LATE CRETACEOUS GUNNARITES AMMONITES EXHIBIT LOCATION-BASED MORPHOLOGICAL DIFFERENCES WITHIN THE JAMES ROSS BASIN, ANTARCTICA

Macrofossils, particularly the Gunnarites genus of ammonites, are important tools for correlating outcrops across different islands in the James Ross Basin (JRB), Antarctica. In the JRB, Gunnarites is the dominant component of ammonite assemblages 9 and 10 (Olivero 2012) and its first occurrence in the basin roughly coincides with the base of the Maastrichtian. Although Gunnarites is currently used for biostratigraphy only at the generic level, previous workers have observed morphological differences in Gunnarites from different islands within the JRB (e.g., Spath 1953), in some cases even retaining distinct species unique to certain localities. Characterizing the scope and pattern of location-specific morphologies may provide important context for the use of Gunnarites as a biostratigraphic tool, or an understanding into environmental controls on ammonite morphology.

To quantitatively evaluate location-based differences in Gunnarites morphology in the JRB, we collected measurements of conch shape and ribbing characteristics for 123 specimens from 7 locations in the basin (sites on James Ross Island, Vega Island, Humps Island, and Snow Hill Island). Each specimen was measured at an average of 6 ontogenetic positions. Measured variables included whorl height, whorl shape, coiling tightness, whorl expansion rates, rib angle, rib sinuosity, and rib count.

We used linear mixed models (LMMs) to estimate location-based differences in morphology and to model the scaling relationships between each measured response variable and specimen size (whorl height) while accounting for between-individual variation. For almost all response variables (all except radius expansion rate), we found strong support for location-based differences in conch morphology and ribbing characteristics. However, our response variables differed in the pattern of variation across sites (e.g., geographically-distant sites had the most divergent rib counts but other traits followed more complex patterns). There are many possible explanations for location-based morphological differences in Gunnarites; some explanations (e.g., environmental plasticity along a depth gradient in a narrow interval of time) are consistent with existing stratigraphic interpretations of the basin, but alternative explanations (e.g., evolution over a longer interval of time) cannot be ruled out.