Morphological variation in extinct Aulacoseira (Bacillariophyta) species from Lake Towuti, with a description of novel species

Recent studies examining the fossil records of ancient Lake Towuti found this tropical lake once had abundant Aulacoseira spp. that no longer exist in the lake. Varying considerably in cell shape and size, these frustules were found deep within the sediments from the early stages of lake formation (>1 My) and formed two population maxima approximately 300 and 400 kya. We examined the morphological variation and taxonomy of the genus Aulacoseira in the fossil record of Lake Towuti and used principal component analysis (PCA) to confirm six different morphological forms of Aulacoseira. A taxonomic investigation identified one novel species (A. towutiensis sp. nov.), one regionally endemic species (A. malayensis stat. nov. com. nov.), and four new varieties of A. sumatrana sp. nov. not previously reported in the literature. The four varieties of A. sumatrana (var. elongata var. nov., var. miniscula var. nov., var. crustula var. nov., var. rectangularis var. nov.) formed a complex morphological continuum. Aulacoseira sumatrana was originally identified as Aulacoseira granulata var. valida from sediments in Lake Toba, but would now appear to be a unique regionally distributed taxon. PCA further revealed two distinct endmembers (A. towutiensis and A. malayensis), based on areola and cell size. The complex morphological variation observed in Aulacoseira in the sediments of Lake Towuti is suggestive of being the result of differing environmental conditions favouring one species over the other. It is concluded that, historically, Lake Towuti supported selected endemic planktonic diatom species that are now extinct.