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. In this study, we recovered three-dimensionally preserved graptolite specimens from four collections of Middle Ordovician limestones made in the eastern and central United States. All four collections yielded mono-specific assemblages of specimens that belong to the genus Amplexograptus . Specimens isolated from the Lebanon Limestone of Tennessee are particularly noteworthy as they are referable to Amplexograptus perexcavatus (Lapworth)—the poorly known type species of Amplexograptus . The presence of three-dimensional graptolites in the Lebanon Limestone was previously mentioned in the literature (Hofstetter, 1965; Conkin and Conkin, 1992), but the specimens were not described. Two other collections, from the Upper Bromide Formation in Oklahoma and the Mifflin Member of the Platteville Formation in Illinois, yielded specimens of A. maxwelli (Decker). Our last sample, from the Kope Formation in Kentucky, yielded specimens of A. praetypicalis Riva. The purpose of this paper is to re-describe A. perexcavatus based on well preserved three-dimensional material, to quantitatively compare the morphology of A. perexcavatus to the better known species of Amplexograptus found in our other samples, and to discuss the biostratigraphic significance of these samples. …