THE CONVOLUTED HISTORY OF PARACRINOIDS AND RHIPIDOCYSTIDS (BLASTOZOAN ECHINODERMS) DURING THE ORDOVICIAN RADIATION
Thecal shapes range from globular to ovoid to lens-shaped in paracrinoids, and are strongly flattened in rhipidocystids. Plates in rhipidocystids and some paracrinoids are polygonal, thin, and smooth; in other paracrinoids, plates are either ridged with covered respiratory slits or surface pores between radiating ridges, or concave with respiratory pores on the raised sutures or in plate centers. The mouth can either be central on the summit opposite the stem facet (most rhipidocystids and early paracrinoids) or tilted off the summit with the periproct opposite the stem facet (most later paracrinoids). 2-5 recumbent or erect ambulacra can radiate from the mouth, with either erect biserial brachioles or uniserial ‘pinnules’ attached to the ambulacra.
Various workers in the late 20th Century, such as Frest and Rozhnov, suggested a sister-group relationship between the two groups, but exactly what synapomorphies link the two groups as a clade remain elusive. We suggest the search for phylogenetic linkage focus on peristomal bordering plates, hydropore–gonopore–periproct arrangement, the 3-plate basal circlet, and proximal stem morphology. These features are likely to be relatively stable compared to highly variable features such as thecal plate number, respiratory structures, and ambulacral system arrangement.