A MORPHOSPACE OF MARINE PLANKTONIC DIATOMS AND ITS SAMPLING-STANDARDIZED OCCUPANCY THROUGH THE CENOZOIC ERA

Molecular clocks and fossil first appearances both suggest that diatoms had evolved most of their current morphological range by the end of the Cretaceous Period. In contrast, a canonical reading of the fossil record suggests a dramatic Cenozoic rise in diatom species diversity that can be interpreted in terms of continuing increase in morphological diversity. We investigated this apparent discrepancy using a discrete-character-based, empirical morphospace of marine planktonic diatoms. The morphospace was resolved both by phylogenetic position and through time, using fossil occurrences from the Neptune database. The analysis shows little correspondence with phylogeny and little Cenozoic change in disparity, as measured by mean pairwise distance. There is, however, a Cenozoic increase in the total (hyper-)volume of morphospace occupied. While this temporal pattern superficially supports a conclusion of increasing morphological variety, there is also a secular increase in sample number. To address sampling bias, we calculated metrics of morphospace occupancy under several algorithms for sample standardization. Under uniform subsampling, the increase in morphospace volume occupied through the Cenozoic Era flattens, supporting the conclusion of underlying morphological stasis inferred from molecular clocks.