UNRAVELING THE MOUTH FRAME OF HEMICOSMITIDS USING PAEDOMORPHIC AMBULACRAL REDUCTION

Hemicosmitids (a clade including Caryocrinites, Hemicosmites, and related forms) are derived blastozoan echinoderms closely allied with blastoids and glyptocystitoids. This clade is noted for an unusual triradial ambulacral system and six-fold thecal symmetry. Confusion in ambulacral identity and homology of the peristomal bordering elements commonly occurs because 1) many hemicosmitids have a tegmen of modified cover plates obscuring the peristomal bordering plates 2) some peristomal elements are lost through paedomorphosis and 3) some proximal ambulacral elements are incorporated into the theca. Here I show that Hemicosmitid peristomal plates can be assigned homology using the same elemental scheme as glyptocystitoids and, by extension, most advanced blastozoans. The tegmen of Caryocrinites is composed of a series of enlarged peristomal cover plates that conceal the peristomal bordering plates. However, in Hemicosmites the cover plates are small and often removed taphonomically from the thecal summit, thereby exposing details of the peristomal border plating. Three of the larger peristomal bordering plates are homologous to O3, O4 and O1 of glyptocystitoids. Two smaller plates that border the hydropore/gonopore in the CD interray are homologous to O6 and O7. Plates O2 and O5, that primitively form the left and right margins of the summit, are lost via paedomorphosis. These plate homologies demonstrate that hemicosmitid ambulacra represent the A, shared BC and shared DE ambulacra of most blastozoan echinoderms because the distal B, C, D, and E ambulacra form via bifurcation of the shared ambulacra when O2 and O5 are inserted. Six large, paired, and radially positioned plates upon which the erectile ambulacra mount are proximal ambulacral floor plates incorporated into the summit. Three small plates positioned between the orals and the proximal ambulacral floor plates are incorporated brachiole mounting plates and mark the position of juvenile, summit-mounted brachioles.