Deline and Thomka—Echinoderm disparity and taphonomy


and what part of the animals is recovered may be as important as recovering the animal at all. Few studies have examined taph- onomy in regard to disparity (seeWebster and Hughes, 1999 for an example), but the loss of soft-tissue characters has been shown to make taxa appear more ancestral in phylogenetic analyses (Sansom and Wills, 2013). This may be especially problematic at higher taxonomic levels owing to greater intrin- sic (e.g., intraclade) variability in the taphonomic sensitivity of the organisms or features being studied. One method that may be particularly susceptible to these biases is the use of discrete characters, which is often used to describe large-scale trends in disparity (Briggs et al., 1992; Foote, 1992; Wagner, 1997; Wills, 1998; Ciampaglio, 2004; Deline, 2015; also see reviews by Foote, 1997a; Erwin, 2007; Hughes et al., 2013). Paleozoic echinoderms represent an ideal group for a case


study of potential bias associated with preservation because they contain a wide array of morphological features (Pawson, 2007); their patterns of disparity are well documented (Foote, 1994, 1999; Deline and Ausich, 2011); and they have predictable and empirically tested patterns of taphonomic degradation (e.g., Kidwell and Baumiller, 1990; Greenstein, 1991). In addition, Deline and Ausich (2017) found that the magnitude and order of relative disparity of crinoid groups changed with varying emphasis on different body regions, which is also likely to occur as skeletal regions are differentially affected by taphonomic alteration. To explore these potential biases, we examined how the quantification of morphology in Paleozoic stalked echino- derms is influenced by variations in preservation and how those variations could potentially alter observed morphological trends in different taphofacies or through time.


Echinoderm taphonomy


The echinoderm endoskeleton consists of a multitude of indi- vidual ossicles, varying in number from dozens in irregular echinoids (e.g., Smith, 1984) to hundreds of thousands in cri- noids (e.g., Macurda and Meyer, 1983; Meyer and Meyer, 1986). These ossicles are bound together primarily by unminer- alized connective tissues, including ligaments and, in some taxa, muscles. This intricate, soft tissue–bound morphology makes echinoderms among the most taphonomically volatile of well-skeletonized macroinvertebrates: in the absence of burial, echinoderms will undergo skeletal disarticulation into isolated ossicles as conspicuously evidenced by the abundance of Paleozoic carbonates composed entirely or predominantly of isolated pelmatozoan columnals (Lowenstam, 1957; Ausich, 1997). Field and laboratory studies of extant echinoderms have repeatedly confirmed the rapid rate of postmortem disarticula- tion under normal environmental conditions (e.g., Blyth Cain, 1968; Meyer, 1971; Schäfer, 1972; Liddell, 1975; Lewis, 1986; Meyer and Meyer, 1986; Allison, 1990; Kidwell and Baumiller, 1990; Greenstein, 1991, 1993; Baumiller et al., 1995; Greenstein et al., 1995; Gorzelak and Salamon, 2013, among many others). Detailed taphonomic evaluation of Paleozoic faunas supports an equally rapid rate of disarticulation for extinct echinoderm groups if not removed from the taphono- mically active zone (e.g., Meyer, 1990; Sumrall, 2000; Dornbos and Bottjer, 2001; Zamora et al., 2013b; Martin et al., 2015; Thomka et al., 2016).


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specific taphonomic pathways recorded by echinoderm skeletal preservation provide accurate and precise information about paleoenvironmental processes and parameters (e.g., Brett and Baird, 1986, 1993; Speyer and Brett, 1991; Thomka et al., 2012), biotic influences and interactions (e.g., Maples and Archer, 1989; Nebelsick et al., 1997), constructional morpho- logy (e.g., Ausich and Baumiller, 1993, 1998; Baumiller and Hagdorn, 1995; Baumiller, 2003), and major biases in the fossil record (e.g., Kier, 1977; Greenstein, 1991, 1992; Donovan, 2001). This interpretive value stems largely from the non- randomness of disarticulation patterns, which directly reflect the distribution and relative lability of connective tissues (for example, muscular vs. ligamentary articulations in different parts of crinoids) and the presence of significant structural fea- tures (e.g., imbricated vs. tessellating plates and presence vs. absence of internal support ‘pillars’ in different groups of echi- noids). Hence, it is well established that different echinoderm skeletal modules and morphotypes respond differently to phy- sical, chemical, and biological processes in operation following death of the individual (Donovan, 1991; Brett et al., 1997; Ausich, 2001). The difference in the relative propensity for echinoderm


Despite the rapid rate of echinoderm disarticulation, the


skeletal modules to undergo total disarticulation was empha- sized by Brett et al. (1997), who identified three ‘types’ of echinoderm. Type 1 echinoderms are characterized by a general lack of rigid skeletal modules; consequently, these are most susceptible to rapid and total disarticulation. These taxa, which include most asterozoans, edrioasteroids, homalozoans, and loosely plated eocrinoids, are generally only preserved as iso- lated ossicles, reflecting only minor exposure prior to final burial in many instances, or as complete, articulated individuals, gen- erally reflecting live burial. Type 2 echinoderms are character- ized by variability in the relative durability of skeletal modules, with some portions of the skeleton capable of resisting dis- articulation longer than others. As a result, these organisms are typically found in a wider range of states of completeness, reflecting differences in rigidity or volume of connective tissues. Examples include regular echinoids, many crinoids, and most blastozoans. Finally, Type 3 echinoderms are characterized by relatively robust skeletal modules that were capable of resisting total disarticulation for more extended periods of time. As with Type 1 echinoderms, these taxa are generally found in a limited number of taphonomic states; examples include sand dollars, blastoids, and most microcrinoids. Moreover, within a single ‘type’ of echinoderm (sensu


Brett et al., 1997), taphonomic variability can be produced by differing the exposure time of skeletons prior to final burial. As described in the preceding, Type 1 and Type 3 echinoderms are less affected by this factor than are Type 2 taxa due to the skeletal fragility and skeletal robustness, respectively, of these groups. Nevertheless, the progression from completely articulated individual to partially articulated modules, and eventually to isolated ossicles, can be documented for each echinoderm type—it simply occurs significantly more rapidly in Type 1 echinoderms and over a more extended interval for Type 3 echinoderms. It is important to note that as disarticula- tion progresses, plating configurations are disrupted and morphology becomes increasingly obscured; hence, features


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