CHONDRICHTHYANS OF THE CRETACEOUS-TERTIARY BOUNDARY SEQUENCE, MONMOUTH COUNTY, NEW JERSEY

Recent biostratigraphic work by Landman et al. (2004, 2006) indicates that in Monmouth County, New Jersey, the K/T boundary is bracketed by the Hornerstown Formation (Danian) and the New Egypt/Tinton Formations (Maastrichtian). Dinoflagellates indicate that the boundary interval may be missing no more than about 100,000 years. We collected chondrichthyan teeth across the K/T boundary at three sites in Monmouth County studied by Landman et al. Teeth are rare to abundant depending on stratigraphic position, locality, and lithology, and there is a wide range in preservational state with some teeth in pristine condition and some reduced to small unidentifiable fragments. Ray teeth are not nearly as common as shark teeth. The boundary sequence contains such genera as: Cretolamna; Carcharias; Odantaspis; Paranomotodon; Palaeogaleus; and Dasyatis. Specimen counts from measured volumes of sediment sampled at measured intervals above and below the boundary indicate that the teeth of some genera are more abundant in the Latest Maastrichtian (e.g., Galeorhinus) while the teeth of other genera (e.g., Dasyatis) are more abundant in the Early Danian. Some forms go across the boundary with no apparent change in abundance (e.g., Carcharias), although different congeneric species may occur on opposite sides of the boundary. Because bioturbation and reworking occur in the exposures we examined, it is not yet clear whether these abundance and diversity changes are precisely correlated with the K/T boundary, but at least some changes appear to take place within 20 cm above or below the boundary. The data gathered to date, when compared to chondrichthyan diversity lower in the New Jersey Late Cretaceous section, suggest a strong chondrichthyan diversity decline at the species level across the K/T boundary in Monmouth County. At the genus level the decline is less marked. Diversity remains low in the Lower Hornerstown Formation (Lower Danian), but increases later in the Paleocene.