Linking paleobiological patterns across geographic scales: An example using Upper Mississippian fossil assemblages from the Illinois and Appalachian basins, USA.

摘要

A fundamental goal of evolutionary paleobiology is to understand how the composition, structure, and diversity of biotic assemblages have varied through space and time, in response to physical and/or biological perturbations. Historically, research on this topic has focused on reconstructing and understanding global biotic patterns throughout the Phanerozoic. This research program has greatly improved our understanding of the global history of marine biodiversity and helped to identify intervals of major biotic turnover throughout the Phanerozoic Eon. However, global level analyses mask important variability in the timing or magnitude of biotic change among regions or habitats and preclude an understanding of the processes that control faunal dynamics at regional and local levels. Work that integrates biotic patterns across spatial scales is needed to better understand the processes that combine to drive long-term ecological and evolutionary trends.;Here, I analyze regional patterns of marine taxonomic and ecologic diversity, turnover, and ecosystem structure across the onset the Late Paleozoic ice age (LPIA), using data that were collected within a highly resolved stratigraphic framework from the Illinois and Appalachian basins. In Chapter 2, I explore how global patterns of biotic turnover are expressed in the structure and stability of regional biotic gradients from the Illinois Basin during the LPIA---an interval noted for low global rates of faunal turnover. Gradient analyses reveal a marked shift in the structure of biotic gradients across the onset of the LPIA: in the pre-LPIA interval, depositional environments are clearly distinguished by ordination analyses and are dominated by distinct associations of taxa with modest habitat ranges. However, during the LPIA interval depositional environments are only weakly differentiated by ordination and are dominated by similar associations of taxa that had broad habitat ranges. Comparisons reveal that the structure of successive LPIA biotic gradients is nearly identical. Our results are consistent with findings from global level studies, which indicate that broadly adapted taxa (eurytopes) increased in importance following the start of the LPIA. However, unlike the global level, the regional increase in eurytopy was not linked to the extinction of narrowly adapted taxa in response to climate change. Instead, eurytopy increased as the geometry of the Illinois Basin shifted from a flat carbonate ramp, comprised of shallower water, higher stress environments in the pre-LPIA interval, to a steeper ramp comprised of deeper water, more stable habitats in the LPIA interval. Because eurytopic taxa tend to be extinction resistant and have lower rates of turnover their increased importance in late Paleozoic assemblages likely drove: (1) a previously documented pattern of decrease in regional-level turnover during the late Paleozoic and (2) a perceived pattern of greater persistence in late versus early Paleozoic biotic gradients.;In Chapter 3, I focus on regional taxonomic richness and compare regional diversity trends for the Illinois Basin to global patterns that have been reported previously in the literature. Sample standardized estimates of regional taxonomic diversity and guild diversity remain nearly constant across the onset of the LPIA, despite a documented 28% decline in global diversity. The transition to the LPIA is also associated with very low levels of turnover: 76% to 92% of taxa persist from pre-LPIA sequences into the ice age interval. The onset of glacially driven high amplitude eustasy failed to affect significantly levels of regional diversity and faunal turnover during the LPIA interval. These results suggest that: (1) global and regional diversity patterns were decoupled across the onset of the LPIA and the timing or magnitude of extinction may have varied geographically across the globe; and (2) the major transition to the LPIA and associated high amplitude glacio-eustasy failed to strongly influence levels of regional diversity and turnover in the Illinois Basin. Thus, faunal persistence, not diversification or extinction, appears to be the normal biotic response to glacio-eustasy in the region, during the LPIA interval.;Chapter 4 provides the first study of its kind to compare geographic and temporal patterns of taxonomic richness, turnover, faunal composition, and ecosystem structure within and between correlative sequences in two separate depositional basins. This chapter builds upon the results displayed in chapters 2 and 3 by providing an important and comparable dataset from the Appalachian Basin, against which Illinois Basin faunal patterns can be compared. Therefore, this chapter makes a significant first step toward understanding the meaning of global LPIA biotic curves by resolving spatial complexity in faunal patterns between regions. Results from rarefaction and faunal turnover analyses indicate that taxonomic diversity, community structure, and faunal turnover did not vary significantly with geography, within or between depositional sequences. I suggest that interregional phenomena, including open connectivity between basins and the presence of similar environmental settings in each region, allowed for the establishment of comparable faunas in each basin; in turn these biotas responded in parallel ways to eustatic fluctuations between sequences. Following my discussion, I outline a research agenda for future regional studies that I believe will enrich the understanding of the biotic consequences of the LPIA and the meaning of LPIA global diversity data.