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Journal of Paleontology 91(4):604–617
some cases may have pores as are known for both adradial and abradial floor plates in different taxa. Later crinoids appear to lose the floor plates and have a large food groove formed along the interior of the brachial elements. The basic morphology of a food groove formed from floor
plates covering an appendage cored with coelomic extension is known from blastozoans, including Eumorphocystis (Fig. 2.9) and solutes (Parsley, 1980), showing that these morphologies are not unique to crinoids as some have suggested (Guensburg and Sprinkle, 2009).
Homologies in Eleutherozoa
Eleutherozoa remains a great challenge for oral/axial skeletal homologies in
Echinodermata.These taxa belong to the pentaradial clade as evidenced by the preponderance of pentaradiate symmetry among taxa, though a few derived clades exhibit unusual ambulacral symmetries. Eleutherozoa is a well-diagnosed group that has been recovered as a clade by several broad analyses supported by a variety of synapomorphies including five-fold symmetry (sensu Sumrall and Wray, 2007), articulating spines, lack of cover plates, jaw structures, inverted posture, vagrant life mode, and others. The structures of the mouth frame and ambulacral system,
however, are highly derived among the various taxa, and there is little understanding of which, if any, of the skeletal elements correspond with elements outlined by the UEH model. Most of these taxa have the derivative of a jawapparatus that bears 10 paired elements (the dipyramids and auricle elements of echinoids, the mouth angle plates of asteroids and ophiuroids, parapharyngeal ring elements of holothuroids [see Sumrall, 1997]). Whether any of these elements are homologous has not been determined. Interestingly, there seems to be nothing clearly corresponding to the peristomial border of most other pentaradiate echinoderms to suggest homology with elements of UEH. This suggests that either UEH peristomial border systems are evolved within the pentaradiate clade or Eleutherozoa has lost these elements secondarily. Given the fragmented but improving Cambrian echinoderm record and an absence of anything that seems to be an Eleutherozoan (Zamora et al., 2013a), it seems likely that a derived loss in Eleutherozoa is most reasonable. The only structures that seem to show promise for UEH
elements in Eleutherozoa are the ambulacral elements in asterozoans (Fig. 4).Here, there are two pairs of elements that form the food groove, called ambulacrals and adambulacrals, that correspond to the double biserial floor plate elements in other pentaradiate echinoderms. The ambulacrals of asterozoans correspond to the adradial floor plate series of other pentaradial taxa as seen by their position along themidline of the ambulacrum. The adambulacral elements of asterozoans correspond to the abradial set. These plates lie abradially to the ambulacral series. Figure 4 shows a cross-sectional comparison between the plates of asteroids and the edrioasteroid-grade echinoderm Kailidiscus.
Alternative views
A recent paper by Guensburg et al. (2016) proposed an uncon- ventional model for the origin of crinoids, which they argued were derived from edrioasteroids or stemmed edrioasteroid-like
m ab ab
Figure 4. Cross-sectional relationships of ambulacral floor plates in edrioasteroids and asteroids. (1) Cross section of the ambulacrum of Kailidiscus showing adradial and abradial floor plates forming the food groove, which is covered by cover plates. (2) Cross section of the asteroid Luidia showing adradial and abradial floor plates forming the inverted food groove, which lacks cover plates (after Blake, 1980). ab = abradial floor plates (ambulacrals in asteroid terminology); ad = adradial floor plates (adambulacrals in asteroid terminology); cp = cover plates; m = mouth.
forms rather than blastozoans. Their argument was based, in large measure, on a rejection of UEH. However, their critique reflects a misunderstanding of both UEH and the modern phylogenetic methodological context in which it was developed. UEH, in and of itself, makes no claimconcerning the ancestry of crinoids. That UEH components can be identified in crinoids (see the preceding) speaks only to the fact that crinoids are nested within the pentaradiate echinoderm clade. Crinoid origins can only be addressed through rigorous phylogenetic analysis and inferred a posteriori from the resulting tree topologies. These misunderstandings merit brief discussion. Most important, the critique presented by Guensburg et al.
(2016) confuses a character state with the polarity of a given state at a given part of a phylogeny. Their argument that the echinoderm-wide ancestral plating conditions identified through UEH were simultaneously plesiomorphic and homoplastic, for example, misconstrues basal conditions for a clade with derived conditions in subordinate lineages. A character state can arise multiple times, even after it is secondarily lost, but whether subsequent reacquisitions are considered plesiomorphic is entirely dependent on where on the tree one is looking. Like other morphological character states, UEH components
cannot be described a priori as plesiomorphy, synapomorphy, or homoplasy in the absence of a phylogeny by: (1) presuming them to be present in the last common ancestor of blastozoans and crinoids, (2) interpreting them as independently acquired on the basis of stratigraphic arguments, or (3) interpreting them as independently acquired because of inconsistent distribution among taxa (Guensburg et al., 2016, p. 258). Character state polarity can only be assessed a posteriori. Furthermore, synapomorphies are clade-diagnosing characters, not autapomorphies found in single terminal taxa (contra Guensburg et al., 2016, p. 258). In fact, the goal of UEH is to identify the homologous struc-
tures on which character states can be identified—not to polarize the states themselves. UEH shows how homologous elements are present in presumed descendant lineages independent of howthose descendent lineages are related to one another. This is the pure
ab ad
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