Deline and Thomka—Echinoderm disparity and taphonomy


potentially more sensitive than other echinoderms to taphono- mically induced variability in observable characters. More important, crinoids (specifically the species included in the current study) are far more abundant and taxonomically diverse in Paleozoic mixed carbonate-siliciclastic systems than are taphonomically comparable blastozoans (Frest et al., 1999). Crinoids occur in an unmatched variety of lithofacies, allowing interpretation of paleoenvironmental controls on preservation at a level exceeding that which is currently possible for most blastozoans. In addition, blastozoans are generally abundant primarily in carbonate environments only (Brett et al., 1997; Frest et al., 1999). The subsampled crinoid data set analyzed at each of the


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selected intervals of the stratigraphic sequence reflects changes in diversity observed in field studies. Given the generalized paleoenvironments analyzed (see the preceding), an equal number of crinoid taxa at each phase of base-level change is an unrealistic assumption. The transgressive systems tract was treated as the interval where crinoid diversity is greatest, owing to the low turbidity of water during sediment deposition, the increased availability of hard substrata (including abundant hardgrounds), and the strong stratigraphic condensation (i.e., ‘time richness’). Fewer crinoid taxa were analyzed as part of the overlying early highstand systems tract, with even fewer representing the upper highstand systems tract. This reflects increasing sedimentation and soft, muddy substrates, which limits the number of crinoids to those capable of occupying such environments. The lowest crinoid diversity is interpreted for the falling stage systems tract, an interval that typically contains few crinoids, except in local lenses or patches (e.g., Frest et al., 1999). This most likely reflects the increases in sedimentation rate, siliciclastic influx, and grain size associated with the falling stage systems tract. Although the diversity of crinoids analyzed at each selected


phase of our model stratigraphic sequence changes in order to better reflect field data, the subsampled data set of specific crinoid taxa analyzed was the same for each of the stratigraphic intervals. That is, the same crinoid taxa were considered for each of the four phases of relative base-level change, but the number of taxa varied at each of the stratigraphic intervals described in the preceding. These taxa were chosen from the larger data set because they represent a range of crinoids that are relatively common in the Middle Ordovician—Early Silurian of eastern North America, resulting in a more realistic match between the lithofacies and taphofacies in the theoretical stratigraphic sequence and at least some of the crinoids encountered within them. Perhaps more important, selected crinoids represent taxa that are known from relatively complete specimens: this resulted in a larger and less initially taphonomically biased set of characters that could be coded (for example, properties of the distal arms and stem could not be coded for many taxa within the larger echinoderm data set due to rapid disarticulation from other skeletal modules identifiable to low taxonomic levels). Since close to the full morphology of these crinoids is known, these taxa are particularly well suited for accurately capturing disparity and determining the effects of taphonomy on this measure. It is somewhat unrealistic to assume that the exact same crinoid taxa will be present in all of the depositional


environments described in our theoretical stratigraphic sequence. For example, taxa that are particularly susceptible to fouling by fine-grained sediments and/or unable to occupy soft substrates will occur primarily or exclusively in carbonate hardground facies, which are best developed during major transgressive intervals in mixed carbonate-siliciclastic systems (e.g., Thomka and Brett, 2015). Conversely, those taxa that are particularly tolerant of turbidity and/or readily able to occupy soft, shifting, or unstable substrata may be restricted to various siliciclastic facies. Nevertheless, use of the same crinoid taxa to test disparity within all of the taphofacies of our stratigraphic sequence is important for this initial exploration of the effects of taphonomic processes on controlling crinoid disparity: if the same taxa are used in all taphofacies, then patterns in disparity should be more strongly developed than what is likely to be observed within any single real-world stratigraphic sequence. Hence, it is worthwhile to test whether any potentially valuable pattern is capable of being discerned at all, and this is best accomplished using the same set of morphologically well- described taxa as standards. The rationale for determining which features are likely to


be preserved within each taphofacies was largely described in the overview of each studied interval of the stratigraphic sequence. It is important to note that characters for comparing disparity among the selected taphofacies were assessed independently from previous analyses. This was done to account for taphofacies where certain crinoid taxa are likely to be preserved as a variety of skeletal modules regardless of whether they, as a whole, are considered Type 1, Type 2, or Type 3 echinoderms. On the basis of the preceding rationale, we quantified local


crinoid disparity through the idealized stratigraphic section. The original data set was culled to only include crinoids and characters that contained low levels of missing data (less than 10%). The resulting data set consisted of 153 characters describing 27 crinoids, all of which had preserved arms and distal attachment structures. These 27 crinoids were then split into two groups, one that would be modeled for taphonomic alteration through a stratigraphic section and an unaltered control group. Thirteen exclusively Ordovician taxa were used to represent a theoretical community that contained all of the


major crinoid groups (camerates, disparids, cladids, as well as a flexible and a hybocrinid); this community was subjected to modeled taphonomic degradation corresponding to each taphofacies within the stratigraphic section. The other 14 taxa were left taphonomically unaltered as the control group. At each interval in the stratigraphic section, characters that are unlikely to be preserved were coded as missing for the taphonomically altered group, whereas all characters were coded as present for the control group at each stratigraphic datum. In addition, random subsets of crinoids were sampled (1,000 times) at some of the stratigraphic datum to assess how the morphological metrics change with a lower chance of recovering all of the taxa in an assemblage. The upper highstand systems tract datum, for example, is modeled as containing five out of the 13 taxa, so the metrics are based on 1,000 random subsets of five crinoids with the corresponding taphonomically reduced character suite. By contrast, this step was skipped for the transgressive systems tract datum in which all 13 crinoid taxa are modeled as likely to be


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