Faunal variation and its potential for sampling bias in the Morgarts Beach Member of the Yorktown Formation (Pliocene)

Abstract

A detailed statistical study was performed on molluscan fossil assemblages collected from the Pliocene Morgarts Beach Member of the Yorktown Formation, located in Isle of Wight County, southeast Virginia. The principal objectives of this study were to examine potential sampling problems and biases inherent in paleontological research. This has been accomplished by statistically testing for the homogeneity, or lack thereof, in species assemblages obtained from samples collected from three localities within the Morgarts Beach Member. Multivariate analysis of variance, two-way analysis of variance, multiple comparison tests and cluster analysis were performed on data collected from a five horizon by five section sampling grid (18 feet [5.5 meters] high, 21 feet [6.4 meters] long) located at Morgarts Beach, Virginia. The results of this analysis demonstrate that the relative abundances of species present in the five horizons are significantly different, whereas the relative abundances of species present in the five sections are not significantly different. Results from cluster analysis show that species assemblages contained in samples collected from the Morgarts Beach Member located at Rushmere are substantially different from the Morgarts Beach type area assemblages, in terms of relative abundances of species. The reason for the lack of faunal similarity relates to the documented facies change between the two localities. The results demonstrate that there is no reliable method to obtain accurate census data (frequency abundance curves) from biostratigraphic or lithostratigraphic units deposited during anything but a restricted time interval. In addition, replicate sampling was found to be unnecessary when attempting to determine the relative abundances of species contained in closely spaced sections within the Morgarts Beach Member. Species accumulation curves were constructed from the data collected from the Morgarts Beach Member. Examination of these curves demonstrate that many rare species will not be found unless extensive collecting is undertaken.