MATTHEVIA (POLYPLACOPHORA) INVADES THE ORDOVICIAN: THE FIRST REPORTED POST-CAMBRIAN OCCURRENCE

Abstract

The polyplacophoran Matthevia Walcott, 1885 has been reported previously only from Cambrian rocks. Recovery of specimens of two different species of Matthevia from the top of the cherty lower Gasconade Dolomite in Missouri extends the known range of the genus into the Ordovician. The presence of the Lower Ordovician trilobite Praepatokephalus Lochman, 1964 in one collection provides the biostratigraphic evidence that the new specimens of Matthevia are Ordovician in age. Matthevia erecta n. sp. is described. An emended diagnosis for the trilobite Praepatokephalus is provided; two species of the genus are briefly described but are left in open nomenclature as P . sp. A and P . sp. B. Collections were made from 10 localities. The 10 specimens of Matthevia described here are from localities 4 and 6 in the upper part of the cherty lower Gasconade Dolomite of eastern Missouri, south of St. Louis (Figs. 1, 2). Locality 4 is at Spring Branch in Meramec State Park. Locality 6, which is just west of the settlement of Old Mines, Missouri, is owned by Guy and Doris Darrough. The species of Matthevia described here are the first undoubted representatives of the genus reported from rocks of Ordovician age. Figure 1 —Stylized stratigraphic section of the Cambrian–Ordovician boundary beds in eastern Missouri. The disconformity between the Eminence and Gasconade marks the traditional Cambrian–Ordovician boundary. The work herein and the definition of the boundary by Cooper et al. (2001) indicate that the boundary in eastern Missouri is at the top of the Gunter Sandstone Figure 2 —Generalized locality map of study area Stinchcomb and Darrough (1995, p. 54) noted the occurrence of Matthevia in the Gasconade Dolomite; however, they did not describe or illustrate any specimens from this formation. The single specimen they figured is from some unspecified level …