Oral Region Homologies in Paleozoic Crinoids and Other Plesiomorphic Pentaradial Echinoderms

The phylogenetic relationships between major groups of plesiomorphic pentaradial echinoderms, the Paleozoic crinoids, blastozoans, and edrioasteroids, are poorly understood because of a lack of widely recognized homologies. Here, we present newly recognized oral region homologies, based on the Universal Elemental Homology model for skeletal plates, in a wide range of fossil taxa. The oral region of echinoderms is mainly composed of the axial, or ambulacral, skeleton, which apparently evolved more slowly than the extraxial skeleton that forms the majority of the body. Recent phylogenetic hypotheses have focused on characters of the extraxial skeleton, which may have evolved too rapidly to preserve obvious homologies across all these groups. The axial skeleton conserved homologous suites of characters shared between various edrioasteroids and specific blastozoans, and between other blastozoans and crinoids. Although individual plates can be inferred as homologous, no directly overlapping suites of characters are shared between edrioasteroids and crinoids. Six different systems of mouth (peristome) plate organization (Peristomial Border Systems) are defined. These include four different systems based on the arrangement of the interradially-positioned oral plates and their peristomial cover plates, where PBS A1 occurs only in plesiomorphic edrioasteroids, PBS A2 occurs in plesiomorphic edrioasteroids and blastozoans, and PBS A3 and PBS A4 occur in blastozoans and crinoids. The other two systems have radially-positioned uniserial oral frame plates in construction of the mouth frame. PBS B1 has both orals and uniserial oral frame plates and occurs in edrioasterid and possibly edrioblastoid edrioasteroids, whereas PBS B2 has exclusively uniserial oral frame plates and is found in isorophid edrioasteroids and imbricate and gogiid blastozoans. These different types of mouth frame construction offer potential synapomorphies to aid in parsimony-based phylogenetics for exploring branching order among stem groups on the echinoderm tree of life

Universal elemental homology.

<p>Camera lucida drawing of the oral surface of the glyptocystitoid rhombiferan <i>Lepadocystis moorei</i> (Meek, 1871) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Meek1" target="_blank">[72]</a> (CMCIP 57349) with plates colored according to Universal Elemental Homology. Red plates (O1–O7) are oral plates that form the border of the peristome. Blue plates (1–5) are primary peristomial cover plates (PPCP). Green plates are ambulacral floor plates (AFP), tan plates are ambulacral cover plates (ACP). A–E are the ambulacral designations based on the Carpenter <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Carpenter1" target="_blank">[5]</a> system. The mouth or peristome is located beneath PPCP1, 3, and 4. The gonopore and hydropore are gp and hp, respectively. S are shared cover plates of the shared ambulacra. (After <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Sumrall10" target="_blank">[57]</a>).</p

Examples of peristomial border systems (PBS) with cover plates removed to expose the mouth frame.

<p>A. PBS A1, interior view of <i>Kailidiscus</i> showing the mouth frame constructed by integrated interradial plates (IIP), or orals, and unfused biserial floor plates, black areas are the elongated peristome in the center and podial pores between plates, modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Zhao1" target="_blank">[48]</a>; B. PBS A2, the diploporan <i>Protocrinites </i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Sumrall2" target="_blank">[4]</a>, note that orals 2 and 5 do not contact the peristome (black); C. PBS A3, the crinoid <i>Palaeocrinus</i>, note the oval peristome that had shared peristomial cover plates, modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Sprinkle9" target="_blank">[89]</a>; D. PBS A4, the coronoid <i>Stephanocrinus</i>, note the round peristome that was covered by only five primary peristomial cover plates, modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Brett1" target="_blank">[90]</a>; E. PBS B1, the edrioasterid <i>Edriophus</i> with both oral plates (O1–O6) and oral frame plates (A–E), an interpretation based on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone-0077989-g004" target="_blank">Figures 4G, I</a>; F. PBS B2, interior view of the isorophid <i>Hypsiclavus</i> showing oral frame plates (A–E) in line with uniserial floor plates, modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Sumrall8" target="_blank">[51]</a>.</p

List of key taxa discussed or illustrated; order follows Table 3.

<p>List of key taxa discussed or illustrated; order follows <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone-0077989-t003" target="_blank">Table 3</a>.</p

Homologous oral-region plates in more-derived blastozoans.

<p>Colored and uncolored views for plate comparison; color key as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone-0077989-g001" target="_blank">Figure 1</a>; geologic ages in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone-0077989-t001" target="_blank">Table 1</a>. A,B, D, E. the eocrinoid <i>Rhopalocystis destombesi</i> Ubaghs, 1963 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Ubaghs5" target="_blank">[73]</a>, A, B, latex cast of paratype PMO A29122 with only orals and floor plates preserved, D, E, latex cast of paratype PMO A29124 with all oral surface plates preserved; C, F. the glyptocystitoid <i>Quadrocystis graffhami</i> Sprinkle, 1982 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Sprinkle8" target="_blank">[74]</a>, holotype OU 8972; G, H. the diploporan <i>Eumorphocystis multiporata</i> Branson and Peck, 1940 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Branson1" target="_blank">[75]</a>, SUI 97598; I, L. the coronoid <i>Stephanocrinus gemmiformis</i> Conrad, 1842 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Conrad1" target="_blank">[76]</a>, SUI 134869; J, K. the paracrinoid <i>Columbocystis typica</i> Bassler, 1950 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077989#pone.0077989-Bassler1" target="_blank">[77]</a>, SUI 134870. Scale bars: 5 mm (A–L).</p