Morphospaces and Databases: Diatom Diversification through Time

Abstract

The diversity of diatom form inspired Art Nouveau designers, an interest renewed by recent advances in biomimetic design. The fossil record provides two windows on the diversification history of diatoms: taxonomic diversity and morphological disparity. Marine planktonic diatom diversity is conventionally interpreted to describe a steep, almost monotonic rise through Cenozoic time. Subsampling methods used to address the associated rise in sampling reveal a more stationary pattern, with peak diversity in the mid-Cenozoic, whether by established methods or a new method (shareholder quorum subsampling, SQS). However, these methods may underestimate diversification if evenness decreases. In order to measure morphological disparity, we constructed an empirical morphospace based on discrete characters. Mean pairwise distance, a disparity metric describing the density of taxa in morphospace, shows little secular change , while convex hull volume, a measure of the extent of occupied morphospace, increases through time. Since we populated the morphospace with occurrence-based data, we can apply subsampling algorithms to these disparity metrics. Mean pairwise distance is largely unaffected, while the increase in occupied volume largely disappears under subsampling. Depending on the metric used, characterizing diatom diversification thus depends upon whether a literal reading of the fossil record or the use of subsampling algorithms is preferred. While this may prompt a reexamination of evolutionary narratives prominently featuring diatom diversification, changes in abundance and silicification may also affect the diatom’s biogeochemical importance. For biologically inspired design, an early exploration of diatom morphospace suggests that fossil forms should be considered alongside extant diatoms.