Exceptionally well-preserved chitinozoans from the Lower Silurian Measley Ridge Roadcut (Ohio, USA)

The Measley Ridge section, a roadcut located in southern Ohio (USA) provides a remarkable example of Silurian strata. The middle and upper part of the Estill Shale is exposed in this outcrop and the Llandovery-Wenlock boundary is tentatively placed low within this unit. The lithology is composed of reddish-brown to maroon shales with lenticular calcareous siltstones at the base, turning into greenish-gray to olive shales at the top and deposited in an open marine outer ramp with high sedimentation rates (McLaughlin et al., 2008). The Llandovery-Wenlock (late Telychian-early Sheinwoodian) age of this section is based upon conodonts (Kleffner, 1987), although it is accepted that this dating is not particularly precise.

The aim of this study is to produce a high-resolution chitinozoan biostratigraphical scheme for the section, in order to provide further age constraint of the unit and to confirm the position of the Llandovery-Wenlock boundary.

Twenty-five samples from the middle and upper part of the Estill Shale in the Measley Ridge section were processed for chitinozoans and yielded a diverse and well-preserved assemblage. Ten genera and sixteen species were recognized with typical Llandovery-Wenlock species such as Pogonochitina djalmai and Ramochitina angusta observed through all the section. Long ranging species such as Ancyrochitina ancyrea, Ancyrochitina primitiva, Margachitina margaritana and several Conochitina and Euconochitina species were also distributed within the unit. However, Cingulochitina burdinalensis is observed in the lower and middle part of the section but does not reach the upper strata. Since this species is restricted to the Llandovery, its last appearance datum in the section may indicate the position of the Llandovery-Wenlock boundary.

The chitinozoan assemblage overall from the Estill Shale in the Measley Ridge section confirms the Llandovery-Wenlock age of the section.