Sumrall—Echinoderm homology
sister taxon was identified when constructing UEH. In fact, six outgroup taxa were used to polarize a crinoid-inclusive ingroup (Ausich et al., 2015). Regardless, describing sister taxon relationships as ‘subjective’ and something that ‘cannot be rigorously tested’ (Guensburg et al., 2016, p. 254) diverges from modern phylogentic methodology and disregards the kind of information available in the literature. UEH data and characters are explicit and published for others to interpret and refine, as were the methods of phylogenetic inference (Ausich, 1998; Ausich et al., 2015). That Guensburg et al. (2016) did not test them by presenting an alternate phylogeny renders them neither subjective nor untestable. Characters are evidence of relationships, not features that
essence of the morphological character. The description of a single character suite as ‘superficially similar’ that ‘can be traced back to the plesiomorphic pattern’ (Guensburg et al., 2016, p. 256) is incompatible; the former describes homoplasy, and the latter describes homology. A character suite cannot be both. Although the casual a priori dismissal of well-documented ontogenetic and phylogenetic similarity as homoplasy has a long history in echinoderm paleontology (i.e., Jefferies, 1986), it is inconsistent with modern systematic methodology. Guensburg et al. (2016, p. 254) claim that no specific
615
taxa provide character transformation data for phylogenetic inference.
Evidence suggests that the two series of plates associated
with the peristome, oral frame plates and oral plates, are each associated with complementary floor plates (adradial and abra- dial, respectively). Oral frame plates and adradial floor plates are internal structures that generally cannot be seen from the thecal exterior. They are found in several edrioasteroid-grade and eocrinoid-grade echinoderms. Oral plates and abradial floor plates are externally expressed and are found in most derived blastozoans and crinoids. Arguments used to dismiss UEH and document the origin
of crinoids are incompatible with data and methods. They con- fuse several issues, including homology versus phylogenetic analysis; the nature of homology; a priori versus a posteriori arguments; homoplasy and plesiomorphy. The simple fact that, barring a few apparently highly derived taxa, UEH is a powerful tool to understand echinoderm morphology suggests that the patterns recognized show a deep-seated underlying homology scheme for pentaradiate echinoderms.
Acknowledgments
define clades. Clades are defined by ancestry and descent (Rowe, 1988). Guensburg et al. (2016, p. 256) suggest that any sister taxon to crinoids should have calycinal coelomic slots, slat-like floor plates, and podial basins with pores. They suggest that these must either be lost and regained by the pertinent ancestor of crinoids (homoplasy) or are present in yet unknown taxa (plesiomorphy). They omit the obvious (and optimal) third possibility—that these are synapomorphies of crinoids. This third option is consistent with these characters only being present in crinoids and absent in other taxa. Temporal arguments about the timing of supposed sister taxa
of crinoids aremeaningless, especially with the patchy fossil record of echinoderms in the late Cambrian (Zamora et al., 2013a). Guensburg et al. (2016) based their argument that crinoids are not descended among blastozoans on a temporal gap of 25 Myr between the earliest crinoid and the earliest blastozoan. This distorts the actual argument because Ausich et al. (2015) used Ordovician, not middle Cambrian, outgroups, and the actual temporal gap was negligible. That Guensburg et al. (2016) used Ordovician forms rather than middle Cambrian edrioasteroids, such as Kailidiscus, suggests an inconsistent approach toward the issue.
Summary
The common morphology seen in the peristomial border of pentaradiate echinoderms indicates that these structures result from a deep-seated homology. The often-confusing nature of peristomial plate names found across pentaradiate echinoderms typically results from concentrating on differences, rather than the overarching similarity, among taxa. It is, therefore, critical that before coding characters, the oral area and ambulacral system of taxa be interpreted using the UEH model to avoid the pitfalls of coding nonhomologous elements simply based on position or on possessing the same name as a historical artifact. Different expressions of these morphologies among subsets of
Numerous people helped in the formation of ideas, including W.I. Ausich,
C.A.Brochu,B. Deline, T.W. Kammer, R.L. Parsley, S.L. Sheffield, A.B. Smith, J. Sprinkle, and S. Zamora. J. Golden, T. Adrain, S.V. Rozhnov, and J.W. Atwood provided specimens for this study. Funding was provided by the National Geographic Society and NSF grant DEB1036260. T.W. Kammer,
J.AWaters, and S. Zamora reviewed this manuscript and provided numerous suggestions for its improvement.
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