Phylogenies provide the context in which hypotheses about character evolution can be tested. But the phylogenetic understanding of many diatom taxa, including some of the more spatially and temporally ubiquitous genera, is insufficient to fruitfully pursue questions about character evolution within them. Consequently, we have estimated the phylogeny of Aulacoseira using a combination of nucleotide sequence data and data derived from morphological characteristics of the silica cell wall. In addition to the use of traditional "discrete" or "qualitative" morphological characters, we have assessed the use in our cladistic analyses of "continuous" or "quantitative" characters, including descriptors of the ontogenetic trajectories of these characters over a species' life cycle (size range). Also, molecular characters gathered from a chloroplast genome marker (rbcL) as well as a nuclear genome marker (18S rRNA) were used simultaneously in the cladistic estimation of the phylogeny of approximately 12 species of Aulacoseira. In order to have more thorough taxon sampling for a better estimate of phylogeny we included several fossil species, as well as taxa where cultures were unavailable, by utilizing the morphologically based characters despite the absence of molecular characters for these taxa.

Key words: Aulacoseira, diatoms, phylogeny, total evidence