Acritarch biostratigraphy of the dicellograptus complanatus graptolite zone from the Vaureal formation (Ashgillian), Anticosti Island, Quebec, Canada

Abstract

Forty‐two species of organic‐walled microphytoplankton (ac‐ritarchs) and a single specimen of a spore‐like microfossil are documented from the Dicellograptus complanatus graptolite Zone in the Vaureal Formation on Anticosti Island, Quebec. Our samples are from cored intervals in the New Associated Consolidated Paper #1 well. These cores contain biostratigraphically useful chitinozoans and graptolites which precisely correlate these intervals with lower to middle Ashgillian. Conodonts, ostracodes, and megafossils from outcrops of the Vaureal Formation on Anticosti Island support this lower to middle Ashgillian correlation for our sampled interval.

The acritarch assemblages obtained from the sampled interval contain abundant well‐preserved specimens. Of these, some forms considered diagnostic of the Dicellograptus complanatus (graptolite) Zone are Multiplicisphaeridium bifurcatum, M. irregulare, Orthosphaeridium insculptum, Poikilofusa spinata, Sylvanidium paucibrachium, and Veryhachium hamii.

Four species of acritarchs were described from these same cores more than twenty years ago. At that time, the rocks were erroneously considered Middle Ordovician and this resulted in an incorrect extension of the ranges for Eupoikilofusa striata, Multiplicisphaeridium bifurcatum, M. irregulare, and Poikilofusa spinata.

Two new genera, Cornuferifusa and Uranidium, and five new species are described: Cornuferifusa bifidipertica, Comasphaer‐idium lanugiferum, Disparifusa borea, Multiplicisphaeridium wrightii, and Uranidium semicalvum. A new combination is proposed for Hogklintia radicosa.

This abundant and well‐preserved lower to middle Ashgillian acritarch assemblage is apparently widespread. Thus this strati‐graphically well‐controlled sampling from Anticosti Island may prove a useful reference for this portion of the Upper Ordovician, as the European stratotype for the Ashgillian has not yielded acritarchs.