SHARKS FROM THE BASAL HORNERSTOWN FORMATION, MONMOUTH COUNTY, NEW JERSEY: IMPLICATIONS FOR A SEVERE CHONDRICHTHYAN EXTINCTION AT OR NEAR THE K/T BOUNDARY

An exposure of the basal Hornerstown Formation in northeastern Monmouth County, recently reported by Landman et al. (2004), contains teeth of five species of lamniform sharks: Squalicorax pristodontus, Cretolamna appendiculata, Scapanorhynchus texanus, Archaeolamna kopingensis, and Carcharias samhammeri. All these species are well known from Maastrichtian deposits in the Atlantic and Gulf Coastal Plains and the Western Interior Seaway. These teeth are relatively rare components of a fossiliferous lag horizon at the base of the Hornerstown Formation that also contains dinoflagellates, and an array of invertebrate fossils, including ammonites. The ammonites are diagnostic of the Discoscaphites iris zone, the highest ammonite zone of the Late Maastrichtian. Dinoflagellates indicate that the lag represents a gap of about 100,000 years and encompasses the K/T boundary. We interpret the co-occurring lamniform shark teeth, all of which are heavily abraded, to represent latest Maastrichtian materials reworked into this lag. None of these species are known from horizons higher in the New Jersey section than this K/T lag, a situation that suggests their extinction at or near the New Jersey K/T boundary. This pattern of chondrichthyan disappearance close to the K/T boundary is also observed in the Arkadelphia Formation of Arkansas, and in the Fox Hills, Hell Creek, and other Late Maastrichtian units of the Western Interior. We interpret this pattern to indicate that North American chondrichthyans suffered a significant diversity bottleneck at or near the K/T boundary, perhaps as a result of marine ecosystem collapse due to the Chixulub impact event.