NORTH AMERICAN AMMONITE STASIS OVER EVOLUTIONARY TIMESCALES

Ammonoid cephalopod mollusks (ammonites) are well known to have experienced significant diversification and increased disparity during the Late Cretaceous; it has even been hypothesized that morphological “plasticity” contributed to their demise at the end-Cretaceous (KPB) mass extinction event (i.e., if disparity was related to specialization and/or geographic isolation). This potential “adaptability,” and their abundant fossil record, make ammonites an excellent group with which to test the hypothesis of punctuated equilibrium. The family Scaphitidae is one of the most common and diverse Late Cretaceous ammonite clades, persisting to the KPB. We focused on two species: Hoploscaphites nicolletii (endemic to the North American Western Interior) and Discoscaphites iris (endemic to North America and abundant in the U.S. Atlantic and Gulf Coastal Plains). Variables capturing variation in whorl width, height, length, and shell compression were measured in over 1500 individual specimens to test for directional change vs. stasis of morphological characters across the entire spatial and temporal range of each species. These data were then compared to broad environmental proxies such as stable isotope reconstructed temperature estimates, dominant substrate type, and paleo- latitude and longitude to investigate potential ecophenotypic drivers of morphological variation. Results from both species demonstrate morphological change over space and time; however, this was reversible and showed no net directional change, supporting the dynamic stasis model of Eldredge and Gould (1972) observed in the fossil record of many other groups. Potential ecophenotypic changes observed within each species are further consistent with stasis driven by differential selection across paleoenvironments and/or stabilizing selection with a fluctuating optimum.