620


Journal of Paleontology 91(4):618–632


that can only be reliably assessed on articulated individuals become impossible to describe with increasing taphonomic alteration. The comparative framework erected by Brett et al. (1997),


and slightly modified by Ausich (2001), serves as a useful tool for assessing taphonomic patterns among echinoderms on multiple temporal scales. Because the record of all three types extends throughout the Phanerozoic (Lefebvre et al., 2013; Zamora et al., 2013a), microstratigraphic to secular taphonomic trends can be recognized on the basis of: (1) the relative pro- portion of each echinoderm skeletal type, and (2) the specific preservational state with regard to discrete physical characters that are identifiable in any given deposit.


Quantification of morphology


Data matrices.—Two discrete morphological character matri- ces were utilized in the current study (Table 1). The first was constructed by Foote (1992) to characterize morphologic trends in blastozoan echinoderms (data set available at: http://geosci. uchicago.edu/~foote/MORPHDAT/BLASTOZOAN_DATA. TXT). The matrix is composed of 56 multistate and binary characters describing features across the entire body with particular emphasis on the ambulacral and feeding structures, thecal morphology, attachment structures, and respiratory features. The data set includes 131 genera encompassing multiple skeletal types (Brett et al., 1997) including Type 1 (eocrinoids), Type 2 (eocrinoids, rhombiferans, paracrinoids, and diploporitans), and Type 3 (coronoids and blastoids) echino- derms. The individual codings were not changed from those of Foote (1992) other than the addition of character contingencies that allowed the distinction between absent and nonapplicable character states. The second data set is the recently constructed crinoid


character suite (Supplemental Data 1) developed by Ausich et al. (2015) and Deline and Ausich (2017). This character suite was built as part of the Assembling the Echinoderm Tree of Life Project (AEToL) and differs from previous data sets (e.g., Foote, 1999) by increasing the overall size of the character suite as well as adding characters that accommodate the unique morphology of the protocrinids (Guensburg and Sprinkle, 2003). In addition, the AEToL data set incorporates characters describing the oral surface according to the Universal Elemental Homology model (Sumrall and Waters, 2012; Kammer et al., 2013). This data set is composed of 178 binary and multistate characters with contingencies describing 198 Ordovician through early Silurian crinoids including cladids, flexibles, camerates, hybocrinids, protocrinids, and disparids (Supple- mental Data 2). Unlike the blastozoan character matrix, the crinoids all represent a single skeletal type (Type 2).


Analytical protocols.—Characters were coded following the methods of Deline (2009) as a given character state (including absent), missing (unknown because of preservation), or non- applicable. Characters or taxa with a large amount of unknown data were excluded from analyses, which reduced the crinoid data set to 163 species and 145 characters. The character states in the two matrices were then serially recoded to assess changes in taphonomic state. This was done in


a crude sense following Brett et al. (1997). Even though taphonomic degradation occurs along a continuous gradient, four distinctive bins (taphonomic grades) were used (see Fig. 1 for an example). Taphonomic Grade A represents rapid burial within hours to days following death, resulting in complete and articulated preservation regardless of the skeletal type of echinoderm. The following grades represent a longer time in the taphonomically active zone from days to weeks (Grade B), weeks to years (Grade C), to more than a year (Grade D). A longer duration of taphonomic alteration results in a greater degree of disarticulation with progressive deterioration from intact skeletons to intact body regions to intact portions of skeletal modules to individual ossicles. In addition, the amount of abrasion on individual elements was modeled to increase with the taphonomic grade. This process features both the loss and gain of characters with grade. The ability to code some characters with an increase in taphonomic alteration may be counterintuitive, but abrasion of plates allows the observation of internal structures (e.g., pores in diploporitans), and the removal of some features can permit the coding of others (e.g., the loss of crinoid arms allows the coding of the tegmen or peristomial surface). The blastozoan and crinoid characters were then classified


as observable or not for each taphonomic grade (Table 1) according to conservative taphonomic interpretation and field observations of various taphofacies (Supplemental Data 3, 4). In crinoids, all interpreted as Type 2 skeletons, characters that are unlikely to be observed were excluded from the analysis such that the size of the character suite decreased with taphonomic grade. Blastozoans required a slightly more elaborate procedure resulting from the inclusion of multiple different skeletal types (1–3). If a character was unobservable for all three skeletal types at a specific state of taphonomic degradation (taphonomic grade), then it was excluded from the analysis, as was done with the crinoid data set. However, if a character is likely to become unobservable at a certain taphonomic grade for one or more, but not all, of the skeletal types, then it was coded as missing and the character was retained. The data sets were then analyzed using Gower’s similarity


coefficient (Gower, 1971). This similarity coefficient allows for the inclusion of missing data along with quantitatively differentiating between nonapplicable and missing or unknown data (Deline and Ausich, 2011). This metric is calculated as the number of shared character states between two taxa divided by the number of characters for which at least one of the two is applicable. The distribution of taxa in the taphonomically altered data sets can be compared to the original data set by calculating the Mantel statistic, which compares random permutations of the distance between individuals. The distribu- tion of taxa can also be visualized using principal coordinate analysis (PCO). This analysis, along with Gower’s similarity coefficient, can potentially create triangle inequalities within multidimensional space, which can be prevented by adding a small value to zero distances within the analysis (Cailliez, 1983). The changing of the character number and states for each taphonomic grade necessitates independent ordinations, which produce unique morphospaces with noncomparable absolute distances. Using ratios of distances such that they can be directly compared circumvented this issue. In particular, we used partial


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