Discussion of Phylogenetic Relationships
Traditionally, crinoids were subdivided into four subclasses: the Paleozoic Camerata, Inadunata, and Flexibilia, and the post-Paleozoic Articulata. However, a fifth subclass, the Echmatocrinea, was added in Moore & Teichert (1978). Phylogenetic relationships among these subclasses have long been poorly understood, with the exception of Flexibilia and Articulata derived from the cladid inadunates. Crinoid phylogeny and its implied classification are now the subject of intensive research and somewhat differing opinions based on differing assumptions of the crinoid outgroup and calyx plate homologies that yield different phylogenies.
Simms & Sevastopulo (1993) recognized that three major groups of crinoids were already distinct by the Lower to Middle Ordovician: the Camerata, Disparida, and Cladida. They recognized the Camerata as traditionally understood. However, they followed the suggestions of Kelly (1982, 1986) and disregarded the Inadunata as a group, because the disparids and cladids were separate, unrelated lineages. The flexibles and articulates both evolved from the cladids, so Simms and Sevastopulo (1993) placed these within the Cladida. Phylogenetic relationships in Simms and Sevastopulo (1993) were based on a limited number of characters with synapomorphies listed on a cladogram. No quantitative analyses were performed, and an emphasis was placed on Articulata characters.
Simms (1994) presented an alternative phylogeny based on a greatly revised scheme of calyx plate homologies. Rather than the radial plates (the upper plates of an aboral cup, sensu Moore & Teichert, 1978) being homologous among all crinoids and the landmark for determining all calyx plate homologies, Simms (1994) proposed that the lowest plates in the aboral cup (the infrabasal plates of three-circlet forms but the basal plates of two-circlet forms) were homologous and the homology landmarks. This substantially altered more traditional views on crinoid phylogeny, as Simms (1994) illustrated using a cladogram with inferred synapomorphies. Few workers have followed Simms (1994).
Ausich (1996a, 1997, 1998a,b) presented a phylogeny based on another alternative calyx plate homology scheme (Ausich 1996b), a differing outgroup, and parsimony-based character analyses. In this scheme, the most primitive crinoid aboral cup construction had four circlets of plates, inherited directly from rhombiferan echinoderm ancestors (Ausich 1999). An early rhombiferan echinoderm is suggested as the putative ancestor to crinoids because of shared characters among some early rhombiferans and some early crinoids. Also, Conway Morris (1993) and Ausich & Babcock (1996, 1998) do not regard Echmatocrinus from the Middle Cambrian as a crinoid. Thus, the oldest known crinoids are Early Ordovician. However, note that Sprinkle & Collins (1995), Sprinkle & Guensburg (1997), and Guensburg & Sprinkle (1997) do not follow this revision. The fourth, lowermost circlet of plates was previously unrecognized. Homologies of Ausich (1996b) only require revisions for four-circlet crinoids and disparids. The phylogeny of crinoids presented above is based on the homology scheme and PAUP parsimony analyses of Ausich (1996a, 1998a). Primitive crinoids have four circlets. Ausich followed Simms & Sevastopulo (1993) by eliminating the Inadunata and recognizing the Disparida and Cladida as separate, early crinoid lineages . The camerates are recognized by Ausich (1998a) as are the Flexibilia and Articulata. Furthermore, the camerates, flexibles, and articulates all evolved from different cladid lineages.
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