The end Ordovician; An ice age in the middle of a greenhouse

Abstract

With millions of years of Earth history to study, it is interesting that so much attention is devoted to the rare and relatively short lived time intervals that represent Earth's major mass extinctions. Perhaps this interest is twofold. On the one hand, there is a fair degree of self-interest in studying extinction considering the present biodiversity crisis we now face. On the other hand, these periods of time have had an incredible effect on life history. These cataclysmic times represent periods of environmental and ecological abnormality amidst millions of years of relative stability. As such, these mass extinctions are times of incredible change, which can be studied both evolutionarily as well as ecologically. When viewed through an evolutionary framework, mass extinction events represent unique time periods in the history of life. These ecological crises prune the tree of life, removing families and killing off entire lineages at random (Raup 1981). Those lineages lucky enough to survive the catastrophe continue and diversify. Often, it is by this seemingly random removal of organisms that large scale evolutionary changes can take place. Take, for instance, the present state of our world, which is primarily dominated by large mammals. Had the non-avian dinosaurs not met with an untimely demise at the end of the Cretaceous, mammals would probably never have been able to diversify into the numerous forms that we see today. It is for this reason that the study of mass extinction events is incredibly important to evolutionary biology. Mass extinctions are essentially historical 'turning points' that affect the evolution of all of the Earth's biota on a gross scale. Mass extinctions can also be studied as ecological experiments. Ultimately, mass extinctions represent times of ecological upheaval in which climate may shift and ecological niche space can be destroyed. By studying both the causes of these ecological perturbations, as well as the affect that these changes have on the biota, we are able to better understand how life reacts under times of ecological stress. This in turn can help us predict the patterns that we might expect in future mass extinctions. This type of study is of particular importance in our present biodiversity crisis. The end Ordovician mass extinction is a unique time period that offers a great deal of study material to geologists interested in both the ecological and evolutionary aspects of mass extinctions. The end Ordovician mass extinction is a time of great ecological upheaval. The cause of this massive die off has long been considered to be a glacial period (Berry and Boucot 1973; Sheehan 1973). Although this interpretation appears to be quite sound, there is still a great deal of debate about the timing of the glacial event as well as its forcing mechanism. The original interpretation proposed by Berry and Boucot (1973) was that the glacial period might have lasted millions of years and that global cooling was gradual. Recent evidence (Melott et al 2005; Brenchley et al. 1994) suggests that the glaciation was incredibly sudden and brief, possibly lasting only a few hundred thousand years. Furthermore, it appears that this glacial period occurred in the middle of a greenhouse climate. The extinction patterns in the end Ordovician glacial period are also intriguing, especially the patterns found in trilobites. Trilobite species with cosmopolitan biogeographic ranges preferentially go extinct while more endemic species are more prone to survive (Chatterton and Speyer 1989). This is contrary to the pattern frequently identified by Stanely (1979), Vrba (1980), Eldridge (1979) and others, who argued that organisms with larger biogeographic ranges tend to have lower extinction rates than those with smaller, more endemic ranges. Yet, in the Ordovician extinction it is the endemic species that tend to survive. This paper will focus on previous research that has been conducted on the Ordovician mass extinction. Furthermore, several of the major unresolved issues concerning the causes of the glaciation as well as the patterns of the extinction will be emphasized; this paper will conclude with a discussion of new research that hints at a possible forcing mechanism for the sudden onset of glaciation. © 2008 Springer-Verlag Berlin Heidelberg.