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