EVOLUTIONARY STASIS, ECOPHENOTYPY, AND ENVIRONMENTAL CONTROLS ON AMMONITE MORPHOLOGY IN THE LATE CRETACEOUS (MAASTRICHTIAN) WESTERN INTERIOR SEAWAY, USA

Quantitative investigations of the fossil record are of fundamental importance for understanding the processes driving macroevolutionary changes through time. Such analyses require large sample sizes, excellent preservation of fossils, and well-constrained stratigraphy. The record of ammonites in the Late Cretaceous Western Interior Seaway (WIS) of the United States meets these criteria. We examined morphological changes in a single species of ammonite (Hoploscaphites nicolletii) across its entire temporal and geographic range, based on a collection of >1500 specimens from the late Maastrichtian Elk Butte Member of the Pierre Shale and overlying Trail City Member of the Fox Hills Formation. Using material from five stratigraphic horizons which occur over ~4000 km² and conservatively represent 0.5 – 0.75 million years, we measured seven parameters which capture shell size and shape, and coupled our morphological investigation with an analysis of the oxygen and carbon isotopic composition of well-preserved shell material to determine environmental conditions. Results reveal Hoploscaphites nicolletii exhibits evolutionary stasis over its lifetime, concurrent with fluctuating seawater paleotemperatures around a relatively stable mean of 18-21°C. Despite some statistically significant changes, morphological traits do not generally show any kind of unidirectional trend in terms of size or shape change from first to last occurrence, and changes that do occur between horizons are ephemeral and reversable. Statistical models linking morphological changes to various environmental parameters did not reveal any significant correlation between morphology and specific environmental regime, including average inferred temperature. Specimens from a single location in the Elk Butte Member are unusually large in size, suggesting development of a local population influenced by enhanced nutrient input. An increase in shell compression in the upper Trail City Member can also be related to concurrent grain-size increase and sandier substrates, consistent with a hydrodynamic response to increasing depositional energy. These data are thus consistent with a plastic response to local environmental changes and a “dynamic” version of evolutionary stasis mediated by population-level selection pressures.