Journal of Paleontology, 91(4), 2017, p. 604–617 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.9


New insights concerning homology of the oral region and ambulacral system plating of pentaradial echinoderms


Colin D. Sumrall Department of Earth and Planetary Sciences, The University of Tennessee, 1412 Circle Dr., Knoxville, Tennessee 37996, USA ⟨csumrall@utk.edu⟩


Abstract.—Universal elemental homology (UEH) underpins recent understanding of peristomial and ambulacral elemental homology of pentaradiate echinoderms by providing a uniform set of terminology to construct phylogenetic characters. Variation in the expression of these elements provides evidence for phylogenetic relationships. Two nonhomologous sets of plates border the peristome and are associated with two nonhomologous sets of floor plates forming the ambulacral food groove. Some edrioasteroid-grade and eocrinoid-grade echinoderms have ambulacral systems formed from oral frame plates and adradial floor plates, whereas most blastozoans and crinoids bear oral plates and abradial floor plates. These plates are expressed in a variety of ways among echinoderms, but nearly all can be reconciled with the underlying model. Arguments against UEH are methodologically flawed and confuse many terms and interpretations.


Introduction


Inferring phylogenetic relationships of any clade using morpho- logical data cannot be done in the absence of an accurate under- standing of homology. In Paleozoic echinoderms, assessing homology is particularly challenging because: (1) the process of evolution oftenmasks homology via character transformation, and (2) a striking proliferation of taxonomic ‘classes’ based on unique combinations of morphological characters has emphasized differences rather than underlying similarities. Unfortunately, many of these classes have unique sets of terminology, which apply different names to homologous structures or identical names to nonhomologous structures (Sumrall, 2010; Sumrall andWaters, 2012). Recently, the universal elemental homology (UEH)model provided a morphologic framework to facilitate the identification of individual echinoderm skeletal elements associated with the mouth region and ambulacral system (Sumrall, 2010, 2015; Sumrall and Waters, 2012; Kammer et al., 2013; Ausich and Kammer, 2016). With this model based on detailed anatomy, symmetry, and development, evolved differences in plate relationships, morphology, and presence/absence of plates can be identified with high precision and utilized to construct useful phylogenetic characters. This paper summarizes various taxa of derived pentaradiate


echinoderms and shows how UEH can be used to describe characters within this high-resolution framework. Of particular interest is the recognition of two nonhomologous sets of floor plates that make up the food groove. A few taxa (early edrioasteroid-grade echinoderms and asterozoans) bear both an adradial and abradial floor plate set on either side of the perradial suture. Many later taxa lose one of these sets, and these losses correlate to the plate series forming the peristomial border. Generally, the skeleton of pentaradial echinoderms can be divided into axial skeleton, associated with the peristome


(mouth opening) and ambulacral system, and the extraxial skeleton, comprising the rest of the body wall (Mooi et al., 1994, 2005;Mooi and David, 1997, 1998, 2008; David and Mooi, 1998; David et al., 2000). TheUEHmodel strictly relates to elements of the axial skeleton of these echinoderms. Within the pentaradial clade, the axial skeleton is constructed from several different plate series (Fig. 1). The peristomial opening is bordered by some combination of radially positioned oral frame plates and interradially positioned oral plates (Kammer et al., 2013). These two circlets are demonstrably not homologous by conjunction (Patterson, 1988) as they occur simultaneously in several taxa, such as edrioasterids. Distal to the peristomial border lie radial or perradial floor plates that form the food groove and often mounting sites for small feeding appendages called brachioles (Fig. 1.1).Most taxa bear some combination of floor plates that are of two types, either the adradial (inner) set or the abradial (outer) set. These two floor plate types are demonstrably not homologous by conjunction (Patterson, 1988) as they occur simultaneously in several taxa, such as Kailidiscus, pyrgocystid edrioasteroids, and


asteroids (Zhao et al., 2010; Sumrall and Zamora, 2011). These floor plate sets form a continuous plate series with either the oral plate series (abradial) or the oral frame plate series (adradial), respectively, suggesting they formpart of the same developmental sequence. Covering the peristome and ambulacral system is a series of plates that protect the food groove. Proximally, the peristome is roofed by interradially positioned primary peristomial cover plates (PPCP) forming the center of the cover plate series (Fig. 1.1, 1.3). Ambulacral cover plates mount directly to the peristomial bordering plates and floor plates forming a roof to the ambulacral system. Fundamental bilateral symmetry of the ambulacral system results in these plates being aligned to the underlying ambulacral designations. This allows individual plates to be precisely identified and traced developmentally and phylogenetically (Sumrall andWaters, 2012).


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