MORPHOLICAL PATTERNS IN THREE ENDEMIC CYCLOTELLA SPECIES FROM LAKE BAIKAL

The top 57m of a 100m core from Lake Baikal has been analyzed to determine patterns of morphological evolution for three endemic Cyclotella species. The core was recovered from the lake's central basin at a depth of 365 m. Microfossil abundance varied greatly within the sampled intervals, ranging from samples devoid of siliceous microfossils to samples with up to 3.49 x 10¹¹ microfossils g^-1 sediment. An unusual pattern of morphological development is revealed for the Cyclotella species. A single linear equation describes the relationship between diameter and specific morphological features for each of the three taxa over the course of their evolutionary history. Evolutionary independence for each of the three Cyclotella taxa is demonstrated by specific size categories and the production of specific auxospore size classes. Over time variance around the line describing morphological relationships of the taxa has descreased suggesting selective forces are acting on the three species. The similarity of the mathematical relationship describing morphological features of the taxa explains the taxonomic confusion associated with delineating specific species. The stratigraphic density of diatoms in the system and specimen abundance make this data set less prone to specific errors caused by sampling and preservational biases in other paleontological data sets. Trends in total microfossil abundance had a statistically significant positive correlation with dry weight percent biogenic silica and dry weight percent total organic carbon estimates. Fluctuations in these estimates appear to reflect trends in the lake's climatic history, with peak levels generally representing climate optima. The development of the Cyclotella species in the upper portion of the core occur despite these environmental fluctuations and suggest the potential selective forces acting on the species are independent of the environmental factors influencing the species composition of the diatom flora over time.