A morphospace of planktonic marine diatoms. II. Sampling standardization and spatial disparity partitioning

Andrew H Knoll,Benjamin Kotrc

doi:10.1017/pab.2014.5

Abstract

AbstractMorphospace occupation through time provides a view of diversification distinct from the more familiar taxonomic tabulations. However, this view is subject to the same geological biases long recognized in studies of taxonomic diversification, where techniques for correcting secular bias in sampling have become standard practice. In this study, we apply sampling standardization techniques to a morphospace investigation to test whether observed stratigraphic trends in morphospace occupation are artifacts of trends in sampling. When sampling bias is corrected by randomized subsampling, all disparity metrics show stationary patterns, or at most directional changes of small magnitude. Metrics describing the average dispersion of taxa in morphospace are less subject to sampling bias than those describing the total extent of morphospace occupied. We also investigate a measure of disparity that is insensitive to sampling intensity, introducing a geographic component of morphological disparity. By analogy to α and β components of taxonomic diversity, we suggest the notions of α and β disparity, and find that α disparity remains roughly constant through time. Our analysis also allows us to present the first taxonomic diversity curve of diatoms under shareholder quorum subsampling (SQS), showing similar results to previously published subsampling methods: a roughly twofold rise over the Cenozoic, with peak diversity around the Eocene/Oligocene boundary. Tests for methodological bias from choices in ordination method and data culling during morphospace construction indicate that our results are relatively insensitive to both factors: Cenozoic occupation of planktonic diatom morphospace is largely unchanging. We find a similarly stationary pattern when we directly analyze the morphological data, seeing no change in the prevalence of taxa with different sets of morphological characters. More broadly, our results make clear that a complete view of morphological disparity must consider sampling biases, which can be addressed with wellestablished, quantitative methods in morphospaces populated using occurrence-level data.

Highlights

Studies of the fossil record make valuable contributions to our understanding of evolution, not least through documenting the diversification history of clades
We use principal coordinates analysis (PCO) to transform the data to continuous form, and binned occurrences into 2-Myr time intervals to calculate four disparity metrics describing the occupancy of this morphospace: the volume of the convex hull encompassing the taxa present, the volume of an alpha shape encompassing the taxa present, the alpha shape volume divided by the number of taxa, and the mean pairwise distance
Those disparity metrics describing the separation among taxa in morphospace do not change substantially compared to the raw (SIB) results, while those metrics describing the volume of morphospace occupied lose the increasing trend seen under SIB when subsampled

Summary

Introduction

Studies of the fossil record make valuable contributions to our understanding of evolution, not least through documenting the diversification history of clades. Much less than half of the extant species of marine planktonic diatoms are known from deep-sea sediments (Sournia et al 1991) This bias may, have been less significant in the past, as a number of lines of evidence point toward a decline in the oceanic concentration of dissolved silica over the course of the Cenozoic (Siever 1991, Maldonado et al 1999, Racki and Cordey 2000, Muttoni and Kent 2007, Lazarus et al 2009).

Materials and Methods

Conclusions

Findings

Literature Cited