The stratigraphic distribution of graptolites in the classic upper Middle Ordovician Utica Shale of New York State: an evolutionary succession or a response to relative sea-level change?

The stratigraphic distribution of graptoloid species within the upper Middl e Ordovician strata of New York State represents a complex pattern of origi nation, extinction, faunal migration, and fluctuating relative abundances. In particular, the observed patterns of species turnover at graptolite bioz one boundaries are apparently strongly correlated with lithofacies, samplin g strategies, and the depositional effects of relative sea-level change. Vertical facies changes that occur within third-order depositional sequence s and fourth- or fifth-order parasequences are mirrored by changes in the g raptoloid faunal composition. Large-scale faunal turnovers at biozone bound aries tend to occur either at sequence boundaries or at flooding surfaces w ithin sequences (e.g., the base of Highstand System Tracts). The base of th e Corynoides americanus and Climacograptus (D.) spiniferus Zones coincide w ith major onlap events, and the Orthograptus ruedemanni Zone fauna arrears just below a Lowstand Systems Tract. Within individual biozones, smaller-sc ale changes such as the fluctuating relative abundances of graptoloid speci es coincide with higher-order parasequence cyclicity. These distribution pa tterns resemble recent computer-generated models for the sequence stratigra phic distribution of hypothetical taxa. By combining good biogeographic control with a detailed sampling program, w e are able to see through the patterns attributable to depositional cyclici ty and identify the different components of faunal turnover (migration, spe ciation, and extinction).