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 2 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. IntroductionThe diversity of diatom form has been a source of fascination and inspiration since diatom frustules were first described by the 19th Century pioneers of micropaleontology (Ehrenberg 1838, Haeckel 1904 and their shapes applied to Art Nouveau architecture and design, like René Binet's design for the Printemps department store (Proctor 2006) or Hendrik Petrus Berlage's jewelry imitating chain-forming diatoms (Netherlands Architecture Institute 2012). Many thousands of extant diatom species have been described (Mann and Droop 1996), their shapes representing a wide range of variations on a basic pill-box Bauplan-from circles to triangles, needles, and curves-with staggering variety in the geometrically arranged, hierarchical pore structure (Round et al. 1990), lending an aesthetic that evidently appealed to turn-of-the-century designers. With biomimetic design advancing from superficial aesthetic inspiration to an application of underlying structural and evolutionary principles, renewed interest in diatoms warrants efforts toward a deeper understanding of their diversification, a cardinal feature of any clade's evolutionary history.The fossil record provides two windows on clade diversification history: taxonom...

The Miocene: The Future of the Past

“…The Neogene diversification of diatoms has been linked to high latitude cooling, the evolution of the Southern Ocean and development of the cold ocean interior (Berger, 2007; Bown, 2005; Cervato & Burckle, 2003). Hypotheses linking ocean cooling to stronger oceanic fronts, increased turbulence and enhanced nutrient availability in the photic zone likely underpin (regional) increases in diatom productivity (Falkowski et al., 2005; Katz et al., 2005; Kemp et al., 1995) However, it has also been suggested that the diversity patterns are partly a sampling artifact of the “pull of the recent” and the greater sampling of more recent sediment cores (Kotrc & Knoll, 2015; Rabowsky & Sorhannus, 2009). The most up‐to‐date syntheses of morphological, genetic, and fossil data (Lazarus et al., 2014; Nakov et al., 2018) indicate that speciation rates have indeed exceeded extinction rates since the early Miocene leading to significant diversification.…”