Morphological differences are predicted by geographic location for Maastrichtian <i>Gunnarites</i> from the James Ross Basin, Antarctica

Abstract The early Maastrichtian kossmaticeratid ammonite Gunnarites serves as an important biostratigraphic index fossil for correlating between localities within the James Ross Basin (JRB), Antarctica. The JRB is the highest southern paleolatitude marine sedimentary outcrop record of the Upper Cretaceous. Gunnarites is abundant within a relatively narrow stratigraphic interval in the JRB, and its presence in outcrops is used to recognize this specific interval, which is coeval with environmental disturbances including cooling temperatures and local sea level regression. Early taxonomic work on the genus (e.g., Spath, 1953) qualitatively described morphological differences between Gunnarites specimens from different locations within the JRB; if real, these location-based differences could have important implications for the utility of Gunnarites for biostratigraphy in the JRB and could contribute to our understanding of controls (e.g., environmental or temporal) on ammonite morphology. To test whether JRB Gunnarites specimens exhibit location-based morphological differences, we collected morphometric measurements (conch morphology and ribbing parameters) from multiple ontogenetic stages on 118 specimens from seven distinct localities within the JRB. We used linear mixed models and generalized additive mixed models to quantitatively evaluate mean differences in morphological variables by location, while accounting for variation in specimen size (i.e., characterizing scaling relationships between size and shape). Every morphological variable except whorl radius expansion rate exhibited significant location-based differences, although patterns of variation across sites differed by morphological variable. These location effects on Gunnarites morphology may be a result of phenotypic variation along an environmental gradient (e.g., depth), a result of temporal effects (e.g., evolution), or a combination of both. Large magnitude morphological differences between nearby locations may also be due, in part, to the influence of stratigraphic repetitions produced by previously hypothesized structural features in the JRB such as faults or folds, which could disrupt spatial or temporal gradients in the basin. Untangling the exact mechanisms behind location-based morphological differences in Gunnarites must await more precise, independent, age-dating of each outcrop.