Rare Earth Element (REE) signatures have been used to test whether mosasaur bones in a basal Hornerstown Formation bonebed (the Main Fossiliferous Layer, or MFL) in New Jersey were reworked from the underlying Maastrichtian beds or deposited synchronously with the bones of other taxa in the Hornerstown Formation. The interpreted age of the bonebed (Maastrichtian vs. Danian) and the position of the K-T boundary in New Jersey are affected by the possible reworking. Statistical techniques, such as ANOVA and Discriminant Analysis, show that signatures of REE in MFL bones are different from those of bones in either the underlying Navesink Formation or the upper part of the Hornerstown Formation, suggesting a unique depositional setting for this bonebed. REE signatures of the MFL mosasaur bones conform with signatures in bones from other taxa within the MFL, suggesting that their deposition was contemporaneous with that of the other taxa. Thus, the MFL bonebed appears to be Cretaceous in age and the K-T boundary must be in or above the MFL, within the Hornerstown Formation.