Left behind – delayed extinction and a relict trilobite fauna in the Cambrian–Ordovician boundary succession (east Laurentian platform, New York)

Abstract 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 ofCordylodus proavusZone conodonts and the lowest post-extinction trilobites (ParakoldinioidiaEndo) 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 catillicephalidAcheilopsUlrich and several dikelocephalid species) are shown for the first time to persist well beyond the extinction as they occur with middleC. proavusZone conodonts (Clavohamulus elongatusor, more likely,Hirsutodontus simplexSubzone). The Ritchie Limestone member of the uppermost Little Falls Formation yields a succession of conodont faunas that spans theC. elongatus–H. simplex–Clavohamulus hintzeiSubzones (middle–upperC. proavusZone). These data prove that the trilobites are a relict fauna that persisted into theSymphysurinaZone 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 gallicusSerpagliet al. 2008 is a synonym ofT. nakamurai(Nogami, 1967), andT.?francisiLanding sp. nov., with a large base and tiny cusp, is a lowerC. proavusZone form. New trilobites areAcheilops olbermanniWestrop sp. nov. andParakoldinioidia maddowaeWestrop 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.