Fossil assemblages consist of individuals that were members of one or more ancient communities. Four types of fossil assemblages (fossil communities, residual or winnowed fossil communities, transported fossil assemblages, and mixed fossil assemblages) may be differentiated on the basis of amount, and in some cases, cause of preburial alteration. The most damaging agents to marine communities are predators, scavengers, and especially bottom currents which selectively remove shells. To assess the preburial alteration of a particular fossil assemblage one must evaluate as many of the following criteria as can be reasonably applied: size-frequency distributions, ratio of articulated to disarticulated valves, ratio of opposite valves, dispersion of fossils, population density, community density, faunal composition and diversity, orientation of clastic particles (including fossils), texture, sorting, and structure of sedimentary aggregate, lithofacies relationships, surface condition of fossils, and ratio of whole to fragmented shells. Replacement, leaching, and crushing of shells are the causes of most postburial alteration and may profoundly influence the criteria for preburial alteration.
Analysis of a fossil assemblage from the Pennsylvanian of Nebraska indicates that the specimens have been subject to considerable postburial crushing which has reduced the significance of some of the criteria for assessing preburial alteration. Nonetheless, right-skewed size-frequency distributions for two species of fossil invertebrates give strong indication that this assemblage is a fossil community. This conclusion is supported by the fact that the elongated shells show no tendency toward parallel alignment and the absence of primary sedimentary structures indicative of deposition from strong directional currents.