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Journal of Paleontology 91(4):618–632
observed in isolated ossicles are coded, the morphospace becomes uninformative with all of the included groups over- lapping in an indistinct cloud of points. The perception of morphologic diversity contained
within a crinoid assemblage through an idealized stratigraphic cycle is shown in Figure 4. The partial disparity of the altered taxa compared with the control assemblage (Fig. 4.4) and the relative distribution of taxa based on the Mantel statistic (Fig. 4.5) are relatively indistinguishable from the original data set through most of the section. However, the condensed assemblages found in the transgressive systems tract are noticeably different because of fewer observable characters even though all of the taxa within the assemblage are likely to be observed. The effect of an exaggerated taphonomic bias in blastozoan
morphologic diversity through the Paleozoic is presented in Figure 5. As was discussed by Foote (1992), blastozoans are characterized by a rapid rise in morphological disparity early in the group’s history (Cambrian–Early Ordovician) followed by a contraction of morphology within groups. Coupled with the extinction of multiple blastozoan classes, this resulted in a steady and significant reduction in disparity from the Silurian to the Mississippian. The only blastozoans present during the Late Paleozoic were the morphologically conservative blastoids, such that disparity remained low. Exaggerating potential taphonomic biases based on the frequency of marine Lagerstätten had little effect on the disparity trends in blastozoans through the Paleozoic. The only difference can be seen during the Devonian, in which the taphonomic alteration makes the drop in disparity appear later and more gradual than in the original data set. This is largely the result of the loss of strong clustering within rhombiferans and blastoids and, more important, the outlier position of the morphologically distinctive blastoid Eleutherocrinus (Millendorf, 1979).
Discussion
The quantification of morphology in order to detect trends in morphological diversity through time is a vital approach in unraveling macroevolutionary processes. Examination of potential biases that influence disparity metrics is as important as studies of the patterns and rates of change in disparity through time. The largest potential bias is undoubtedly variation in pre- servation, which can alter biodiversity as well as nonrandomly distort observed morphological features. However, the role of taphonomy has been understudied such that it is unknown whether the prominent patterns in disparity (Hughes et al., 2013) are the result of evolutionary processes or, to some degree, an artifact of taphonomic alteration. Echinoderm taphonomy is well studied, which allows a system in which the potential effects of variable preservation can be exaggerated in order to explore the magnitude of this theoretical potential bias.
In addition, these simulated effects represent a ‘best case scenario’ of sorts for documenting which taphofacies are best for capturing echinoderm disparity, which taphofacies are most likely to produce no or spurious patterns, and which taphofacies are less than ideal but still capable of producing informative patterns.
Examination of the distribution of taxa within morpho-
space and the relative contribution of individual groups to overall disparity indicate the influence of taphonomy on mor- phological signals is very different for crinoids and blastozoans (Figs. 2, 3). This difference is important in stressing how the fossil record for even similar appearing and related groups can be quite distinctive and nonuniform. For blastozoans, the loss of taphonomically sensitive characters produces morphological patterns that are well within those produced by merely reducing the size of the character suite. In other words, the characters that are influencing the major axes and, therefore, are contributing to the differentiation between morphotypes are taphonomically resistant. The characteristics that define different taxonomic groups are largely defined by respiratory structures in addition to the size and type of thecal plates, all of which can potentially be recognized from isolated ossicles (e.g., Paul et al., 2016). The features within blastozoans that are rapidly lost following death, such as the distal stalk, anal pyramid, and feeding appendages, are fairly consistent across blastozoan morpho- types such that they contribute little to the overall disparity. This is not the case with crinoids, where the characters that define the morphological groups of crinoids are lost with increasing taphonomic degradation. The characters that are important in structuring morphospace primarily relate to the structure of the cup, anal series, fixed rays, and interareas, which all require the preservation of most, if not all, of the calyx. For example, an isolated basal plate does not necessarily help distinguish one crinoid subgroup from another, with the possi- ble exception of morphologically aberrant forms such as the calceocrinids. If patterns in disparity as they relate to taphonomy are
examined through either an idealized stratigraphic section (for crinoids; Fig. 4) or through the Phanerozoic (for blastozoans; Fig. 5), results are encouraging. However, patterns of crinoid disparity through time might appear to be suspect given the results of Figure 3, but previous analyses conducted by Foote (1999) should alleviate those concerns. Foote (1999) found that disparity derived from exclusively cup characters, which could be easily discerned in Taphonomic Grades A–C, mimicked the overall disparity pattern. In addition, Foote (1999) noted that the pair-wise distance was not strongly influenced by missing data. Therefore, the current study aims to explore potential biases rather than correcting previous studies in an attempt to elucidate the true history of echinoderm disparity. Patterns of local dis- parity among crinoids are only significantly altered within the most taphonomically altered portions of the stratigraphic section (transgressive systems tract). This part of the sequence features
Figure 3. The effect of taphonomic degradation on morphological characterization of crinoids. (1) Mantel statistic comparing the distribution of taxa based on the Gower dissimilarity coefficient between the taphonomically altered and original crinoid data sets. (2–4) Partial disparity, in black, of the major groups of crinoids included in the data set (camerates, disparids, and cladids, including flexibles) as they are progressively degraded taphonomically. Error bars and the comparison to randomized character loss follow the methods described for Figure 2. (5–8) Morphospaces showing the change in morphologic distribution with the loss of information from taphonomic alteration. Cladids/flexibles are indicated by an open dash; camerates are indicated by an open circle; protocrinids are indicated by a plus; hybocrinids are indicated by a gray asterisk; disparids are indicated by a gray triangle.
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