The age of the Euconochitina symmetrica Zone and implication for Lower Ordovician chitinozoan and graptolite zonations of Laurentia

An Euconochitina symmetrica Zone was defined in the G-locality at Lévis, Quebec, within an interval belonging to Zone A of Raymond that contains Paratetragraptus approximatus. The interval was consequently attributed to the Floian. E. symmetrica was also recognized in Northern Gondwana, and the species was considered an undisputed marker for the base of the Floian. During the past decades, several occurrences of E. symmetrica have been reported from Avalonia, southern China, and even from Northern Gondwana in Tremadocian strata casting doubt on the Floian age of the Zone. Revisions of the graptolites in the G locality and N section at Lévis provides a frame on which the age of the E. symmetrica Zone can be precisely documented. At both localities the E. symmetrica chitinozoan assemblage is older than the base of the Paratetragraptus approximatus graptolite Zone. Therefore, the presence of E. symmetrica can no longer be correlated with the graptolite P. approximatus Zone as it was initially suggested. It rather characterizes the upper Tremadocian Sagenograptus murrayi Zone, its upper part could however reach the Floian. An older assemblage characterized by a Lagenochitina gigas n. sp. assemblage (previously designated Lagenochitina maxima) occurs in strata correlated with the Tremadocian Aorograptus victoriae graptolite Zone. Correlation of the biostratigraphic ranges of the chitinozoan and graptolite faunas in the lower part of the Lévis Formation allowed for a better positioning of the Lower Ordovician Laurentian chitinozoan zones on the Ordovician time scale.