 Caron and Jackson found that about 25 % of the 172 known species were abundant and widespread throughout the time range of the GPB , while the majority of species were rare , and occurred in a small area for a short period of time . In most layers the five most abundant species accounted for 50 % to 75 % of individual animals . The species that had wide ranges in time and space may have been generalists , while the rest were specialists in particular types of environment . Alternatively some wide @-@ ranging species may have been opportunists that were quick to recolonize the area after each burial event . The 6 species that appeared in all layers were very probably generalists . 
