Abstract

For many years the earliest record of the class Crinoidea was a single late Tremadocian genus. In the past decade, five crinoid genera were described from the early and middle Tremadocian, near the base of the Ordovician. Together these six genera represent a diverse assemblage with all but one expressing existing subclass apomorphies. Two of the recently described genera were initially assigned to their own order (plesion) Protocrinoida but not to a subclass. Here they are placed in the camerates based on apomorphies of the tegmen complex. Protocrinoids exhibit plesiomorphies unlike typical camerates. Two genera group with cladids, one expressing dendrocrinine apomorphies and the other cyathocrinine. One genus is placed within disparids, with iocrinid apomorphies.

Based on its ancient age and trait mosaic, the protocrinoid Titanocrinus is designated as outgroup in a phylogenetic analysis using all other Early Ordovician and select Middle Ordovician taxa as an ingroup. Character compilation and phylogenetic analysis posit early class-level plesiomorphies inherited from an unknown ancestry but lost during subsequent crinoid evolution. Class-level apomorphies also emerge, some of which were subsequently lost and others retained. Results are generally robust and consistent with earlier subdivisions of the class, but supporting lower rank reorganizations. Strong support for the camerate branch low in the crinoid tree mirrors findings of earlier workers. Cladids branch from a series of intermediate nodes and disparids nest highest. Branching of disparids from cladids could be homoplastic.

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