrupt event like a mass extinction has a gradual pattern of last occurrences leading up to the time of extinction, rather than a tight clustering of last occurrences at the time of extinction (Signor and Lipps 1982). To account for this bias, paleontologists collected larger data sets (Sheehan et al. 1991, 2000; Marshall and Ward 1996), employed methods that standardize for sample size (Pearson et al. 2002; Wilf and Johnson 2004), and developed methods to put confidence intervals on last occurrences of fossil species in stratigraphic sections (Marshall 1995). Paleontologists now nearly unanimously agree that the K-Pg boundary records abrupt extinction of many species around the world.
This basic argument over how to interpret the fossil record, exemplified by the K-Pg controversy, has been repeated countless times across a wide array of paleontological studies on macroevolutionary patterns, morphological evolution, community ecology, and biostratigraphy. It is among the oldest issues in paleontology: whether the fossil record should be read at face value or, instead, presents a distorted view of the history of life (Gould 2002). In the early 1800s, Georges Cuvier argued for a literal interpretation of the fossil record, specifically that it recorded multiple catastrophes in the history of life, each with widespread extinction followed by radiation of new forms (Cuvier 1812). Shortly thereafter, Charles Lyell advocated a gradualistic view of Earth's history, one that required a more cautious and less literal interpretation of the fossil record (Lyell 1833). Lyell's view, of course, highly influenced Charles Darwin's theory of evolution by natural selection in that it supported gradual, continuous change in organisms (Darwin 1859). To reconcile his theory with the fossil record, Darwin pointed to the imperfections of the geologic record and argued that long intervals of time are not recorded in sediment, so that gradual transitions among species are not observed.
Even today, this tension between Cuvier's literal reading of the record and Lyell's more interpretive view of the record remains relevant to many issues at the forefront of paleontology. How to interpret the fossil record lies at the heart of interpreting extinction and origination patterns and their causes, ecosystem persistence and turnover, and patterns of morphologic change and modes of speciation. How literally the fossil record should be interpreted as the history of life could easily be considered the most fundamental issue in paleontology.
WHAT IS STRATIGRAPHIC PALEOBIOLOGY?
Stratigraphic paleobiology holds that any interpretation of the fossil record must be based on a modern understanding of the principles of sediment accumulation. It is built on the premise that the distribution of fossil taxa in time and space is controlled not only by processes of evolution, ecology, and environmental change, but also by the stratigraphic processes that govern where and when sediment that might contain fossils is deposited and preserved. Teasing apart the effects of these two suites of processes to understand the history of life on Earth is the essence of stratigraphic paleobiology.
Stratigraphic paleobiology is rooted in traditional biostratigraphic methods of carefully collecting fossils from measured stratigraphic sections. The rise of community paleoecology in the 1960s and 1970s heightened interest in the ecology of extinct organisms, but it also led to a greater awareness of potentially formidable biases in the fossil record, such as the mixing of non-contemporaneous individuals in fossil collections, a process known as time-averaging. An important conceptual advance in the 1980s linked biostratigraphy with community paleoecology. Called dual biostratigraphy (Ludvigsen et al. 1986), this approach recognized that what governs the distribution of fossil organisms in the fossil record requires distinguishing the spatial distributions of organisms, controlled by ecology, from their temporal distributions, controlled by evolution. This key point is also at the heart of stratigraphic paleobiology.
Recent advances in how we interpret Earth history define the scope of stratigraphic paleobiology. First, sequence stratigraphy has revolutionized our understanding of how sedimentary basins are filled and, in particular, how to recognize features of the record, such as erosional unconformities and stratigraphically condensed intervals, that shape the fossil record. Second, event stratigraphy has considerably improved our ability to correlate events in Earth history and our understanding of unusual environmental conditions by identifying sedimentary layers produced by extreme events (e.g., widespread volcanic ash falls, rapid and large climatic fluctuations) that can be traced for long distances and that severely affected ancient ecosystems. Third, new analytical methods and increased computational power permit us to ask questions of the fossil record that were essentially impossible to answer until recently. Finally, a realization that global biodiversity is controlled by processes operating over a range of spatial and temporal scales highlights the importance of local—and regional—scale studies for answering fundamental questions in paleontology.
We define stratigraphic paleobiology as the intersection of sequence and event stratigraphy with paleobiology. As a field, its fundamental questions concern the interpretation of changes through time in ecology and evolution based on the fossil record. We restrict our definition to this set of questions because examining all points of intersection between stratigraphy and paleontology (e.g., taphonomy, biostratigraphic methods, reconstructing depositional environments) would be too much to cover in a single volume. Furthermore, recent advances in sequence and event stratigraphy lie at the very center of how to interpret the stratigraphic record, and therefore how to interpret ecologic and evolutionary patterns drawn from the fossil record. We believe that the implications of these advances for interpreting the fossil record are not yet widely recognized and that many opportunities for groundbreaking discoveries lie ahead. A major goal of this book is to fully convey these advances.
THE CORE QUESTIONS IN THE HISTORY OF LIFE
Four core questions about the history of life drive much of the research in paleobiology.
First, how can we describe ecological niches of fossil taxa, and why might they change or remain static? For both species and higher taxa, we are only beginning to understand the extent to which taxa persist in their habitat of origination or spread to other habitats. This question lies at the heart of onshore-offshore patterns of diversification of higher taxa (Sepkoski and Sheehan 1983; Sepkoski and Miller 1985; Jablonski and Bottjer 1991; Jablonski et al. 1983), the correlation of age and geographic area among taxa (Miller 1997a), and changes in the abundance and geographic extent of taxa over geologic time (Liow and Stenseth 2007; Foote et al. 2007).
Second, what is the tempo and mode of evolutionary change, and what are the main processes that drive this change? Documenting the tempo and mode of evolutionary change was the central role of paleontology in the Modern Synthesis (Simpson 1944, 1953). A tacit acceptance of the importance of phyletic gradualism was jolted by the idea of punctuated equilibria, that species are morphologically static for long periods of time and that speciation events are short-lived branching episodes (Eldredge and Gould 1972). Even today, understanding the relative importance of phyletic gradualism and punctuated equilibrium remains a central concern (Hunt 2008; Webber and Hunda 2007; Hannisdal 2007).
Third, how has the diversity of life at different spatial scales changed through time, and what key processes controlled this change? The history of global diversity on Earth has long been a central question (Phillips 1860; Valentine et al. 1978; Raup 1972; Sepkoski et al. 1981; Sepkoski 1981; Benton 1995; Stanley 2007; Alroy et al. 2008), and global diversity has been viewed as a proxy for the health of Earth's ecosystems (Raup and Sepkoski 1984). Many have also recognized that global diversity must be built from the diversity histories of individual provinces or ecosystems, and they have sought to understand how the processes that shape diversity at smaller spatial scales combine to build the global signal (Valentine 1971; Miller 1997b; Bambach 1977; Sepkoski 1988; Miller et al. 2009). These questions likewise arise in understanding how diversity is assembled within landscape-scale regions (Patzkowsky and Holland 2007; Heim 2008; Layou 2007; Scarponi and Kowalewski 2007).
Fourth, what is the tempo and mode of change in ecosystems through time, and what role does the ever-changing Earth environment play in effecting these changes? Paleontologists have long been aware that regional ecosystems display a characteristic pattern of long intervals of relatively little turnover and ecological change, separated by brief intervals of substantial turnover and ecological reorganization (Olson 1952; Vrba 1985, 1993; Boucot 1983). More recently, the hypothesis of coordinated stasis proposed that such a pattern is ubiquitous (Brett and Baird 1995), a claim that has spurred numerous studies (e.g., Westrop 1996; Patzkowsky and Holland 1997; Bonuso et al. 2002a, 2002b; Ivany et al. 2009). The cause of turnover and ecological change has always been a central issue, and from an early date, sea-level change has been suspected as a prime driver of turnover in marine ecosystems (Chamberlin 1898a, 1898b; Moore 1954; Newell 1962, 1967; Bretsky 1969b; Johnson 1974; Hallam 1989; Peters and Foote 2002; Peters 2005).
The answers to all of these core questions require paleontologists to extract the signal of true biological change from a fossil record filtered by the stratigraphic record. This is the domain of stratigraphic paleobiology.
OUR PHILOSOPHICAL POINT OF VIEW
In this book, our approach to using stratigraphic paleobiology to address these core questions is founded on five guiding principles, points of view that we have come to as we have watched the field of paleobiology develop.
First, understanding the history of life requires an investigation of patterns over a wide range of spatial and temporal scales. For example, global diversity is assembled from diversity patterns at local, regional, and provincial scales, and an understanding of what controls global diversity must be reconciled with what is observed at these lower levels. The stratigraphic record forms a natural hierarchy of units from the sedimentary bed to the depositional basin. This hierarchy of sample units therefore allows the investigation of patterns and processes that shape the fossil record over many spatial and temporal scales.
Second, we believe that although the fossil record is deeply affected by processes of sediment accumulation, it is not hopelessly biased and it does preserve important biological signals. Even so, the pattern of fossil occurrences in stratigraphic sections cannot be taken at face value. For example, our intuition is that the massive declines in diversity at the Permo-Triassic boundary, the Cenomanian-Turonian boundary, and the Ordovician-Silurian boundary reflect real perturbations of the global biota. However, as we will discuss later, the stratigraphic architecture of these boundaries suggests that species ecology and the availability of suitable facies for preservation strongly overprint the expression of these events within stratigraphic sections, such that the fossil record cannot be read literally as the history of life.
Third, any interpretation of the fossil record must be rooted in a sound event stratigraphic and sequence stratigraphic interpretation, because the architecture of the stratigraphic record determines where fossils are found. For example, early efforts to confront the completeness of the fossil record—such as recognition of the Signor-Lipps effect and the derivation of confidence limits on stratigraphic ranges—were based on an assumption of an equal probability of collection of a taxon through a stratigraphic section. It is now understood that sequence stratigraphic architecture makes this assumption unlikely. Likewise, several decades of bed-scale research in sedimentology and paleontology has demonstrated that any paleontological question must focus first on the mechanics by which sediment and fossils accumulate and by which a bed is deposited. Making robust ecologic and evolutionary interpretations from the fossil record requires knowing how the architecture of the stratigraphic record affects the distribution and abundance of fossil species.
Fourth, paleontologists should characterize amounts and rates of change in the fossil record rather than simply choosing between the alternatives of a false dichotomy. Paleontologists have debated too many false dichotomies, such as whether turnover is continuous or pulsed, and whether evolution is gradual or punctuated. A more useful and more informative course would be to characterize and compare turnover rates or rates of morphological change through time. In addition, we need a strong grasp of the variance in the patterns we observe and the sources of that variation among taxonomic groups, across environments, and through time.
Finally, paleontologists need to shift away from classical statistical hypothesis testing and instead estimate the magnitude of effects and place confidence limits on them. Paleontology has been greatly aided by increased quantification and statistical rigor, and it leads other areas of the geosciences in this regard. However, this can easily devolve into an emphasis on statistical significance rather than biological importance, an issue that ecologists are confronting as well (e.g., Anderson et al. 2000; Seaman and Jaeger 1990; Yoccoz 1991). For example, one could ask if turnover rates in two time intervals differ, but the answer to this question is almost always yes: the difference might be exceptionally small, but it is exceedingly unlikely that the two intervals have exactly the same turnover rate. Detecting this difference statistically (e.g., obtaining a low p-value) is therefore a function of sample size: a statistical difference will be found if sample size is large enough. The result is that a significant p-value does not necessarily indicate a biologically important difference in turnover rates. Rather than ask whether turnover differs among two time intervals, or whether the difference is statistically significant, one should measure the difference in turnover and use confidence limits to convey the degree of certainty in the estimate.
THE ORGANIZATION OF THE BOOK
This book is organized around a stratigraphic approach to reading the fossil record and investigating the core questions listed above. Chapters 2, 3, and 4 address the nature and architecture of the stratigraphic record and how environmental gradients determine the distribution of species. Chapter 5 builds on this foundation by describing a numerical model that predicts many features of the fossil record arising as a result of stratigraphic architecture. Armed with this understanding, the book then pivots to considering the core questions in the history of life, in particular, how to answer these questions without being misled by stratigraphic overprints. Chapters 6 and 7 provide bases for understanding how the ecology and morphology of individual taxa change through time in a stratigraphic context. Chapters 8 and 9 address regional ecosystems, how they change through time, and their relationship to processes that govern diversity.
Excerpted from STRATIGRAPHIC PALEOBIOLOGY by MARK E. PATZKOWSKY STEVEN M. HOLLAND Copyright © 2012 by The University of Chicago. Excerpted by permission of The University of Chicago Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of ContentsPreface
2 The Nature of a Sample
3 The Stratigraphic Framework
4 Environmental Controls on the Distribution of Species
5 Stratigraphic Controls on Fossil Occurrences
6 The Ecology of Fossil Taxa through Time
7 Morphological Change through Time
8 From Individual Collections to Global Diversity
9 Ecosystem Change through Time
10 From Beginnings to Prospects
Common Sequence Stratigraphic Terms