Introduction

The biogeochemical cycling of elements in the Earth's crust and shallow mantle by the interaction of physical and biological entities has had a profound effect on the Earth's climate and its biota and preservation. Over relatively short time scales, the assumption of steady-state dynamics of these cycles is no doubt true. But although the cycles are described in textbooks as if they have always existed in their present steady-state form, they actually evolved, and gave rise to secular megabiases in preservation. And, as will quickly become apparent, many of these secular trends are intertwined with each other.

Types of secular megabiases

Kowalewski and Flessa (1996) recognized four types of secular megabiases in the fossil record. A within-taxon megabias is a change in the quality of a single taxon's record that results from evolutionary, environmental, or geological trends that affect the taxon's preservation potential (Figure 9.1). Such gradients may occur within taxa along: (1) depth gradients, such as in nautiloid cephalopods (e.g., Hewitt, 1988), echinoids (Kidwell and Baumiller, 1989), and articulate brachiopods (Patzkowsky, 1995); (2) with latitude (e.g., changes in temperature, rates of predation, organic matter reactivity, and CaCO3 saturation), all of which affect preservation (Kidwell and Baumiller, 1989; Martin and Liddell, 1991); and (3) through time.