THE ONES THAT GOT AWAY: A RELICT, POST-EXTINCTION TRILOBITE FAUNA IN THE CAMBRIAN—ORDOVICIAN BOUNDARY INTERVAL OF NEW YORK

The trilobite extinction at the end of the Late Cambrian (late Furongian) Sunwaptan Stage in Laurentian North America has been documented at several sites across the continent. The faunal change consistently involves turnover of genera and species through an interval comprising the Eurekia apopsis and Parakoldinoidia depressa Zones. By the end of this interval, which spans 6–26 m of strata at different localities, taxa from pre-extinction biofacies disappear across the Laurentian shelf. In eastern New York, the Ritchie Limestone Member (Skullrockian Stage; Clavohamulus elongatus-C. hinztei Subzones; uppermost Little Falls Formation) is a shallow-water carbonate that post-dates the extinction, but it yields a unique, relict, low diversity fauna of new species of the trilobites Calvinella Walcott (Dikelocephalidae) and Acheilops Ulrich ("Catillicephalidae"). Elsewhere, these genera do not survive beyond the E. apopsis Zone in the lower part of the extinction interval, and their occurrence in the Ritchie is a significant range extension. The data indicate that that the extinction was more protracted than previously believed, with some taxa persisting in local refugia. The presence of refugia is not unexpected, as some clades that disappear in the extinction interval, including the Remopleurididae, Norwoodiidae, and "Catillicephalidae" reappear as Lazarus taxa in younger strata. The record of Cambrian extinctions in Laurentia is undersampled, and most of our knowledge comes from the western and south-central parts of the continent. Finally, the relict fauna has biostratigraphic implications. Until now, the mere presence of dikelocephalid trilobites would indicate an unequivocal Sunwaptan to basal Skullrockian age for these strata, but this is no longer tenable.