Journal of Paleontology, 91(4), 2017, p. 618–632 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.129
The role of preservation on the quantification of morphology and patterns of disparity within Paleozoic echinoderms
Bradley Deline,1 and James R. Thomka2
1Department of Geosciences, University of West Georgia, Carrollton, GA 30118, USA ⟨
bdeline@westga.edu⟩ 2Department of Geosciences, University of Akron, Akron, OH 44325, USA ⟨
jthomka@uakron.edu⟩
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 distribu- tion 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.
Introduction
There are multiple methodologies that can be used to explore macroevolutionary trends. The most common metrics are taxo- nomic diversity and phylogenetic analyses, both of which are fundamental within paleontology. However, each focuses on a fairly narrow subset of evolutionary processes: phylogenetic analysis based on parsimony attempts to reconstruct evolu- tionary relationships, whereas taxonomic diversity tracks the changes in the number of biologically distinctive units, which is a function of the relative rates of speciation and extinction. The amount of morphological change within a lineage or during a speciation event, as well as the type of extinction event (selec- tive or random in relation to particular morphotypes), is not prominently captured using these methods. Morphological diversity—disparity—is a much more encompassing metric in the exploration of large-scale macroevolutionary patterns, and consequently, its interpretation can be more problematic and varied (Foote, 1997a; Lloyd, 2016). In addition, disparity requires the quantification of morphology, which can be labor- ious and time consuming such that disparity is relatively understudied with regard to both pattern and potential biases. Ideally, multiple methods (diversity, disparity, and phylogenetic reconstruction) would be used in concert (e.g., Gorscak and O’Connor, 2016) to gain an even more expansive view of evo- lutionary histories. However, this further expands the labor, data, and exploration of biases required for reliable results, and
reliable phylogenies are elusive for many groups of fossil organisms, especially at higher taxonomic levels. The characterization of morphology can be accomplished
using multiple techniques, including the use of morphometrics (e.g., landmarks or outlines analysis) in two dimensions (Crônier et al., 1998; Crampton, 2007; Webber and Hunda, 2007) or three dimensions (Eble, 2000; Goswami et al., 2011), discrete characters (Briggs et al., 1992; Wagner, 1997; Wills, 1998; Foote, 1999), and GIS analysis (Sheffield et al., 2012; Wilson et al., 2012; Knauss and Yacobucci, 2014). Each of these methods has its strengths and focus, and in some cases, they have been shown to produce similar morphologic patterns (Villier and Eble, 2004; Hetherington et al., 2015). Studies of the potential biases in detecting morphological diversity are fairly limited, especially with the myriad of available approa- ches, but the influences of metric (Ciampagio et al., 2001), environmental distribution (Hopkins, 2014), community struc- ture (Deline, 2009), and taxonomic or geographic scale (Butler et al., 2012; Deline et al., 2012) have been investigated. Of particular interest are the effects of variable preservation and taphonomy on the characterization of morphology. This poten- tial bias is especially concerning given that it may result in the loss of taxa, loss of features, loss of entire body regions, and/or alteration of the features that are preserved. Foote (1997b) explored the sensitivity of disparity to taxon sampling, which captures part of the taphonomic bias on estimates of disparity, i.e., differential recovery of organisms. However, how much
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