Person:

Kotrc, Benjamin

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.

The tests and scales formed by protists may be the epitome of lightweight bioconstructions in nature. Skeletal biomineralization is widespread among eukaryotes, but both predominant mineralogy and stratigraphic history differ between macroscopic and microscopic organisms. Among animals and macroscopic algae, calcium minerals, especially carbonates, predominate in skeleton formation, with most innovations in skeletal biomineralization concentrated in and around the Cambrian Period. In contrast, amorphous silica is widely used in protistan skeletons, and a majority of the geologically recorded origins of silica biomineralization took place in the Mesozoic and early Cenozoic eras. Amorphous silica may be favored in protist biomineralization because of the material properties of both silica itself and the organic molecules that template its precipitation. The predominace of carbonates and phosphates in macroscopic skeletons may, in turn, reflect the low quantities of dissolved silica in fresh and marine waters. The evolutionary success of diatoms has depleted silica levels in surficial waters since the Cretaceous Period, and fossils show that other biological participants in the silica cycle have responded both through altered habitat preferences and reduced use of silica in test construction. These natural instances of doing more with less might serve to inspire continuing innovations in biomimetic design.

The post-Paleozoic history of the silica cycle involves just two groups of marine plankton, radiolarians and diatoms. I apply paleobiological methods to better understand the Cenozoic evolution of both groups. The Cenozoic rise in diatom diversity has long been related to a concurrent decline in radiolarian test silicification. I address evolutionary questions on both sides of this coevolutionary coin: Was the taxonomic diversification of diatoms accompanied by morphological diversification? Is our view of morphological diatom diversification affected by sampling biases? What evolutionary mechanisms underlie the macroevolutionary decline in radiolarian silicification? Conventionally, diatom diversification describes a steep, monotonic rise, a view recently questioned due to sampling bias. For a different perspective, I constructed a diatom morphospace based on discrete characters, populated through time using an occurrence-level database. Distances between taxa in morphospace and on a molecular phylogeny are not strongly correlated, suggesting that morphospace was explored early in their evolutionary history, followed by relative stasis. I quantified morphospace occupancy through time using several disparity metrics. Metrics describing average separation of taxa show stasis, while metrics describing occupied volume show an increase with time. Disparity metrics are also subject to sampling biases. Under subsampling, I find that disparity metrics show varied responses: metrics describing separation of taxa into morphospace are unaffected, while those describing occupied volume lose their clear increases. Disparity can have geographic components, analogous to (\alpha) and (\beta) taxonomic diversity; I find more evidence of stasis in an analysis of (\bar{\alpha}) disparity. Overall, these results suggest stasis in Cenozoic diatom disparity. The radiolarian decline in silicification could result from either macroevolutionary processes operating above the species level (punctuated queilibria) or anagenetic changes within lineages. I measured silicification in three phyletic lineages, Stichocorys, Didymocyrtis, and Centrobotrys, from four tropical Pacific DSDP sites. Likelihood-based model fitting finds no strong support for directional evolution, pointing toward selection among species, rather than within species. Each lineage shows a different trajectory, perhaps due to differences in the ecological role played by the test. Because Stichocorys shows close correspondence to the assemblage-level trend, abundance may be an important factor through which within-lineage changes can influence the macroevolutionary pattern.