MORPHOMETRIC CHANGES IN AN ANTARCTIC NASSARIID GASTROPOD DURING THE EOCENE

The impact of climate change on marine faunas globally has received significant attention. The Eocene of Antarctica offers the opportunity to examine how global cool-down impacted marine mollusks in a shallow, marine, high-latitude environment. This assemblage is especially interesting because these stratigraphic units record a decline in durophagous predators like crabs due to the temperature changes. This shift in the food web has a cascading effect on the whole community in particular mollusk prey organisms. Thus, we hypothesize that a reduction of predation pressure could result in prey organisms allocating less energy to the costly building of thick, well-defended shells which would be reflected by changes in shell morphology.

Here we concentrate on the gastropod taxon Sudonnassarius that is found throughout Seymour Island’s La Meseta Formation. Over 300 well-preserved specimens from museum collections have been photographed and the images were landmarked using tpsDig2 software. Ten landmarks and ninety semilandmarks tracing the outline of the aperture and outside of the body whorl were used to define the outline of the shells. The data was processed using MorphoJ and PAST software. Generalized Procrustes Analysis was performed and Principal Component Analysis was used to visualize shape changes.

Initial results suggest that there is not a significant morphological change through time, but we were able to observe three distinct morphological groups in the samples. Over 70% of the variation is introduced by the difference of a slender morphotype, which develops a narrow aperture and slim high spire in comparison to a group of more stout specimens with a shorter spire and a wide and round body whorl. The third morphotype falls in between these two groups with the maximum curvature of the outer lip of the aperture and body whorl is located towards the anterior end of the shell. All three morphotypes are present throughout the Eocene, but adding more samples will help us better understand their taxonomic context and stratigraphic range. Furthermore, an expanded sample size will enable us to examine these groups individually to understand if they responded morphologically to the climate-induced shifts in the marine communities of Antarctica’s Eocene.