CHONDRICHTHYAN DIVERSITY AT THE CRETACEOUS-TERTIARY BOUNDARY, WESTERN MONMOUTH COUNTY, NEW JERSEY

In Monmouth County, New Jersey, the Cretaceous-Tertiary Boundary (KTB) is closely approximated by the contact between the Paleocene Hornerstown Formation and the Maastrichtian Tinton Formation (eastern Monmouth County) or New Egypt Formation (western Monmouth County). Evidence in the form of an Ir-rich layer, probable impact spherules, and in-place bivalves and other benthic animals, characterize the boundary (Landman et al. 2004, 2006). At the base of the Hornerstown Formation is a thin lag deposit known as the Main Fossil Layer (MFL). Dinoflagellate distributions suggest that there is a hiatus associated with the KTB and MFL of no more than about 100,000 years (Landman et al. 2004). At a site in western Monmouth County at which the KTB is well exposed, we collected chondrichthyan teeth at decimeter intervals from 1 m below the KTB to 1 m above. Our aim was to generate evidence that bears on the question of whether diversity in these animals changes appreciably close to or at the KTB. We collected about 2000 chondrichthyan fossils, mostly teeth, or tooth fragments. Our results are: 1) chondrichthyan diversity in this interval is low (only 7 species); 2) Maastrichtian species are found in the MFL, but only one (Carcharias samhammeri)is found above; and 3) Tertiary species do not occur in the MFL, and only 1 is found above. These results indicate that in New Jersey: 1) chondrichthyan diversity was low prior to the KTB mass extinction event; 2) New Jersey Maastrichtian species, with one exception, do not seem to have crossed the KTB into the Tertiary; 3) the recovery of chondrichthyan diversity in the New Jersey Paleocene did not begin immediately after the KTB. We infer from this that although factors other than an end-K impact reduced New Jersey chondrichthyan diversity in the Late Maastrichtian, the end-K mass extinction seems to have finished off the impoverished chondrichthyan fauna that remained, perhaps as a consequence of marine ecosystem collapse. Only one species got through this event, and apparently that went extinct soon after. Appearance of new Tertiary chondrichthyans in New Jersey was delayed, and did not occur until a time after the deposition of the study interval.