Geographic and temporal morphological stasis in the latest Cretaceous ammonoid Discoscaphites iris from the U.S. Gulf and Atlantic Coastal Plains

We examine temporal and spatial variation in morphology of the ammonoid cephalopod Discoscaphites iris using a large dataset from multiple localities in the Late Cretaceous (Maastrichtian) of the United States Gulf and Atlantic Coastal Plains, spanning a distance of 2000 km along the paleoshoreline. Our results suggest that the fossil record of D. iris is consistent with no within species net accumulation of phyletic evolutionary change across morphological traits or the lifetime of this species. Correlations between some traits and paleoenvironmental conditions as well as changes in the coefficient of variation may support limited population-scale ecophenotypic plasticity, however where stratigraphic data are available, no directional changes in morphology occur prior to the Cretaceous/Paleogene (K/Pg) boundary. This is consistent with models of ‘dynamic’ evolutionary stasis. Combined with knowledge of life history traits and paleoecology of scaphitid ammonoids, specifically a short pl...