DISTINGUISHING SAMPLING AND ECOLOGIC EFFECTS ON GEOGRAPHIC RANGE AND DURATION IN GRAPTOLITES

Although the fossil record is inherently biased and incomplete there is a potential that differences in measures of completeness, rather than being driven primarily by abiotic factors, contain an ecological signal for intensively sampled groups. One such group are graptolites. These macrozooplankton of the Ordovician-lower Devonian have been studied for over a century for use in biostratigraphy. A global database of horizon by horizon occurrences data from over 400 stratigraphic sections and 600 graptolite species spanning the group’s entire existence provides an opportunity to distinguish effects of sampling and ecological limits on the spatiotemporal distribution of taxa.

One means of detecting taxa with unusual sampling patterns is to look for outliers in a plot of the number of sites at which a taxon occurs at versus the number of horizons from which a taxon is sampled. This plot shows a general trend where most species are observed on five horizons for every section at which they occur. However, there are a number of conspicuous outliers that fall far outside the scatter of other species, most of them appearing on a greater number of horizons than expected indicating that they are persistently present within a section, possibly representing generalist taxa. More rarely, some taxa instead occur at many sections but on relatively few horizons, possibly indicating taxa that were either consistently rare or appeared only occasionally in swarms.

Unique ecological groups, termed biotopes, have also been recognized in graptolites but only for a small set of Ordovician taxa. An attempt to expand this classification to a greater number of taxa based on a taxa’s occurrence in continental shelf deposits show that while there is not a distinct break in shallow-occurring versus other taxa all species that exhibit unusual occurrence patterns are known to occur in shallow sites. The ecological meaning of this result is unclear but may indicate that graptolite taxa found in shallow environments are more ecologically malleable than their deeper counterparts. Further information on abundances within horizons would be informative to detect further differences between taxa.