Chitinozoans from the upper Tremadocian (Lower Ordovician) Watch Hill Formation of the Lake District, northern England

摘要

The Global Stratotype Section and Point (GSSP) for the Lower Ordovician Floian Stage is defined by the first appearance datum (FAD) of the graptolite Tetragraptus approximatus at Diabasbrottet Quarry, Hunneberg, southern Sweden. Correlation of sections with the Floian GSSP when graptolites are absent, however, must rely on other criteria. Among other fossil groups with potential for biostratigraphical correlation at this level are chitinozoans. Here we describe a chitinozoan assemblage from the Watch Hill Formation in the Skiddaw Group of northern England. The Watch Hill Formation is dated on graptolite evidence as late Tremadocian, but the assemblage contains Euconochitina symmetrica, a species previously considered to indicate the eponymous lowermost Arenig (Floian) chitinozoan biozone in a long-established Gondwanan Ordovician biozonal scheme. The hitherto earliest Floian index species has also been reported recently from Tremadocian successions in South China and Morocco. We here confirm its extended range into the Tremadocian and its continued significance as an index species for the Tremadocian-Floian boundary interval.