FOSSIL VERTEBRATE ASSEMBLAGES AND IRIDIUM ANOMALY IN THE CRETACEOUS-PALEOGENE SECTION OF NEW JERSEY

A program of core drilling in the Cretaceous-Paleogene (K/P) stratigraphic interval of the New Jersey coastal plain has established iridium (Ir) excursions at six different sites at the K/P boundary. Several of these sites have associated vertebrate faunas in nearby outcrops and excavations, so for the first time the relationship of vertebrate fossil discoveries to well-documented Ir excursions can be established in this section. The Inversand pit has long been one of the primary sources of vertebrate fossils in the New Jersey K/P interval, and we can now associate a 0.5 ppb Ir anomaly (5x background) with the basal part of the Hornerstown Formation in a core taken adjacent to the pit exposures. The Ir anomaly occurs at the base of the stratigraphic horizon that contains the Main Fossiliferous Layer (MFL). The platinum-iridium ratio of this anomaly is 1.35, indicating an extraterrestrial source. The association of the Ir anomaly with the MFL suggests that the MFL concentration of vertebrates and invertebrates represents immediate post-impact mortality in the wake of deteriorating environmental conditions after bolide impact. Vertebrates found in the MFL include chondrichthyans, osteichthyes, chelonians, crocodylians, mosasaurs, and bird remains. Rare crocodylian remains occur above this, and another fossil assemblage is found 2-3 m above the MFL.

Just below the basal Hornerstown horizon, the New Egypt Formation yields mosasaurs, plesiosaurs, and dinosaurs. Dryptosaurus aquilunguis (Cope 1866) came from the top of the New Egypt; other dinosaur discoveries in this bed are the remains of hadrosaurs. On biostratigrphic evidence, the New Egypt is now given a late Maastrichtian age. These specimens are the youngest known dinosaurs in the Late Cretaceous of eastern North America, located just below the Ir anomaly level.

At the Meirs Farm site, an Ir anomaly was detected in the base of the Hornerstown Formation. This site was a significant source of mosasaur remains for O. C. Marsh, including the type specimens of several taxa. These and more recently discovered mosasaur and ammonite specimens are from the underlying New Egypt Formation.

In summary, the K/P section in the New Jersey coastal plain shows the response of the biota to impact-driven K/P environmental changes, with extinction of Mesozoic forms and the subsequent dominance of a more modern marine fauna.