A FACIES-PRESERVATION MODEL OF DIVERSITY: EXPLORING PRESERVATIONAL BIASES DURING THE LATE DEVONAIN BIOTIC CRISIS

Tabulations of rock volume show striking correlations to marine diversity through time, suggesting that preservational processes have biased the record of preserved diversity. Researchers have seldom studied these biases at small scales nor their effect on mass extinctions. Preservation biases may explain the unusual decrease in origination rates that seems to cause much of the diversity loss during the Frasnian-Famennian biotic crisis.

We present a null model to predict the effect of differential preservation among facies on preserved origination and extinction rates. The model is based on Sepkoski's (1991) onshore-offshore diversity model, adding preservation rates. True diversity remains constant, while preservation rate may change independently between facies and time intervals. Preservation rate changes alone causes changes in observed overall taxonomic rates for the whole facies gradient.

We used biostratigraphically constrained data from the Middle to Late Devonian of Iowa to calculate empirical preservation rates, by gap analysis or proportional amounts of each lithofacies preserved in a time interval, which we then used in the model to generate the predicted pattern. The model predicts that a progressive decrease in preservation quality in nearshore facies (those with the highest species extinction rates) produces a pattern of observed taxonomic rates remarkably similar to that seen in the Late Devonian, when origination rates decreased dramatically relative to extinction rates. Eustatic sea level changes during the late Devonian may have affected nearshore facies more than offshore facies. Extinctions and ecological changes occur across the F/F boundary, so we do not suggest that preservation alone causes the observed pattern, but that the trend may be influenced by rock record biases. One of the most important implications of this model result for the fossil record is that variable rates of preservation among facies may exert a strong control on overall observed taxonomic rates and diversity.