Abnormal forms of acritarchs (phytoplankton) in the upper Hirnantian (Upper Ordovician) of Anticosti Island, Canada

A detailed study of Late Ordovician–early Silurian acritarchs (Palaeozoic phytoplankton) from Anticosti Island (Québec, Canada) revealed an unusually high abundance of abnormal forms from the upper Hirnantian carbonate strata (uppermost Ordovician) of the Ellis Bay Formation in the western part of the island (member 6, Spinachitina taugourdeaui chitinozoan Biozone). The objective of this paper is to describe these abnormal forms in detail. Two species are particularly affected: Disparifusa psakadoria Loeblich and Tappan, 1978 presents abnormally hypertrophied central vesicles, whereas Peteinosphaeridium laframboisepointense nov. sp. has appendices that are fused along their length. The abnormal forms of acritarchs occur in rocks deposited during periods that are near time-equivalents to those of maximum ice-sheet extensions on Gondwana during the Hirnantian glaciation. Although this stratigraphic level corresponds to an interval of strong perturbations of the global carbon cycle, the exact factors causing the observed malformations remain unknown. ► We observed abundances of teratological acritarchs in the Hirnantian of Anticosti. ► Rocks were deposited during near time-equivalents of maximum ice-sheets extension. ► The stratigraphical level corresponds to an interval of perturbations of the δ13C. ► Two species are particularly affected and are consequently described. ► The origin remains unknown but similar events occur at other periods.