a1 New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA
a2 Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73072, USA
a3 Museum für Naturkunde, Humboldt Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany
a4 Chevron Gulf of Mexico Business Unit, 5750 Johnston St., Lafayette, Louisiana 70503, USA
Two completely dissimilar faunal changes occur between the Sunwaptan and Skullrockian Stages (Ptychaspid and Symphysurid ‘Biomeres’) in the uppermost Cambrian on the east Laurentian craton. An undolomitized section in the Little Falls Formation in Washington County, New York, shows a typical ‘biomere’ extinction, with highest Sunwaptan trilobites followed by the abrupt appearance of Cordylodus proavus Zone conodonts and the lowest post-extinction trilobites (Parakoldinioidia Endo) 5.0 m higher. This stage boundary interval is very condensed by comparison with coeval Great Basin and Texas sections. Approximately 70 km southwest, typical pre-extinction taxa (the catillicephalid Acheilops Ulrich and several dikelocephalid species) are shown for the first time to persist well beyond the extinction as they occur with middle C. proavus Zone conodonts (Clavohamulus elongatus or, more likely, Hirsutodontus simplex Subzone). The Ritchie Limestone member of the uppermost Little Falls Formation yields a succession of conodont faunas that spans the C. elongatus–H. simplex–Clavohamulus hintzei Subzones (middle–upper C. proavus Zone). These data prove that the trilobites are a relict fauna that persisted into the Symphysurina Zone of the Skullrockian Stage. The massive (burrow-churned), mollusc-dominated Ritchie Limestone, with the second Upper Cambrian cephalopod locality in east Laurentia, represents an inner-shelf refugium for Sunwaptan trilobites that has not been previously encountered. Final extinction of typical Sunwaptan clades is at least locally diachronous, and a simple, genus-based approach to trilobite biostratigraphy in the Cambrian–Ordovician boundary interval is untenable. The relict fauna appears to be distinct at the species level, so it is likely that a viable, species-based biostratigraphy can be developed. Teridontus gallicus Serpagli et al. 2008 is a synonym of T. nakamurai (Nogami, 1967), and T.? francisi Landing sp. nov., with a large base and tiny cusp, is a lower C. proavus Zone form. New trilobites are Acheilops olbermanni Westrop sp. nov. and Parakoldinioidia maddowae Westrop sp. nov. The lowest Ordovician ‘Gailor Dolomite’ is a junior synonym of the Tribes Hill Formation, and the Ritchie Limestone is assigned to the top of the terminal Cambrian Little Falls Formation.
(Received April 12 2010)
(Accepted September 10 2010)
(Online publication November 19 2010)