MICROSTRATIGRAPHIC ANALYSIS OF FOSSIL DISTRIBUTION IN THE LOWER HORNERSTOWN AND UPPER NAVESINK FORMATIONS AT THE EDELMAN FOSSIL PARK, NJ

The Maastrichtian–Danian fossiliferous sediments of the Navesink and Hornerstown formations exposed at the Jean and Ric Edelman Fossil Park at Rowan University in Mantua Township, New Jersey, have long been of interest to paleontologists. Within the basal portion of the Hornerstown Formation occurs the Main Fossiliferous Layer (MFL), a rich bonebed assemblage investigated previously by numerous studies on taphonomy, taxonomic description, and chronostratigraphy. Yet over the last 50 years, debate has ensued as to the stratigraphic and geochronologic position of this fossil layer, adding to debate over the taphonomic origin of this bonebed. Herein, we define the taxonomic composition and distribution of vertebrate and invertebrate fossils within the lower Hornerstown Formation. Through methodical top-down excavation, we recorded the position of every fossil encountered in three dimensions and then analyzed the results statistically and spatially. Three-dimensional visual displays and analyses of these data made using ArcGIS Pro yielded an unprecedented look at this fossil bonebed. We found that most reported discrepancies about the stratigraphic placement and thickness of the MFL can be explained by the presence of two distinct fossil assemblages within this interval that are occasionally combined into a single bonebed. The stratigraphically-lower fossil assemblage, which we herein term the “oyster layer”, exhibits low taxonomic diversity, high abundance of the oyster Pycnodonte dissimilaris, moderate taxonomic richness, and is generally tabular in geometry. The stratigraphically-higher fossil assemblage, the MFL, occurs approximately 9 cm higher in section and exhibits high values of taxonomic diversity, fossil abundance, and taxonomic richness. Importantly, our formal definition of the MFL and detailed analysis of its stratigraphic placement is essential for future studies on the taphonomic origin and chronostratigraphy of this bonebed. Our work demonstrates that all researchers must use the same definition of the MFL, as proposed herein, if we are to elucidate the exact chronostratigraphic positions of precise geochronologic indicators (e.g., shocked quartz, iridium spike) within it or correctly estimate the degree of time averaging of its fossils.