From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition

By Donald R. Prothero; Linda C. Ivany et al. | Go to book overview

CHAPTER 29
Boundaries, Turnover, and the Causes of
Evolutionary Change:
A Perspective from the Cenozoic

Warren D. Allmon


ABSTRACT

Biostratigraphic boundaries have been fundamental to stratigraphic geology since its founding, but their biological interpretation has lagged. After taking into account the effects of preservation, such boundaries, particularly those marked by coincident appearance and disappearance of taxa, suggest that evolutionary origination and extinction are not just coincidentally associated, but are dynamically connected as evolutionary processes. The incompleteness of the geological record will always limit our resolution on such questions, but we may nevertheless be able to learn about evolution at these boundaries if we examine faunal turnover using explicit evolutionary models as hypotheses for testing. Specifically, we may be able to understand more about the relationships among environmental change, speciation, and extinction. Environmental change may dynamically link the processes of speciation and extinction through the effects of disturbance, which at intermediate levels can stimulate speciation but at higher levels can increase extinction.

Boundaries between epochs of the Cenozoic (such as the Paleocene/Eocene, Eocene/Oligocene, and Pliocene/Pleistocene) are frequently marked by high rates of apparently roughly coincident appearance and disappearance of many marine taxa, especially mollusks, and may therefore offer good cases for such analysis. These were clearly times of accelerated environmental change in at least parts of the marine realm, and also perhaps accelerated origination and extinction in several higher taxa of marine organisms.

Such insights may be of particular relevance to understanding the causes of diversity patterns, reaction of the biota to massive environmental change, recovery from mass extinction, and putative examples of “coordinated stasis.”


INTRODUCTION

Biologically defined boundaries—times of appearance and/or disappearance of taxa—have been fundamental to stratigraphic paleontology and geology for almost 200 years (Berry, 1968; Hancock, 1977; Schoch, 1989). Defining such boundaries has traditionally been one of the principal tasks of paleontology—it is crucial to the establishment of all types of biostratigraphic units or zones (Hedberg, 1976)— and recognition of boundaries remains an extremely important element of integrated approaches to chronostratigraphy today (Remane et al., 1996).

Only relatively recently, however, have paleontologists devoted comparable attention to exploring what may be happening biologically and/or geologically at these boundaries. That is, why exactly do taxa appear or disappear? It has long been acknowledged that biostratigraphic boundaries may be created by both the evolutionary processes of origination and extinction as well as nonevolutionary phenomena such as changes in facies and degree of preservation (e.g., Neaverson, 1955; Ager, 1973). Thus any observed pattern of first and last appearances in the stratigraphic record may potentially be amenable to a variety of causal interpretations. If we can by whatever means control for the nature of the stratigraphic record, and interpret at least part of the observed patterns of appearance and disappearance as representative of the actual processes of evolutionary origination (speciation) and extinction, then biostratigraphic

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