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

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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 previous 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 addition, 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

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 reference 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 perhaps most influential answer to this question, and may be largely credited with establishing the classic textbook Darwinian response. In his book “Climate and Evolution” (Matthew, 1915), Matthew described how terrestrial mammals, such as horses, had evolved in response to climate change, specifically that they had developed a series of morphological features (larger body size, reduction in toe number, and increased tooth height) in response to selection pressure caused by mid-Tertiary cooling and drying of the mid-continent of North America. As the climate changed, grasses replaced trees, and grasslands expanded where woodlands had once dominated. Forest-dwelling mammals that fed by browsing on relatively soft vegetation were replaced by plains-dwelling forms that fed on tougher grasses (and so needed taller teeth) and needed to run faster to avoid predators. What might be called “Matthew’s Law” thus says that the primary connection between climate change and evolution is that climate change creates new selection pressures that lead to the development of morphological adaptations to enhance chances for survival and reproduction in the new conditions. This is an example of what Eldredge (1979, 1982) has called “transformational” evolutionary change, which he contrasted with “taxic” evolutionary change (Allmon, 1994), which focuses on the origin and disappearance of species as discrete entities.

The Stratigraphic Record

What constraints and opportunities does the Paleogene GCP stratigraphic record present for evolutionary studies? Like all stratigraphic records, it is incomplete, in part because, like all shallow-marine sections, it contains significant unconformities related to changes in sea level (Hazel et al., 1984; Allmon, 1989; Baum et al., 1994; Miller et al., 2008). Many of these hiatuses are also intervals of considerable faunal turnover (i.e., numer- ous simultaneous first and last appearances of species (Dockery, 1986). Because of the lack of record, or pro- nounced facies change associated with it, such unconformity-related turnovers are sometimes (and sometimes rightly) dismissed as artifacts (Jablonski, 1980; Holland, 1995, 2000). Yet the sea-level changes that cause these hiatuses may themselves have been potential influences on the evolution of marine faunas (Allmon, 2003; Peters, 2006, 2008). More detailed evaluation of these stratigraphic boundaries is therefore necessary to determine to what degree they can be used as sources of insight, rather than or in addition to sources of uncertainty. Speciation and extinction sometimes co-occur (or appear to) at stratigraphic boundaries. This is commonly called “turnover” and episodes of high faunal turnover (sometimes called “bioevents”) are frequently targeted for studies of environmental causality (Walliser, 1986, 1996; Hallam, 1989; Brett and Baird, 1995; Allmon, 2003, and references therein). Most recently, Peters (2006, 2008) demonstrated that not only do depositional hiatuses reflecting sea-level change correlate with times of evolutionary turnover but the relationship may not be simply one of stratigraphic artifact (cf. Holland 1995, 2000). He maintained that associated changes in shelf area and shelf habitats were a major (though not the only) driver for extinction and diversity in the marine realm. It may not always (or even very often) be possible to determine with high precision/resolution the duration of the interval of time represented by the major unconformable boundaries in the GCP section (e.g., between the deposition of the uppermost macrofossil beds of the upper Paleocene Bells Landing and lowermost beds of the lower Eocene Bashi; or between the deposition of the Upper Lisbon, Gosport, or Moodys Branch formations). In such cases, we will only be able to bracket the possible durations with biostratigraphy, graphic correlation, and/or

Allmon and Ivany physical stratigraphy. Thus, in such cases, we will only have “before and after” assessments of conditions, separated by 105-106 years, and can only use these to constrain possible environmental events associated with (and therefore potentially causing) observed faunal changes.

Paleoenvironmental Data

Are adequate paleoenvironmental data available? Despite the abundance of paleoenvironmental reconstructions in the GCP, there have been no detailed isotopically-based paleotemperature curves for the region until relatively recently (Ivany et al., 2000, 2003; Kobashi et al., 2001; Miller et al., 2008). Despite the important role the GCP played in the development of global eustatic sea-level curves (Vail et al., 1977; Haq et al., 1987; Baum and Vail, 1988), arguments about local and regional sea-level changes have persisted (Baum et al., 1994; Miller et al., 2008). In addition to the most oft-discussed parameters of mean temperature and sea level, more difficult-to- assess variables such as seasonal variation in range of temperature (Ivany et al., 2000), shelf area (Peters and Foote, 2001; Peters, 2006), water-mass stability (Kobashi et al., 2004), and productivity (Allmon and Ross, 2001; O’Dea et al., 2007; Haveles and Ivany, 2008, this volume) have all been invoked as potential drivers for evolutionary change. None of these factors is straightforward to reconstruct. Clearly we cannot investigate the relationship of evolutionary to environmental change if we do not understand what the environmental changes were.

(Non)correlation of Environmental and Evolutionary Change

What if there is no correlation between environmental and evolutionary change? All of these approaches assume that the faunal and environmental changes co-occur, and that the magnitude of evolutionary change observed in the fossil record is in some way connected to the magnitude of environmental change. In this view (“Matthew’s Law”), faunal or morphological change is driven by external environmental changes, and low levels of faunal turnover are correlated with relative environmental quiescence. There is also, of course, the possibility that the environmental and evolutionary change are completely uncorrelated, for example, that the environment may change, between or across stratigraphic boundaries, but during this interval we observe no measurable morphological or taxonomic change. This condition—evolutionary stasis—may apply in the transformational mode to morphological change in single lineages (“morphological stasis” [Eldredge and Gould, 1972]), or in the transformational mode in multiple lineages (“coordinated stasis” [Brett and Baird, 1995]). In either case, long intervals of relative evolutionary stability (morphological or faunal) apparently exist despite stratigraphically resolv- able evidence for some type of environmental change (Lieberman et al., 1995; Brett et al., 1996).

A CLASSIFICATION OF EVOLUTIONARY QUESTIONS

A matrix for looking at the general issue of connections between environmental and evolutionary change might thus look something like the following in Table 1.

Table 1. Potential relationships between environmental and evolutionary change.

Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast Each one of these possible cases represents a different research problem that potentially requires a different research strategy. Cases of presence/absence of transformational change with or without change in environment (T1-T2, P1-P2, S1-S2, T5-T6, T6-P6, and S5-S6 in the table above) are, of course, part of the 30+ year debate over punctuated equilibrium or “evolutionary tempo and mode” (Gould, 2002; Gould and Eldredge, 1977, 1993). Surprisingly, few studies of tempo and mode have been carried out in the GCP. (More such studies should be done, but we shall not consider the issue further here.) More work, however, has focused on the relationship between taxonomic and environmental change (T3-T4, P3-P4, S3-S4, T7-T8, P7-P8, and S7-S8 in the table above) in the GCP, and this is our focus in the remainder of this paper.

BEGINNING TO ADDRESS THE PROBLEM

By generating and integrating high-quality records of marine temperature with those of macrofaunal composition, diversity, ecology, and morphology, we can begin to explore the nature of the relationship between environment and evolution in this natural laboratory setting of the GCP. The related questions are broad and complex, and the work of one group of researchers alone surely will be insufficient to resolve them completely. Criti- cal to this endeavor is the generation of high-quality, internally consistent, and quantitative records of environmental change against which morphological and faunal data can be compared. In addition, studies of diversity and taxonomic turnover require not only a consistent taxonomy, but that the complexities associated with sampling that diversity can be worked out so as not to overwhelm actual evolutionary patterns. We have taken the first steps in this effort by beginning to compile new and existing data for both the stable-isotope-based record of climate change and the macrofossil-based record of faunal change. Here, we relate progress to date and lay out a plan for future work. Our working stratigraphic column for the following is depicted in Figure 1, with ages updated to Gradstein et al. (2004). Early Oligocene units are as per Miller et al. (2008).

Stable Oxygen Isotope Analyses

Temperature change has long been suspected to be an important driving factor behind GCP faunal turnover (Hansen, 1987, 1992), but until recently, a quantitative long-term climate record specific to the GCP was not available. General patterns of climate change in different parts of the section exist, based on pollen assemblages (Frederiksen, 1988; Oboh et al., 1996; Harrington, 2003; Oboh-Ikuenobe and Jaramillo, 2003; Yancey et al., 2003; Harrington et al., 2004) and leaf margin analysis (Wolfe and Poore, 1982) in terrestrially-derived material, and nannoplankton records (Siesser, 1984) and isotopic analysis of bulk carbonates from core (Baum et al., 1994; Abreu et al., 1999; Hurley and Fluegeman, 2003) in marine sediments. But while these types of analyses provide a valuable overview of the record, it is difficult to know how to compile results from different studies with different methodologies to produce a consistent and quantitative measure of climate through the whole section. Over the past 6 years, advances in high-resolution microsampling and stable isotope analysis of accretionary carbonates have provided data on mean temperature and seasonality from multiple Paleogene GCP units (Andreasson and Schmitz, 2000; Ivany et al., 2000, 2003, 2004a, 2004b; Kobashi et al., 2001, 2003, 2004), re- sulting in the best-constrained quantitative record of mean temperature and seasonal range of temperature variation that exists for a marine shelf system anywhere. The potential now exists to bring these records together, standardize them for taxon and facies, and identify and augment stratigraphic intervals that require additional data so as to produce a single, comprehensive, internally consistent record of isotope-derived paleotemperatures for the entire Paleogene in the GCP. Isotope values from the aforementioned studies come from a variety of macrofaunal taxa (bivalves, gastropods, fishes, corals). Different taxa may precipitate carbonate at different times of the year (and hence show different mean values) and at different rates (and so may time-average the seasonal cycle to a greater or lesser degree) (Wilkinson and Ivany, 2002), and so changes in time that also correspond to taxon shifts can be difficult to evaluate. Many existing data come from fish otoliths (Ivany et al., 2000, 2003), venericard bivalves (Kobashi et al., 2001; Ivany 2004a; Sessa et al., 2006), and turritelline gastropods (Andreasson and Schmitz, 2000; Kobashi et al., 2001); the gastropod Conus has been shown to produce reliable, multi-year records (Kobashi et al., 2003) as has the bivalve Callista (Ivany et al., 2004b). All taxa are not present at every sampling locality, and so using a combination of sources maximizes the chances that at least one taxon will be present in each locality to enable

Allmon and Ivany

Figure 1. Working stratigraphic column for major fossiliferous units in the Alabama and Mississippi GCP. Timescale is updated to Grad- stein et al. (2004), and Oligocene units have been delimited as per Miller et al. (2008). Durations of unconformities at boundaries have not been depicted for simplicity at this time.

Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast maximal coverage of the section. For all taxa, co-occurring specimens are examined to see how their isotope values compare in order to ensure that the composite record is internally consistent. Coral data will not be used due to the propensity for precipitation out of equilibrium with seawater (please see Ivany et al., 2004b, for a GCP example). Existing isotope data are augmented with new unpublished data from specimens in undersampled units coming from a combination of our own new field-collected material, existing bulk collections in our labs, and specimens from museum collections that have unambiguous stratigraphic and lithologic information. All specimens have more than one isotope value associated with them, so as to assess intra-shell variation. In most cases, specimens have been serially microsampled so as to reveal not only mean composition but also seasonal variation in δ18O—this allows for not only a rigorous examination of intra-shell variation, but also the evaluation of additional important climate variables (minimum, maximum, seasonal range). The significance of observed variation among individuals within horizons can be difficult to assess. Within-horizon variation can be due to real time-averaging of variable temperature conditions (and so the range should be roughly the same at different correlative localities), but could also be due to facies variation between localities (in which case different places would show different mean values and spreads). The former provides meaningful climate information when compared through time; the latter does not, and can obviate recognition of any real stratigraphic trends that may be present. We have made a concerted effort to recover isotope data from more than one locality in each stratigraphic horizon so as to assess these options, and continue to obtain and sample specimens that will ultimately achieve this goal. The dataset presently includes values from 170 otoliths and mollusks, 126 of which have also been microsampled to yield seasonality estimates.

Diversity Data

Taxon lists by formation are readily available (e.g., Toulmin, 1977; Dockery, 1986), providing a first-order approximation of compositional change through time, but these lists are subject to a variety of biases, including sampling intensity and taphonomic and environmental heterogeneity. New techniques in data collection and analysis have been developed and/or become more commonplace since the seminal works of Dockery (1986) and Hansen (1987) broke ground on large-scale integrated studies of faunas and environment in the GCP. Whole- fauna studies now routinely employ bulk sampling and sample standardization to circumvent such biases, and the influences of evenness and beta diversity on overall trends are being examined regularly (Powell and Kowalewski, 2002; Olszewski, 2004; Peters, 2004; Kosnik, 2005). Stratigraphically restricted, bulk-sample data with taxon abundances are therefore much preferred, but such data are only published for a few units and have not been integrated in time. We are compiling existing abundance data and supplementing them with our own field-collected data for less well-represented intervals to generate a master database that can be sample- standardized for use in diversity and turnover studies. Existing data (including those from unpublished theses) come from Toulmin (1977), Elder (1981), Hansen et al. (1993), Haasl (1993), CoBabe and Allmon (1994), Gar- vie (1996), Haasl and Hansen (1996), Ivany (1997), and Visaggi (2004). Ongoing thesis research by collabora- tors Heather Wall (please see Wall and Ivany, 2008, this volume) and Jocelyn Sessa will provide additional collections. Currently, the database contains 156,707 individuals representing 1383 species of bivalves, gastropods, scaphopods, corals, bryozoans, and echinoderms distributed among 365 distinct collections. The wealth of taxon occurrence data reported by locality in Palmer and Brann (1965-66) are also presently being compiled so as to provide additional information about geographic distribution of taxa. At this point, data have yet to be vetted taxonomically, facies variations have yet to be taken into account, and some datasets do not include all major taxa; therefore, the patterns reported below should be considered very preliminary and subject to change. For this reason, we resist the temptation to over-interpret at this juncture.

PRELIMINARY RESULTS

Stable Oxygen Isotope Analyses

Figure 2 demonstrates that bivalves, gastropods, and otoliths all yield δ18O data that generally overlap each other—none are consistently more positive or more negative than the others. This suggests that all taxa are pre-

Allmon and Ivany

Figure 2. Stable oxygen isotope values of individual GCP fossils reported by taxon. Values are midpoints between the maximum and minimum values in specimens sampled serially for seasonality (most data points), or mean values from specimens sampled in bulk (generally three time-averaged samples per specimen). Each data point reflects a unique fossil individual, for a total of 170 specimens. cipitating carbonate effectively in equilibrium with seawater and hence giving reasonable approximations of carbonate δ18O. When plotted cumulatively by stratigraphic horizon (formation, member; Fig. 3A), the data show the expected increase in δ18O through time seen in the global ocean records (Zachos et al., 2001), reflecting some combination of global cooling and the growth of ice sheets at high latitudes. At present, these data are presented only as δ18O values. The δ18O value of marine carbonate is due to both the temperature of precipitation (our desired climate variable) and the δ18O value of the water from which the carbonate precipitated (which is primarily affected by salinity and global ice volume). Paleotemperatures from δ18O of aragonite can be calculated using the paleotemperature equations of Grossman and Ku (1986), corrected

Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast

Figure 3. (A) Mean stable oxygen isotope values for all taxa in each stratigraphic interval sampled. Reported values are the means of all midpoints (including bulk sampled individuals), and isotope maxima (cool temperature extremes) and minima (warm temperature extremes) for those individuals sampled serially. (B) Mean amplitude for all specimens sampled serially so as to recover a seasonal cycle (124 individuals). (C) Change in mean δ18O values reported in panel A; negative values indicate that the mean δ18O value of the indicated stratigraphic interval is more negative than the previous. Negative values may indicate warming temperatures, a shift toward more brackish water, or melting global ice sheets, while positive values suggest cooling temperatures, a return from more brackish conditions, and/or increasing global ice volume. for seawater composition (Kobashi et al., 2003) for mollusks and Patterson et al. (1993) for otoliths, but a significant limitation of oxygen isotope paleothermometry is that calculating paleotemperatures from the isotopic composition of carbonate requires knowledge of the isotopic composition of the water in which the carbonate was produced, and this cannot be measured directly for ancient systems. There are several estimates available for global seawater δ18O though the Paleogene based on knowledge of the extent of glacial ice on Antarctica (Zachos et al., 1994), and factoring out the influence of temperature from δ18O using Mg/Ca paleotemperatures from foraminifera (Nurnberg et al., 1996; Lear et al., 2000, 2004). In addition, Huber et al. (2003) presents results of a climate model in which variation in seawater δ18O is resolved over the world oceans for the Eocene. Ultimately, paleotemperature values will be calculated using each of these seawater estimates, and other options for con- straining the composition and/or temperature of seawater will be explored. Several features of the data at present stand out. First, particularly negative values in the early Paleogene need to be evaluated for the potential contribution from freshwater (Ivany et al., 2004a). While the Paleocene

Allmon and Ivany points are constrained by only a few analyses and need to be verified by additional work, the Hatchetigbee For- mation at ~53 Ma is represented now by 14 shells that have all been micromilled, all of which yield very similar means and ranges, unlikely if the primary influence was mixing with fresh water (Keating-Bitonti and Ivany, in press). The possibility therefore exists that conditions in the early Eocene GCP were warmer than previously presumed. Second, the marked positive shift at the Eocene-Oligocene boundary is readily apparent at ~34 Ma, and reflects the rapid growth of a large ice sheet on Antarctica at the time (Zachos et al., 1992). This enrichment, however, does not persist later into the Oligocene, suggesting that the ice sheet was short-lived and/or that GCP temperatures warmed in the Oligocene. Third, amplitude of seasonal variation (Fig. 3B) is generally between 1 and 2 parts per mille (‰) (roughly 4-8°C [39-46°F]). Three intervals of high amplitude are apparent. The first is associated with the unusually de- pleted values of the early Eocene Hatchetigbee Formation, and offer a cautionary note about whether one should interpret mean values as entirely temperature-driven (seasonal runoff could increase the amplitude of variation; Ivany et al., 2004a). The second is associated with the Gosport Sand (~39 Ma). This is a unit not marked by unusual mean values, but the seasonal range is well-constrained by microsampled data from a number of taxa. An ususually high seasonal range in temperature variation might indicate seasonal upwelling (O’Dea et al., 2007) or water mass instability with depth (Kobashi et al., 2004), and both of these phenomena are consistent with higher primary production, a hypothesis supported by the unusually rapid growth of mollusks in that unit (Haveles and Ivany, 2008, this volume). The third increase in amplitude occurs in the Oligocene, and may be related to cooler, more seasonal conditions following the Eocene-Oligocene boundary. Lastly, an examination of the change maximum, mean, and minimum δ18O values through time (Fig. 3C) shows generally higher variation in δ18O minima (warm season) than in maximum values. This may reflect the tendency for warm seasons to be characterized by more runoff of fresh water, and if so would imply that the overall record is compromised by changes in local δ18O of seawater, making reconstruction of a climate signal more difficult. While this is a possibility that needs to be evaluated, other indicators do not support this interpretation. The strong variation in the early Paleogene again is poorly constrained at this time, but suggests the possibility of significant warming and cooling in the Paleocene. Interestingly, δ18O across the Paleocene-Eocene boundary (~55 Ma) is virtually constant, an unexpected result given demonstrated global perturbations (Zachos et al., 2005, 2008; Sessa et al., 2006). At the Eocene-Oligocene boundary, δ18O maxima show a more significant decline in values than do mean or minimum δ18O values, consistent with cooler winters immediately following this transition (Ivany et al., 2000).

Diversity Data

A preliminary compilation of the diversity data based upon bulk-sampled collections is presented in Figure 4. Collections from the same stratigraphic horizon (formation or member) have been pooled for this first-pass look at pattern. Diversity through time based on raw data shows a broadly similar pattern to that based on collections rarefied to 200 individuals, though the standardized data understandably show a smaller range of variation. Evenness as measured by Pielou’s J is reasonably consistent, but drops significantly into the Oligocene. Low values also characterize the middle Eocene Tallahatta/Reklaw formations, but preservation in these units is noto- riously poor, and needs to be considered as an influence. Interestingly, if data from the upper Paleocene Tusca- homa Formation and lower Eocene Hatchetigbee formation are separated out into individual members, evenness values for all of those are surprisingly high (not shown). It is unclear at this point whether that reflects idiosyn- crasies of those collections or a potential boundary-related phenomenon.

DIRECTIONS FOR FUTURE WORK

Assembly of a comprehensive dataset consisting of stratigraphically-constrained abundance data spanning the Paleogene will allow for a much more rigorous analysis of diversity trends through time, and the exploration of a number of ecologically significant patterns that could not be addressed before. The relationship between abundance within bulk samples, consistency of species occurrence among samples and localities, and species

Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast

Figure 4. Summary statistics on faunal diversity through the GCP. (A) Raw richness based on 156,707 specimens representing 1383 species of bivalves, gastropods, scaphopods, corals, bryozoans, and echinoderms counted from 365 bulk samples of varying sizes, and standardized richness when stratigraphic horizon totals were rarefied to 200 individuals. (B) Shannon’s diversity statistic (a measure of combined richness and evenness), and Pielou’s J statistic reflecting the evenness of the distribution of individuals among taxa.

Allmon and Ivany longevity can be investigated in a comparative way among different taxa and ecologies. Taxa can be classified in a variety of ecological categories to the degree possible, including trophic strategy, substrate relationships/ mobility, larval type, eurytopy/stenotopy, metabolic rate (categorical based on trophic level and mobility), and K (carrying capacity) versus r (intrinsic rate of increase in a population) selection strategy, using the literature and nearest living relatives (same genus where possible, otherwise family level). Compositional changes can be as- sessed at the species and genus levels, with turnover calculated as per Foote (2000). Maximum size can be compiled from the literature, as this measure correlates with average sampled size (Kosnik et al., 2006) and so can be applied to published datasets for which original specimens are unavailable, and will contribute yet another ecological variable that may have influenced evolutionary pattern. Lastly, biogeographic ranges of taxa as reported in the literature and in the Paleobiology Database (2008) will provide insight into their thermal preferences and allow identification of subsets of typically cool-water temperate taxa and warm-water tropical indicator taxa. Integration of faunal data and the isotope-based climate record will ultimately allow the beginnings of an investigation into the complex relationships between evolution and environment. Questions that can be considered include: (1) Is there a correlation between the magnitude and direction of temperature change and the amount of ecological change? (2) Are there thresholds of tolerance for temperature change below which a fauna does not respond, or re- sponds only ecologically, such as via migration or changes in relative abundances? (3) Are intervals of warming (i.e., the abrupt PETM [Paleocene-Eocene Thermal Maximum] and the more protracted EECO [Early Eocene Climatic Optimum]) consistently associated with particular suites of ecological changes (such as the preferential disappearance of temperate taxa or a decline in the relative abundance of stenothermal taxa)? One might predict that turnover associated with the PETM and EECO (if any can be documented) would favor warm-water taxa over cool. (4) Are intervals of cooling (i.e., the Eocene-Oligocene boundary, and potentially the mid-late Eocene) like- wise associated with particular characteristic ecological changes (such as the preferential disappearance of tropical taxa or a decline in the relative abundance of stenothermal taxa)? Hansen (1987) has suggested that warm- water taxa should suffer more at the mid-late Eocene and the Eocene-Oligocene boundary. One might also hy- pothesize that higher trophic level and/or higher metabolic rate organisms would not fare as well during times of cooling due to energetic constraints (Valentine, 1968; Clarke, 1990, 1993). (5) Does ecological response to changes in mean annual temperature differ from ecological response to changes in seasonality? (6) How is evenness affected by turnover? Other studies have suggested that rare taxa are more likely to disappear during turnovers (e.g., Lockwood and Baugh, 2003). If so, evenness should go up following extinctions, and then drop through time as more rare taxa accumulate. Is this supported by GCP data? And last, but not least: (7) How might environmental change (temperature, sea level) at the famous boundaries in the GCP have affected the taxic patterns of turnover (Allmon, 2003) that have attracted so much paleontological attention over the past 150 years? Further, by combining comprehensive ecological and climatic analyses with an as yet unexplored investigation of morphological trends in dominant Gulf Coast lineages, we may have the capacity to produce a synthetic picture of ecological and evolutionary change within a long-lasting and biogeographically restricted fauna, placed into and integrated with a climate record derived from those same specimens. Perhaps the most significant limitation of this sort of research, presuming one can recover meaningful data on the most important environmental variables, is how to compare the various environmental and faunal records? A simple visual comparison of time series, while occasionally illustrative, is not analytically sufficient; correlation is minimally needed, both for raw values as well as first-differences between values. Measures of climate (mean and minimum temperature, seasonal range, etc.) will be available for each stratigraphic horizon for which there are faunal data, because the climate record will in most cases be derived from subsets of the same specimens. Faunal data for each horizon may be represented by simple numerical values (diversity, % infaunal bur- rowers, etc.), but will often be better described as a position in some multidimensional space (morphology, faunal composition). In this latter case, ordination and plotting scores for individual horizons/samples on the primary and/or secondary axis (e.g., Holland, 2005) against the appropriate climate variable will help to determine the correlation. First differences are important to determine whether the nature of change in one variable is consistent with the direction of change in the other, even if the overall secular trend is controlled by another variable.

Testing for Causal Relationships between Environmental and Evolutionary Change, Marine Paleogene, U.S. Gulf Coast This approach can be used to detect tendencies in the faunal data in response to climate change. There are those familiar with the GCP who may read this discourse with some skepticism, thinking, first, that the coastal plain record is not continuous enough to do this sort of analysis, and, second, that facies variation within the section will make comparisons among horizons less than meaningful. We acknowledge that any record of shelf sedimentation is generally discontinuous and lithologically variable and hence not optimal for this sort of study, but we reject the assertion that we must throw up our hands and walk away from meaningful long- term studies of these topics. Our interest lies in the macrofossil record, and as macrofossils are recovered from shelf settings, we must work with what we have. With respect to continuity of record, because of hiatuses associated with sequence boundaries, we will not be able to produce a continuous, uninterrupted record of climate and faunal change of the sort only possible in deep sea settings. Nevertheless, focusing on differences before and after particular environmental transitions that are well-represented in the record allows us to see the effects of that change in the composition and ecological characteristics of the faunas of shell beds from before and after. As for facies variation, one can make an effort to limit data to a consistent subset of the lithologies available, so as to minimize variation contributed by environmental heterogeneity. Isotopic analyses from multiple samples and multiple localities along strike where possible will help to reveal spatial variation in environments. In addition, it is helpful to sample in a sequence stratigraphic framework so as to better characterize environments from the perspective of relative water depth and clastic influx, and minimize the potential for artificially inflated faunal turnover associated with facies change at sequence boundaries and transgressive surfaces (e.g., Holland, 1995, 2000). The potential here is too big to let the spectacular GCP sequences go unmined for evolutionary insight any longer. The time is ripe for synthetic, multidisciplinary, and collaborative work to address this most funda- mental of questions.

ACKNOWLEDGMENTS

A number of individuals are involved in this large-scale project to compile faunal and environmental information for the GCP Paleogene. Most notably, Heather Wall (HW) and Jocelyn Sessa (JS) are integral members of the project, and their Ph.D. theses will each report different aspects of this study in detail (please see for example the contribution by Wall and Ivany, 2008, this volume). Rowan Lockwood is a principal investigator on the project with LCI and WDA, and together with WDA is investigating morphological evolution and phylogeny in selected GCP taxa. This work will be summarized elsewhere. LCI, HW, and JS all provided new bulk sample data for this analysis based on field work. HW and JS compiled the published faunal data, with the help of Emily Feinberg on data entry. JS compiled the stratigraphic section, with additions by LCI. Preparation of samples for isotopes was accomplished by Shea Lambert, Caitlin Keating-Bitonti (CKB), Andrew Haveles (AH), LCI, and JS. Microsampling of specimens for isotopes was done by LCI, JS, CKB, and AH. Please see also related contribution by Haveles and Ivany (2008, this volume), and abstract by Keating-Bitonti and Ivany (in press). Stable isotope analyses were run by Lora Wingate at the Uni- versity of Michigan Stable Isotope Laboratory. We are grateful to Louis Zachos for the invitation to participate in this symposium. We thank Carlton Brett and Paul Morris for stimulating discussion of some of these issues over the past 20 years. This work has been supported in part by NSF grants EAR-0719642 to WDA and EAR-0719645 to LCI, and by PRF grant 40418-G2 to Ivany.

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