A MECHANISM FOR REPEATED EVOLUTION OF AMBULACRAL COVER PLATE ELEMENTS IN ECHINODERMS

Universal Elemental Homology is a model of skeletal homologies of the echinoderm ambulacral system that are conserved throughout most taxa. The peristomial border and food grooves are formed from a combination of oral, oral frame and floor plate elements that, except in Eleutherozoa, are covered by primary peristomial cover plates and ambulacral cover plates. In most pentaradiate taxa including some crinoids, shared cover plates are present between the laterally positioned PPCPs 2 and 5 and the three central PPCPs 1, 3, and 4. In several edrioasteroid taxa this has been shown to be acquired through ontogeney with juveniles having the lateral plates in contact with the central plates and shared cover plates intercalated later in ontogeny. This occurs with a concomitant reduction in relative size of the PPCPs. The neotenic oral area of Carneyella has extremely large PPCPs and no shared cover plates, whereas the closely related Isorophus has shared cover plates lacking size differentiation of the PPCPs. Similar paedomorphic differentiations occur in other pentaradiate taxa including parablastoids, coronates, some blastoids, camerate crinoids, and Paleozoic microcrinoids.

In some taxa, such as diplobathrid crinoids, the cover plate elements loose distinction in the oral region. Although it can be interpreted that these elements do not exist, it is far more likely that they loose distinction with maturity e.g. 1) development of five PPCPs in contact 2) development of the cover plate system 3) development of oral region filler plates 4) loss of distinction of the cover plate system within the oral region filler plates. In some cases, however, character state optimization suggests that from taxa bearing undifferentiated cover plate systems, lineages with recognizable PPCPs and cover plates arise. This is especially true in camerate crinoids with greatly reduced tegmen plating. The general paedomorphic cover plate pattern in other echinoderms illustrates a mechanism by which clades diagnosed by complex oral regions and undifferentiated cover plate elements can give rise to lineages with simple cover plate systems. Assuming that a complex oral region develops like most other pentaradiate echinoderms, juvenilization of this pattern will re-evolve the ambulacral cover plate system in derived taxa via paedomorphism.