Three-dimensionally preserved specimens of Amplexograptus (Ordovician, Graptolithina) from the North American mid-continent: taxonomic and biostratigraphic significance

Abstract

Graptolites are relatively uncommon fossils in the Middle and Upper Ordovician carbonates of the North American mid-continent. With the notable exception of the Viola Springs Formation in Oklahoma, most units contain a single species, which occur within narrow stratigraphic intervals (often single beds). While uninteresting in terms of their diversity, these monospecific collections commonly yield excellent three-dimensionally preserved specimens that can be isolated from their matrix and studied. In particular, the large number of well-preserved specimens allows for accurate evaluations of intra-specific variation and reliable descriptions of taxa that are generally known from flattened or otherwise distorted material. Additionally, these graptolite collections can provide important biostratigraphic ties between disparate facies—shallow mid-continental limestones and the deeper-water black shales characteristic of continental margins.