628


Journal of Paleontology 91(4):618–632


Figure 4. The effect of taphonomic alteration on local crinoid disparity through an idealized stratigraphic sequence. (1) Idealized third-order stratigraphic sequence in a mixed carbonate-siliciclastic system. Even though echinoderms can be preserved throughout the section, four representative datums (transgressive systems tract, early highstand systems tract, late highstand systems tract, falling stage systems tract) were used to illustrate potential changes.(2) Changes in the number of crinoid taxa used at each datum. Note the highest diversity in the transgressive systems tract and the lowest diversity in the falling stage systems tract. (3) Changes in the number of observable characters at each of the stratigraphic datums. (4–7) Changes in observable morphological changes throughout the idealized stratigraphic section. For each datum where all 13 crinoid taxa are unlikely to be recovered, the partial disparity and mantel statistic are based on 1,000 random subsamples of the number of altered crinoids. (4) Partial disparity of the taphonomically altered sample compared to the complete data set of 27 crinoids; the partial disparity of the unaltered data set is shown by the vertical gray line. (5) Mantel statistic comparing the 13 taphonomically altered crinoids to their original morphological distribution. (6) The distribution of the altered (black) and unaltered (gray) crinoids in morphospace for the upper highstand systems tract taphofacies. The morphospaces for the lower highstand systems tract and falling stage systems tract are not shown, but are virtually indiscernible from this morphospace. (7) The distribution of the altered (black) and unaltered (gray) crinoids in morphospace for the transgressive systems tract.


extremely low rates of sedimentation such that echinoderm carcasses remain exposed on the seafloor for prolonged periods, resulting in complete disarticulation. Several previous studies have successfully focused exclusively on disarticulated remains (e.g., Meyer et al., 2002; Hunter and Zonneveld, 2008; Thomka et al., 2012), but the assemblages described in such studies are unlikely to be prime candidates for inclusion in studies of local disparity. Distinctive crinoid faunas may be contained within these environments, and these should be factored into studies of alpha diversity, but their inclusion in studies of crinoid disparity would be problematic given the potential bias associated with such taphonomic alteration. In a similar manner, blastozoan disparity as modeled by considering ‘expected’ taphonomic grades through the Paleozoic showed very little change when the taphonomic bias was artificially exaggerated (Fig. 5). The only interval that was altered with a decrease in preservation potential was the Devonian, which made the contraction of disparity appear later and more gradual than it would otherwise. This change was the result of the aberrant and preservable features within a morphological outlier (Eleutherocrinus), which can be easily recognized by the large increase in error bars during that interval.


The current analyses suggest that the bias associated with


taphonomic degradation could be potentially predicted given a detailed understanding of the anatomy and taphonomy of the group in question. The effect of taphonomy is more pronounced within particular taxonomic groups, and that should be a point of consideration during experimental design in the choice of either characters or metric(s) used to quantify morphology. In addition, if the bias is fairly limited, then the inclusion of genera or species from disparate taphofacies is worthwhile. For exam- ple, one of the most diverse Laurentian assemblages of Silurian crinoids is the Hopkinton Dolomite (Witzke and Strimple, 1981), which is characterized by preservation as internal molds (Frest et al., 1999); this is also typical of preservation in the Silurian of the British Isles (Donovan et al., 2008). Inclusion of these organisms can greatly increase the amount of missing data within an analysis resulting from the unlikely preservation of the arms; despite imperfect preservation, these taxa retain enough of the morphologically valuable characters to allow relatively confident placement within morphospace. Conversely, crinoids known exclusively from isolated ossicles in highly condensed transgressive deposits, such as the columnal morphospecies Floricolumnus in the widespread ‘bead beds’ of the lower


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