The role of preservation on the quantification of morphology and patterns of disparity within Paleozoic echinoderms

Abstract

Abstract.-The loss of information resulting from taphonomic degradation could represent a significant bias in the study of morphological diversity. This potential bias is even more concerning given the uneven effect of taphonomy across taxonomic groups, depositional facies, and stratigraphic successions and in response to secular changes through the Phanerozoic. The effect of taphonomic degradation is examined using character-based morphological data sets describing disparity in Paleozoic crinoids and blastozoans. Characters were sequentially excluded from the analyses following progressive taphonomic loss to determine how morphologic metrics, such as the relative distribution of taxa in morphospace and partial disparity, changed with increasing taphonomic alteration. Blastozoans showed very little change in these metrics with decreasing preservational quality, which is a result of characters that create distance in morphospace being recognizable in isolated plates. The opposite result is present in crinoids as the characters that are important in structuring the morphospace require intact modules (i.e., the calyx) to accurately assess. Temporal and stratigraphic trends produced encouraging results in that patterns could be largely recovered even with exaggerated taphonomic biases. However, certain parts of a stratigraphic sequence should be avoided and morphological outliers could potentially play a larger role through time, though both of these biases can be easily identified and avoided. The methods presented in this study provide a way to assess potential taphonomic biases in character-based studies of morphological diversity.

Does exceptional preservation distort our view of disparity in the fossil record?

“…As such, the distribution and type of lagerstätten in time and space may have heterogeneous effects on the disparity of different clades. Here, we document a clear impact of lagerstätten on ichthyosaur disparity, whereas a study of Palaeozoic echinoderms [46] found that lagerstätten had virtually no effect on their overall disparity (strong effects were noted because of taphonomic degradation, however, highlighting the problem of missing data). In turn, this poses difficulties for associating changes in disparity within distinct groups to external causes or to evolutionary interactions between those groups.…”

Evolution and Development at the Origin of a Phylum

“…Echinoderms are characterized by a mineralized skeleton that is rich in morphological characters [11], such that the loss of data associated with the degradation of soft tissues during fossilization is minor compared to other taxonomic groups [8,26,27]. Therefore, the recovered trends are not subject to significant preservational biases [28], and skeletal features have been vital for reconstructing evolutionary relationships in early echinoderms. The wide diversity of forms among early echinoderms necessitates character-based studies to accurately quantify morphology.…”

How well does a part represent the whole? A comparison of cranidial shape evolution with exoskeletal character evolution in the trilobite family Pterocephaliidae

“…trilobite appendages), or are differentially preserved across clades (e.g. plating configurations in echinoderms, Deline & Thomka in press). This is potentially problematic: for example, single traits within the same measured lineage frequently show different rates or modes of evolution (Hopkins & Lidgard ; Wagner ; Adams ; Hunt et al .…”