DOI: 10.5061/dryad.1ns1rn8q5
title: Evolutionary stasis, ecophenotypy, and environmental controls on ammonite morphology in the Late Cretaceous (Maastrichtian) Western Interior Seaway, USA

description: 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.
description: Using a large collection of specimens of <em>Hoploscaphites
 nicolletii </em>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
 (δ<sup>18</sup>O and δ<sup>13</sup>C) 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.
description: <strong>Witts et al Supplementary Appendix -
 </strong>Including supplementary figures S1 and S2, a detailed
 taxonomic description of <em>Hoploscaphites nicolletii</em>,
 and expanded methodology for stable isotope analysis.
 <strong>Witts et al Supplementary Dataset S1
 </strong>- raw morphometric data for <em>Hoploscaphites
 nicolletii </em>specimens from the Pierre Shale and Fox Hills
 Formation. <strong>Witts et al Supplementary
 Dataset S2</strong>- summary and raw output of multiple linear
 regression models testing relationship between morphological traits and
 environmental variables. <strong>Witts et al
 Supplementary Dataset S3 </strong>- full carbonate
 (δ<sup>18</sup>O and δ<sup>13</sup>C) stable
 isotope dataset for specimens of <em>Hoploscaphites nicolletii
 </em>and information on preservation index (PI).