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Journal of Paleontology 91(4):618–632


recovered during sampling. The local disparity was then assessed by calculating a Mantel statistic between the taphono- mically altered subset and their original distribution in morpho- space as well as the partial disparity of the taphonomically altered crinoids compared to all 27 crinoids.


Paleozoic trends in blastozoans.—In addition to the small-scale temporal patterns observable within stratigraphic cycles, broader temporal trends in preservation could alter estimations of morphological diversity and prevent an accurate evolutionary assessment of clades. Secular changes in oceanic chemistry, tectonic setting, and/or evolutionary paleoecology could all alter the distribution of preservational processes through time. This is evident in the non–randomly distributed frequency of Lager- stätten through time, largely based on occurrence of soft-tissue preservation (Allison and Briggs, 1993; Schiffbauer and Laflamme, 2012). Articulated echinoderm remains require unique conditions not necessarily correlative with those of soft- tissue preservation, but a comparable study of Phanerozoic- scale echinoderm-specific Lagerstätten has yet to be conducted (but see Hess et al., 1999 for a review for crinoids). Therefore, the frequency of soft-tissue Lagerstätten compiled by Schiff- bauer and Laflamme (2012) was used as a first-pass proxy to test how varying preservation through the Phanerozoic may alter our perceptions of the overall disparity of a clade. The Foote (1992) blastozoan data set was used to examine


how temporal trends could alter perceptions of disparity through time. This data set is optimal because of its long temporal coverage compared with the AEToL crinoid data set; however, the shifting proportions of different echinoderm skeletal types (1, 2, and 3) through the Paleozoic provides an additional level of complexity. Although these biases are already contained within the original echinoderm data set to some degree, the preservational differences were amplified in an attempt to see whether a discernable bias exists. The frequencies of Lager- stätten were scaled with taphonomic grade according to the broad underlying assumption that geologic intervals that contain a large number of soft-tissue Lagerstätten would also be associated with high-fidelity preservation of blastozoans,whereas intervals that contain few soft-tissue Lagerstätten are assumed to be associated with more taphonomically altered blastozoans. For example, blastozoans that occur in geologic periods such as the Cambrian, with a large number of known Lagerstätten, were universally coded as if they were preserved in TaphonomicGrade A (as was done in earlier analyses). By contrast, blastozoans that occurred in intervals with few known marine Lagerstätten, such as the Silurian or Permian,were universally coded as if theywere preserved in Taphonomic Grades C or D. Trends in disparity through time were then analyzed in both the original and the taphonomically altered data sets.


Results


The morphospaces produced by these two data sets have been previously discussed to some extent (Foote, 1992; Deline and Ausich, 2017), but their structures warrant a brief discussion (Figs. 2.5, 3.5). The blastozoan morphospace shows a gradation of morphologies with clusters of major taxonomic groups, but with little space differentiating those morphotypes. For exam- ple, the blastoids all group tightly together (gray diamonds), but the area they occupy overlaps with other blastozoans such as the diploporitans and eocrinoids. Major groups of crinoids are more prominently separated into distinctive areas of morphospace, which likely reflects the greater number of characters assessed for crinoids compared with the blastozoan data set. In addition, there are suites of characters that establish the major groups; for example, dozens of characters associated with fixed rays distinguish the camerates from the disparids and cladids. The different groups of crinoids occupy various amounts of mor- phospace; the disparids, in particular, occupy a large area, which is related to having two distinctive forms with the inclusion of the bilaterally symmetrical and recumbent calceocrinids. Both of these data sets were systematically degraded as


would be expected if those animals were only known from Taphonomic Grades A–D. Both in the distribution of taxa (Fig. 2.1) and the partial disparity of different morphotypes (Fig. 2.2–2.4), the taphonomically altered blastozoans showed little difference from what would be expected given random character loss. This is also the case with the distribution of taxa within the blastozoan morphospaces (Fig. 2.5–2.8); as the taphonomic grade increases, the included taxa become more dispersed and the detail contained within the ordination is lost, but the relative positions of the major morphologies and taxonomic groups are retained. The crinoid data set shows a very different pattern from that


of the blastozoans. Both the distribution of taxa as seen by the Mantel statistic (Fig. 3.1) and the partial disparity (Fig. 3.2–3.4) of the major taxonomic groups change dramatically with increased taphonomic grade and far beyond what is expected given random character loss. The change in partial disparity is also concerning given the different trajectories of the different taxonomic groups. For example, the removal of disparids from calculations of disparity is much more influential at higher taphonomic grade while the inverse is seen in camerates. As with the blastozoans, the trends seen in the disparity metrics are also apparent in the morphospaces (Fig. 3.5–3.8). The morpho- spaces produced at Taphonomic Grades B and C still retain the separation of the major taxonomic groups of crinoids, but there are shifts in position as well as introduction of individuals that appear intermediate in morphology between groups. At Taphonomic Grade D, in which only characters that can be


Figure 2. The effect of taphonomic degradation on morphological characterization of blastozoan echinoderms. (1) Mantel statistic comparing the distribution of taxa based on the Gower Dissimilarity Coefficient between the taphonomically altered and original blastozoan data sets. (2–4) Partial disparity, in black, of the three echinoderm skeletal types (Types 1–3 of Brett et al., 1997) as they are progressively degraded taphonomically. Disparity is calculated as the squared distance of taxa within morphospace. Partial disparity as defined by Foote (1992) is the disparity excluding the group in question divided by total disparity. To assess the change in metrics associated with generalized loss of information (reduction of included morphological characters), 1,000 randomized samples with added randomized missing characters and character states matching each taphonomic grade were analyzed and the median (gray line) as well as 5th and 95th percentiles (gray dashed lines) are reported. Error bars are calculated on the basis of the standard error of the resampled data. (5–8) Morphospaces showing the change in morphologic distribution with the loss of information from taphonomic alteration. Skeletal Type 1 blastozoans (some eocrinoids) are indicated by a plus; Skeletal Type 2 blastozoans (some eocrinoids, rhombiferans, diploporitans, and paracrinoids) are indicated by an open circle; Skeletal Type 3 blastozoans (coronoids and blastoids) are indicated by a gray diamond.


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