SAMPLING METHODS AND MEASURES OF DIVERSITY AND PALEOCLIMATE IN FOSSIL FLORAS

Can a transect approach be used to accurately reconstruct diversity, paleoclimate and preservation in fossil plant assemblages? To answer this question, we examined five fossil sub-samples that were collected along a transect, from the same stratigraphic layer, from the 46Ma Green River Formation of Douglas Pass, Colorado. Over seven hundred fossil angiosperm leaves were collected from sub-sampling stations that were spaced 35m apart along the transect. We first used all leaves collected from the entire transect and leaf margin analysis to calculate mean annual temperature (MAT) and mean annual precipitation (MAP) for the overall site. We then calculated MAT and MAP for each of the individual sub-sampling stations. Plant species richness and relative abundance were also measured for the combined sample and for each sub-sampling station.

We calculated a MAT of 14.85^oC and a MAP of 58.19cm using all leaves combined from all sub-sampling stations. MAT ranged from 12.59 to 17.78^oC and MAP ranged from 59.82 to 62.47cm for the individual sampling stations. The presence/absence of rare taxa played an important role in the calculation of these estimates. Overall there were 24 plant species identified in the combined site. Individual sub-samples yielded richness values that were up to 29% lower. Using rarefaction we found that the size of the sub-samples was important in our measures of diversity and that the number of species appeared to asymptote near 200 specimens. Overall, we found that both how one samples and how much one samples strongly influences the characterization of fossil plant assemblages. Using a transect approach allows one to capture rare taxa, which are important to both estimates of paleoclimate and measures of diversity.