Over the last 50 years, paleobiology has made great strides in illuminating organisms and ecosystems in deep time through study of the often-curious nature of the fossil record itself. Among fossil deposits, none are as enigmatic or as important to our understanding of the history of life as Konservat-Lagerstätten, deposits that preserve soft-bodied fossils and thereby retain disproportionately large amounts of paleobiological information. While Konservat-Lagerstätten are often viewed as curiosities of the fossil record, decades of study have led to a better understanding of the environments and circumstances of exceptional fossilization.Whereas most types of exceptional preservation require very specific sets of conditions, which are rare but can occur at any time, Seilacher noted the problem of “anactualistic” modes of exceptional preservation, defined as modes of fossilization that are restricted in time and that no longer occur. Here, we focus on anactualistic preservation and the widely recognized overrepresentation of Konservat-Lagerstätten in the Ediacaran and early Paleozoic. While exceptional fossil deposits of Ediacaran, Cambrian, and Early Ordovician age encompass a number of modes of fossilization, the signal of exceptional preservation is driven by only two modes, Ediacara-type and Burgess Shale–type preservation. Both are “extinct” modes of fossilization that are no longer present in marine environments. We consider the controls that promoted widespread anactualistic preservation in the Ediacaran and early Paleozoic and their implications for the environmental conditions in which complex life first proliferated in the oceans.

Over the last 50 years, paleobiology has made great strides in illuminating organisms and ecosystems in deep time. Sometimes, these advances have come by interrogating the actual nature of the fossil record itself, specifically, the factors that govern how and why fossils are preserved. Among fossil deposits, none are as enigmatic or as important to our understanding of the history of life as deposits that preserve soft-bodied fossils and thereby retain unusually large amounts of paleobiological information. The great German paleontologist Adolf Seilacher called such deposits “Lagerstätten,” a term now taken to signify paleontological “mother lodes.” These fossil deposits typically represent precise sets of conditions that occur extraordinarily rarely but throughout the geologic record. Some types of these extraordinary deposits, however, represent essentially extinct modes of fossilization, which no longer occur within marine environments. Here, we consider these “anactualistic” styles of fossilization that once were widespread in Earth's oceans, but only for a geologically brief period of time. We conclude that the circumstances that caused these lost pathways of fossil preservation resulted from specific suites of conditions that dominated ancient oceans during the rise of animal life. The same conditions that promoted anactualistic fossilization may have important implications for the circumstances in which complex life first proliferated in the oceans.