Stability of regional brachiopod diversity structure across the Mississippian/Pennsylvanian boundary

Paleobiology, Summer 2009 by Heim, Noel A

Abstract.-

The middle Carboniferous was an interval of global change when the climate was transitioning from greenhouse to icehouse conditions. Field collections of paleotropical brachiopod assemblages across the Mississippian/Pennsylvanian boundary reveal a taxonomic turnover event in which the overall diversity structure is conserved, despite an apparent regional extinction of 63% of latest Mississippian genera and an apparent regional origination of 50% of earliest Pennsylvanian. An analysis of the global ranges of the brachiopods encountered in the field reveals that turnover was driven primarily by extirpation and immigration rather than true extinctions and originations. Taxonomic richness and evenness are indistinguishable between the latest Mississippian and earliest Pennsylvanian stages. Additive diversity partitioning shows that the within-collection, between-collections (i.e., within-bed), and between-bed diversity components do not change across the Mississippian/Pennsylvanian boundary for richness or evenness. Rank-abundance plots of genera show the same distribution for both stages, but with no correlation between the Mississippian abundances of range-through genera and their abundance in the Pennsylvanian. Detrended correspondence analysis shows a major change in taxonomic composition across that Mississippian/Pennsylvanian boundary and consistency in the general grathent along which genera were distributed. An estimation of spatio-temporal heterogeneity of taxonomic composition within each stage reveals that the earliest Pennsylvanian was significantly more homogeneous. These results suggest that middle Carboniferous brachiopod assemblages from tropical shallow-water carbonate platform settings were organized by some factor that was independent of the specific taxa present. Furthermore, the increased homogeneity in taxonomic composition in the Morrowan did not affect the overall diversity structure. Strong competitive interactions among taxa do not appear to be important in determining the taxonomic compositions and abundances of brachiopod stage-level assemblages.

Introduction

An important question in ecology and paleoecology is how local and regional communities are assembled from a larger pool of available taxa. Despite the potential shortcomings associated with time-averaging, the fossil record is rich with natural experiments where faunas were decimated and allowed to reform in the aftermath of large environmental perturbations, such as sealevel change. Examining the ecological structure of a fossil assemblage before and after such an event may provide insights into how biological communities are assembled generally. Fossiliferous strata of the Mississippian/ Pennsylvanian boundary interval provide one such natural experiment. The Mississippian/ Pennsylvanian boundary coincides with a major change in global climate and a global shift in biodiversity dynamics, particularly of the brachiopods. This boundary is associated with a global extinction event (Raymond et al. 1990; Kelley and Raymond 1991), reduced rates of origination and extinction (Stanley and Powell 2003), a shift from narrowly to widely distributed genera (Powell 2005), and a weakened latitudinal diversity grathent (Powell 2007). The purpose of this research is to test the hypothesis that global climate change associated with the Mississippian/ Pennsylvanian boundary induced the reorga- nization of paleocommunity diversity struc- ture on a regional scale in the paleotropics. The hypothesis is tested with brachiopod assemblages from southern Ozark highlands of Arkansas and Oklahoma, which were situated on a shallow carbonate platform between 8° and 13° south paleolatitude during the middle Carboniferous. Diversity structure is analyzed for richness and even- ness patterns as well as the processes of migration, all of which have implications for our understanding of interspecific competition over evolutionary time. The results of this study elucidate regional biotic responses to global climate change and regional paleocommunity assembly mechanisms. Although the biodiversity of the middle Carboniferous is fairly well understood at the global scale, regional dynamics are not, and regional biodiversity patterns do not always parallel global trends (Miller and Mao 1998; Heim 2008).

The interval from the latest Mississippian through the early Permian (Ziegler et al. 1997; Batt et al. 2007) was characterized by an ice house climate in which much of Gondwana was near the south pole and covered by continental ice sheets (Veevers and Powell 1987), the so-called late Paleozoic ice age. Although the onset of Gondwanan glaciation occurred in the Serpukhovian Stage of the late Mississippian (Mii et al. 1999; Smith and Read 2000; Batt et al. 2007), geological (Frakes 1992) and geochemical (Mii et al. 1999; Batt et al. 2007) paleoclimate proxies indicate that the ice volume during the Bashkirian, the first stage of the Pennsylvanian, was much greater than in the Serpukhovian. Globally, stratigraphie and paleontologie data indicate that the boundary between the Mississippian and Pennsylvanian subsystems was marked by a global fall in sea level (Saunders and Ramsbottom 1986), an increase in glacial deposit frequency (Frakes et al. 1992), and an equatorward shift in the latitudinal extent of coal deposits and carbonate deposition (Frakes et al. 1992). Stable isotope ratios measured from brachiopods collected in the North American mid-continent indicate that the middle to late Serpukhovian was glaciated, but warm relative to the Bashkirian (Mii et al. 1999). A positive shift in δ^sup 18^O at the Mississippian/Pennsylvanian boundary suggests a decrease in global temperature, and a positive shift in δ^sup 13^C suggests increased organic carbon burial, from which a decrease in atmospheric pCO^sup 2^ is inferred.