Stratigraphically significant acritarchs in uppermost Cambrian to basal Ordovician strata of northwestern Algeria

In Cambro-Ordovician transitional beds of the Hassi R'mel area, northwestern Algeria, three stratigraphically successive acritarch assemblage zones can be recognized: HM/A (uppermost Cambrian), HM/B (basal Tremadoc), and HM/C (lower Tremadoc; with associated graptolites of the Rhabdinopora flabelliformis sensu lato group). Thus, the boundary between assemblages HM/A and HM/B is considered to approximate to the Cambrian-Ordovician boundary in the study area. Six morphologically distinctive acritarch species that facilitate recognition of zonal boundaries are described. One genus and four species are newly instituted: Phenacoon imperfectum gen. et sp. nov., Impluviculus araneus sp. nov., Ladogella saharica sp. nov., and Multiplicisphaeridium attenuatum sp. nov. Revised circumscriptions of two other species—Acanthodiacrodium baculatum and Baltisphaeridium verutum—incorporate morphological details not previously available. P. imperfectum and L. saharica are abundant at the top of assemblage zone HM/A; A. baculatum characterizes the base of assemblage zone HM/B; and B. verutum, I. araneus and M. attenuatum are characteristic components of the lower part of HM/C assemblage zone