Evolution of Crinoidea
The four-circlet crinoids, disparids, cladids, and camerates all arose during the Early Ordovician; the flexibles arose during the Middle Ordovician, and the articulates evolved during the earliest Mesozoic. Camerates, disparids, flexibles, and cladids were extinct by the end of the Paleozoic Era, about 225 million years ago. The Articulata are the only post-Paleozoic crinoids. Either they evolved during the Permo-Triassic mass extinction or immediately thereafter.
Major evolutionary steps in the crinoid phylogeny discussed above include the following (see phylogenetic tree below): Crinoids diverged from rhombiferan echinoderms through (1) loss of pore rhombs, gonopore, and biserial brachioles; development of true arms with extension of the ambulacra; movement of the anus to the tegmen; addition of anal plates; and better pentameral symmetry. The resulting primitive crinoids were four-circlet forms constituting a low diversity, basal group (Ausich, 1998a). From this four-circlet construction, disparids arose through loss of the basal circlet (2). The cladid lineage, arose by loss of the lintel circlet (3) and gave rise to three additional very successful lineages: camerates (4) with fixed brachials and fixed interradials incorporated into the calyx, symmetrical posterior plating, and rigid plate sutures; flexibles (5) with the mouth exposed on the tegmen and loose plate sutures; and the articulates (6) with loss of the anal plate and an entoneural system enclosed within the calyx plates. The monophyletic nature of the subclass Articulata has been argued by Simms & Sevastopulo (1993).
Phylogenetic hypothesis of crinoid relationships, based on Ausich(1996b, 1999).
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