Utility of Lacustrine Deposits for the Study of Variation within Fossil Samples

Abstract

Variance within samples of fossil species includes biological and taphonomic components and is affected by methodological artifacts. The biological component is intrinsically interesting and important in tests of evolutionary hypotheses using fossils. Varved deposits, often produced by meromictic lakes, in which stable bottom water inhibits turbation offine bottom ooze, provide excellent opportunities to isolate the biological component of variance within fossil samples. This is because varves can provide an internal time scale for temporal stratification of samples, and specimens on a single varve represent one or a few consecutive generations. We discuss taphonomic processes that may contribute to fossil sample variance, emphasizing timeaveraging because it is characteristic of fossil samples and produces a positive correlation between character variance and stratigraphic change. Random samples of the fossil stickleback fish, Gasterosteus doryssus, were scored for dorsal-fin ray number. Dorsal-fin ray number is ontogenetically stable over the size range used. Comparisons of fossil stickleback samples to samples of extant Gasterosteus aculeatus and temporal stratification of fossil samples provide no evidence that taphonomic processes or methodological artifacts inflated variance in the fossil samples. This result may reflect inherent difficulty of estimating variances or absence of taphonomic and methodological effects on variance in these samples. However, time-averaging within fossil samples is a viable hypothesis to explain associations between high rates of change through time and high variance in fossil samples.