Invertebrates in Pennsylvanian Correlations

Abstract

Comparatively few Pennsylvanian fossils are known to have precise zonal significance, and consequently most species at present have no more than general correlative value. This is partly because the Pennsylvanian stratigraphic sequence is uninterrupted, except locally, by important breaks separating easily distinguished faunas and partly because most Pennsylvanian species are too poorly understood to be recognized with precision and separated from closely related forms. Complications are introduced by the extreme diversity of Pennsylvanian environments, which resulted in the close stratigraphic association of almost wholly different faunules. The faunal approach to Pennsylvanian correlation has produced results that are unsatisfactory except in a most general way. Detailed correlations can be made much more accurately on the basis of lithologic sequence, and studies of this type have revealed notable errors in correlations based on fossils. Moreover, faunal studies have resulted in the misidentification of certain diagnostic species and confusion regarding the ranges of others. The detailed study of closely related fossils from many different stratigraphic zones in an effort to work out evolutionary trends gives great promise of much more useful and significant results. The establishment of sequences of evolutionary stages is now known to be possible and offers a means of correlation far superior to that which is dependent on the mere presence or absence of individual index species. All such studies are still in their preliminary stages, but more or less progress has been made with several groups, particularly the fusulinids.