A series of intermittent changes in Stephanodiscus (Bacillariophyta) valve morphology was observed in sediment core materials collected from Lake Biwa, Japan. A constant sedimentation rate reveals a continuous but punctuated pattern of morphological evolution. Morphological parameters, including maximum diameter and fascicle number v. diameter ratio (fascicle density), showed long periods of stasis and less than 10 000-year directional changes. Specifically, directional decreases in fascicle density occurred at 200–190 and 160–150 ka, and increases in maximum diameter occurred at 130–120 ka. According to a morphology-based species definition, a non-branching phylogenetic lineage originating from a cosmopolitan ancestor can be presumed from a morphology-based cladistic analysis and the biostratigraphic record. Evolutionary modes were selected from directional change, random walk, stasis and combined models by a statistical test, paleoTS, to clarify that the sequential changes are composed of relatively long stasis modes and short directional change modes. Global climate change was probably an influence during the early stages of the history of Lake Biwa.
Time-series data to discriminate the tempo and mode of morphological evolution are available at http://www.geolsoc.org.uk/SUP18825