We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analyzed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analyzed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play, however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilising selection under a fluctuating optimum (likely reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shell-specific δ¹⁸O, despite a correlation between shell size and δ¹⁸O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data.

Using a large collection of specimens of Hoploscaphites nicolletii from the Cretaceous (Maastrichtian) Fox Hills Formation of the US Western Interior Seaway held four institutions ((Yale Peabody Museum (YPM), 1,137 specimens), American Museum of Natural History (AMNH), 3 specimens), Museum at the Black Hills Institute of Geological Research (MBHI), 164 specimens) and the Timber Lake Museum (TLM), 69 specimens)) we took a series of morphometric measurements. From each measured specimen we calculated ratios that capture the size, shape and degree of compression of these shells throughout their geographic and stratigraphic range. Statistical analyses were used to compare the significance of changes in each trait through time. Stable isotope data (δ¹⁸O and δ¹³C) was collected from analysis of well-preserved shell material, and other palaeoenvironmental information from the Fox Hills Formation was obtained from the literature. Multiple linear regression analysis was used to compare traits to environmental data. All analyses were conducted in R.

Witts et al Supplementary Appendix - Including supplementary figures S1 and S2, a detailed taxonomic description of Hoploscaphites nicolletii, and expanded methodology for stable isotope analysis.

Witts et al Supplementary Dataset S1 - raw morphometric data for Hoploscaphites nicolletii specimens from the Pierre Shale and Fox Hills Formation.

Witts et al Supplementary Dataset S2- summary and raw output of multiple linear regression models testing relationship between morphological traits and environmental variables.

Witts et al Supplementary Dataset S3 - full carbonate (δ¹⁸O and δ¹³C) stable isotope dataset for specimens of Hoploscaphites nicolletii and information on preservation index (PI).