Succession and repetition of Late Ordovician fossil assemblages from the Nicolet River Valley, Quebec

Abstract

Analysis of stratigraphically separated collections of Late Ordovician fossilized marine invertebrates suggests that our understanding of faunal changes through time may be enhanced by considering them in terms of a successional framework. The faunal succession in the Nicolet River Valley, St. Lawrence Lowlands, Quebec, began with the initial colonization of an offshore barren mud by two or three presumably opportunistic deposit-feeding species. Diversity, both of species and of trophic types, increased gradually, peaked, and then decreased slightly with some faunal shifts, especially in the numerical predominance of the epifaunal element. A three-fold repetition of this quiet-water succession seems in each case to have been terminated by an external physical perturbation. There also appears to be a second, separate successional sequence which became localized on a somewhat coarser-grained and perhaps less stable substratum as the paleoshoreline was approached, due to sediment filling of the basin.