A MORPHOMETRIC EVALUATION OF THE LATE CRETACEOUS ANTARCTIC GUNNARITES GENUS USING LINEAR MIXED MODELS

The Late Cretaceous ammonite genus Gunnarites was abundant in the earliest Maastrichtian and is commonly used as a biostratigraphic marker across Late Cretaceous outcrops in the Southern Hemisphere (especially in Antarctica, Patagonia, South Africa, and Oceania). In the James Ross Basin (JRB) of the Antarctic Peninsula, Gunnarites is the dominant component of two local ammonite assemblages (assemblages 9 and 10; Olivero 2012) and is a useful tool for correlating isolated outcrops across the different islands in the JRB. However, despite the abundance of Gunnarites in the JRB and its use at the generic level for biostratigraphic correlation, species level distinctions have not been systematically evaluated, and little is understood about its ontogeny, taxonomy, or evolution. At least 18 species or varieties of Gunnarites have been described in the JRB, but it is unclear whether these represent valid taxonomic distinctions, or whether these described morphotypes may instead be capturing differences between ontogenetic stages, sexual dimorphs, or changes to morphology through time.

To characterize the morphological variation of Antarctic Gunnarites, we collected morphometric measurements of 130 Gunnarites specimens from nine localities across the James Ross Basin (including ImageJ measurements of 37 specimens figured in the literature). Each specimen was measured at multiple ontogenetic positions, when possible, and the full dataset includes measurements from more than 900 total measured positions. We used Linear Mixed Models (LMMs) to determine the relative contributions of within-individual variation (ontogeny) and inter-individual variation (e.g., sexual dimorphs, taxa) to the total morphological variation within Gunnarites. LMMs were also used to evaluate changes in Gunnarites morphology through time and space (across the JRB) and to characterize scaling relationships between morphometric variables and whorl height (size).