Resolution analysis: A new approach to the gaps in the fossil record

Abstract

Fine-scale sampling and analysis of fossiliferous sequences have been used in the debate over gradual vs. punctuated evolutionary transitions between species. The time scale and completeness of these studied sequences can be evaluated using criteria based on compilations of sedimentation rates over different time spans. A set of procedures, herein termed “resolution analysis,” provides the means for estimating the time scale and quality of sequences from which evolutionary patterns are distilled. Seven such published studies are evaluated with these procedures. In general, most fossil sequences are too incomplete on fine time scales to show changes operating within a standing population. Short segments of some sequences have the potential to document nearly complete morphological histories on time scales approaching generation-to-generation processes. Resolution analysis is a necessary step in inferences regarding fossil evidence of evolutionary tempo and mode.Stratigraphic incompleteness necessarily results from the episodic nature of sedimentation. Many stratigraphic gaps result from minor, temporary shifts in sediment distribution, though other, more profound gaps result from changes in habitat conditions that must have had an effect on local biotic distribution. Analyzing paleontological patterns involves not only the positive evidence provided by fossiliferous strata, but also the negative evidence left as gaps by shifting habitat conditions. Thus, incomplete sequences may not be “flawed” records of continuous populations. Incomplete sequences may be a faithful, literal record of separate populations separated in time by local extinctions and re-invasions.