Examining Valve Shape Variation in the Freshwater Diatom Genus Eunotia over Time and Space

A geometric morphometrics landmark analysis was used to investigate evolutionary shape change of Eunotia, a cosmopolitan freshwater diatom genus. Modern records from temperate and tropical habitats were compared with fossils originating from a locality in the Northwest Territories of Canada. The Giraffe Pipe sediment core from a middle Eocene tropical lake contains abundant siliceous remains of diatoms and other taxa, including Eunotia, making these the oldest known representatives of the genus. The purpose of this study was to investigate the shape of Eunotia when comparing modern tropical specimens to analogous fossil specimens. Over 600 specimens were examined in total, utilizing type material and monographic plates from the literature. Linear measurements such as valve length, width and striae density were recorded for each specimen alongside associated collection data, such as habitat and geography. With morphometrics analysis, 4 permanent and 76 semi-permanent landmarks were used to characterize the valve shape change of Eunotia. Data subsets were used to investigate shape change due to climate, geography and age. Specimens underwent a General Procrustes analysis, and were then displayed along principal components. Valve linear measures underwent linear regression and were then compared. Results show significant differences in the shape of fossil and modern Eunotia, as well as between modern tropical and temperate specimens when compared against fossil representatives. Overall however, Eunotia morphology appears to have been relatively consistent, suggesting evolutionary stasis, since at least the middle Eocene