Variation in Shell Morphometry of Neothauma in Lake Tanganyika, Africa

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

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Abstract:

Analysis of morphometric variation of shells is one critical component of reconstructing environmental change recorded in surficial shelly accumulations because it can be used to quantitatively analyze how species grow, live, and the conditions they may have endured. By comparing shell-size distributions of shells in different sub-environments, paleobiologists can use these data to interpret environmental or biological changes that might vary by space or time. However, when shells have accumulated over time (time averaging), the size distributions may also be affected by post-mortem processes, which may vary because of changing conditions as the shells accumulated.  

The focus of this research is on modern shell beds found in Lake Tanganyika, Africa. This is the largest of the African rift lakes which liely formed in the Miocene. Within the lake there are vast sub-modern shell beds comprised of the shells of the aquatic snail, Neothauma tanganyicense. Collections of these shell beds were taken from several offshore transects along four widely separated study sites between 9 and 20 m water depth. For this study two samples from 9 and 20 meter water depths at each study site were analyzed for morphometric characteristics. For each sample, 30 randomly selected shells were chosen; however, if a selected shell was fragmented or damaged to a set criteria, it was eliminated from the analysis. Using a digital caliper, the height, width, aperture width, and number of whorls of the gastropod were recorded. The results indicate the average shell length per site varies from 15.3 mm to 44.3 mm with the mode in the mid-thirties. Histograms of the shell height in each sampling location are bimodal in some sites, but unimodal in others. A cluster analysis exhibited differences in the morphometrics of the samples between the 9m and 20m water depths samples as well as samples at the same water depth but different locations along the lake shore. These differences likely reflect differences in taphonomic processes and possibly changes in recent sedimentation patterns along the sites rather than differences in population structure of the gastropods. Further analysis is needed to determine how exactly these factors may have caused these differences.