Testing for Causal Relationships Between Environmental and Evolutionary Change in the Marine Paleogene of the U.S. Gulf Coastal Plain: the Nature of the Problem

Testing for Causal Relationships between Environmental and Evolutionary Change in the Marine Paleogene of the U.S. Gulf Coastal Plain: The Nature of the Problem Warren D. Allmon1 and Linda C. Ivany2 1Paleontological Research Institution and Department of Earth and Atmospheric Sciences, Cornell University, 1259 Trumansburg Rd., Ithaca, New York 14850-1398 2Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244 ABSTRACT Determining the relationship between environmental and evolutionary change is a research problem as old as evolutionary biology, and one that has occupied many paleo- biologists. The long and rich Paleogene macrofossil record of the U.S. Gulf Coastal Plain (GCP) would seem an excellent laboratory for pursuing this question, for stratigraphy and taxonomy have been worked out in great detail. Yet despite the long history of study, obtaining answers that are not underdetermined in these assemblages is a major challenge. Any such effort must deal with several basic substantive and methodological issues, even before working out the potential effects on biotas of changes in major paleoenvironmental variables such as sea level, temperature, and productivity. These include evolutionary tempo and mode, completeness of the record, nature of stratigraphic and event boundaries, obtaining accurate and sufficiently detailed paleoenvironmental data, and distinguishing evolutionary from biogeographic change. To facilitate this line of research in the Gulf, we are compiling the most complete record to date of stable-isotope-based paleotemperature data through the Paleogene sequence, based both on the literature and on new analyses. Preliminary comparison of these new data and existing sea-level curves with first and last appearances of GCP mol- lusk species suggests that there is only a weak overall correlation, implying that the physical environment alone, as represented by these variables within the Gulf setting, may not be the primary driver for all evolutionary change in these faunas. Potentially fruitful lines of research might focus now on individual clade responses to environmental changes, and on how faunas respond ecologically to such perturbation. A review of pre- vious and ongoing work on evolution of macrofaunas in the Paleogene of the GCP indi- cates that few studies have investigated the role of the environment in affecting evolutionary tempo and mode in individual lineages, although the potential for such research is considerable. INTRODUCTION For much of the century-and-a-half since the publication of “Origin of Species” (Darwin, 1859), climatic change has been considered a major driving force behind evolutionary change, yet the nature of this relationship Allmon, W. D., and L. C. Ivany, 2008, Testing for causal relationships between environmental and evolutionary change in the marine Paleogene of the U.S. Gulf Coastal Plain: The nature of the problem: Gulf Coast Association of Geological Societies Transactions, v. 58, p. 25-45. 25 Allmon and Ivany has remained unclear (e.g., Vrba, 1993, 1995; papers in Ross and Allmon, 1990; Allmon and Bottjer, 2001; O’Dea et al., 2007). How large a role do changes in the physical environment, such as temperature, productivity, and shelf area, really play in determining the course of evolutionary and ecological change? This is not only a substantive question but a methodological one as well. How are we to go about investigating these questions? What will a successful answer look like? The Paleogene section of the U.S. Gulf Coastal Plain (GCP) has been a paleontological treasure trove since the initial papers describing the faunas appeared more than a century and a half ago (Conrad, 1893; Lea, 1841; Lyell, 1849; Wheeler, 1935; Harris, 1895, 1919, 1937). The region offers what is arguably the best-preserved and most-studied series of Paleogene marine macrofaunas in the world. Mollusks dominate these assemblages; they are particularly diverse and well described (Palmer, 1937; Harris and Palmer, 1946-47; Stenzel et al., 1957; Toulmin, 1977; Dockery, 1980, 1982, 1986, 1998; MacNeil and Dockery, 1984; Garvie, 1996; Dockery and Lo- zouet, 2003), and have been standardized into a consistent taxonomy (Palmer and Brann, 1965-1966). In addi- tion, the importance of the Gulf Coast to the oil and gas industry has ensured that stratigraphic relationships have been well documented, and also that surface outcrops have been integrated with the subsurface and placed into a modern sequence stratigraphic framework (e.g., Baum and Vail, 1988, Mancini and Tew, 1993, 1994; Yancy et al., 1993; Yancy and Davidoff, 1994; Baum et al., 1994; Ivany, 1998; Echols et al., 2003). Petroleum-related research has also fostered considerable research (much of it unfortunately still unpublished) on paleoenvironments represented in the GCP Paleogene section (e.g., Gardner, 1957; Davis, 1962; Choung, 1975; Breard, 1978). Given the extensive stratigraphic, paleoecologic, and taxonomic attention lavished on these faunas, however, surprisingly few studies have used them as subjects for investigating evolution, especially the details of the relationship between environmental and taxonomic and/or morphological change. Although a respectable number of studies have addressed patterns and causes of, and recoveries from, mass extinction in these and related faunas (e.g., Dockery, 1986; Hansen, 1987, 1992; Hansen et al., 1999; Lockwood and Baugh, 2003; Lockwood 2004, 2005), they have generally not included explicit consideration of other important evolutionary processes, such as speciation and morphological change (Stenzel, 1949; Fisher et al., 1964). There are several potential explana- tions for this paucity of evolutionary attention. For example, because so much of the research on the GCP has been driven, directly or indirectly, by the petroleum industry, there has been less focus on the biology of these fossils as opposed to more applied studies of their stratigraphy and paleoenvironments. Most of the systematic workers who described the bulk of the faunas made little or no mention of evolution, even in their major end-of- career monographs or valedictories (Harris, 1937; Toulmin, 1977; Palmer, 1979), and on G. D. Harris’s almost total silence on evolution through his 60-year career (please see Brice, 1996). We suggest that this paucity of evolutionary research in the GCP may also be in part a result of inadequate attention to conceptual and methodological issues. In this paper, we summarize what we see as the most important of these issues. We suggest that the GCP Paleogene is indeed a potentially fertile source for insights into evolutionary pattern and process, but only if explicit attention is given to exactly what questions are being asked, exactly what data should and can be gathered, and exactly how that data can be used to test particular evolutionary hypotheses. By way of facilitating this research program, we describe progress to date on an effort to compile new and existing records of both climate and faunal composition that will provide the raw materials for more sophisticated analyses in the future. We then offer some modest suggestions for and examples of studies that may point the way forward. THE NATURE OF THE PROBLEM Research into patterns and processes of evolution—especially the connection between environmental and taxonomic and/or morphological change—in GCP Paleogene macrofaunas must confront at least four separate questions before individual hypotheses can be put forward and tested (the same may or may not be true for micro- faunas, which we do not consider further here). Evolutionary Tempo and Mode What are the potential tempos and modes of evolution that we might expect to detect and examine? Al- though it has become common for textbooks to refer to both “punctuated” and “gradual” evolutionary patterns in 26 Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast the fossil record, in practice anagenesis has not usually been distinguished from cladogenesis in discussions of evolutionary change in GCP Paleogene faunas, and the presence or absence of stasis has hardly been mentioned (please see Kelley, 1983, for such a study in Atlantic Coastal Plain Neogene mollusks). Studying stasis, gradual change, and punctuated equilibrium patterns frequently requires different approaches; if one is not prepared, one may see only what one expects. A related issue is the explicit connection between evolutionary pattern and process. A number of studies have examined patterns of faunal (diversity) change in GCP mollusks (Hansen, 1987; Dockery, 1986; Kosnick, 2005), but have left unexplored what potential evolutionary processes (other than refer- ence to “origination” and “extinction”) might have caused them. Similarly, some workers have remarked on particular morphological features or trends, but have not addressed their possible evolutionary causes beyond brief discussions of “adaptation.” Recent major studies of trends in predation and “escalation” in GCP faunas (Kelley and Hansen, 1993; Hansen et al., 1999) have not specifically explored in detail the connection between micro- and macroevolutionary processes (Kelley et al., 2001). Very different approaches can be taken to explaining evolutionary trends in the fossil record, depending on which tempos and modes one is amenable to seeing. Vertebrate paleontologist William Diller Matthew (1871- 1930) offered the most obvious and

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