On defining limits to paleoecological interpretation in the fossil record

Abstract

Attempts to describe ancient biological relationships from a stratigraphic record that reflects processes acting over geologic time must include a determination of the «nature of the stratigraphic section in question. «Stratigraphic resolution is a procedure to quantitatively define the nature of a stratigraphic sequence by determining the amount of time represented by the thickness of individual beds in a rock unit. This study of the Upper Ordovician Nicolet River Formation (St. Lawrence Lowlands, Quebec, Canada shows that at least 60% of the time represented by this sequence is missing, but that the acuity of individual beds ranges from high (preservation of instantaneous depositional events) to low (leaving a record of time-averaged fossil assemblages and reworked storm deposits). These values of completeness and acuity lead to the following conclusions: (1) Biological relationships that are observable over short time spans (e.g., faunal structure of a community, competition and predation) may be recorded in beds that have high acuity; (2) Biological processes that act over a longer time period (e.g., evolutionary change) cannot be distinguished accurately in individual beds; (3) When considering the entire stratigraphic sequence, patterns of long-term biological relationships are more complete than the record of short-term relationships. Therefore, the level of observation (either a single bed or the entire stratigraphic sequence) dictates the nature of the data from which we create our paleoecological scenarios.