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Journal of Paleontology 91(4):829–846
tentatively followGuensburg and Sprinkle (2009) and Guensburg et al. (2016) by including both Cambraster and the edrioblastoid Cambroblastus as additional external taxon specifiers. Identifying synapomorphies of the Clade Crinoidea
requires a phylogenetic hypothesis of their position within the broader echinoderm clade. As discussed above, this remains an open question. Basal members of both the Camerata and Pentacrinoidea have a dicyclic calyx with an irregular field of plates intercalating between fixed proximal brachials, suggest- ing these may be plesiomorphic traits (cf. Apektocrinus, Cnemecrinus, Glenocrinus) (Guensburg, 2012, Ausich et al., 2015; Cole, 2017; Wright, 2017), but a definitive list of shared derived traits cannot be provided here. Moreover, it is challenging to propose a list of apomorphies that unambigu- ously differentiate crinoids from other echinoderm taxa because many traits are not exclusive to crinoids. Crinoids have been traditionally recognized as distinct from blastozoan-grade echinoderms in having true ‘arms,’ where arms are defined as coelomic extensions of the body cavity (Sprinkle, 1973). However, morphologic observations of solute and diploporitan echinoderms such as Eumorphocystis and the discovery of various Cambrian ‘blastozoans’ with arm-like appendages strongly suggest that arms may not be an apomorphy unique to crinoids (Clausen et al., 2009; Zamora and Smith, 2011; Sumrall, 2014; Zamora and Rahman, 2014). We anticipate future phylogenetic research will help
resolve these broader issues in echinoderm phylogeny and evolution. Improved knowledge of relationships among extinct pentaradiate echinoderms may also help refine our definition of the Clade Crinoidea by removing pleonastic external specifiers. We await its refinement.
Camerata Wachsmuth and Springer, 1885
Definition.—The Camerata is stem-defined as the most inclu- sive clade containing Actinocrinites triacontadactylus Miller, 1821 and Rhodocrinites verus Miller, 1821 but not Pentacri- nites fossilis Blumenbach, 1804.
Remarks.—Camerate crinoids represent a diverse, morphologically distinct ‘stem clade’ (sensu Sereno, 1999, 2005) ranging from the Lower Ordovician to Permian and contain all taxa traditionally placed within the Diplobathrida and Monobathrida (Moore and Teichert, 1978; Cole, 2017). Camerates are most easily differ- entiated from pentacrinoids in having calyx plates united by rigid sutures, a heavily plated tegmen surface covering the mouth, and a medial plate (or series of plates) in the posterior (i.e., CD) interray. Unlike pentacrinoids, the camerate posterior plate series has no proximal topographic affinity with the C ray, although some camerate posterior plates may be homologous with those of pentacrinoids (see Jaekel, 1918, p. 46; Moore and Laudon, 1943; Brower, 1973, p. 301–304; Guensburg and Sprinkle, 2003). In addition, typical camerate species have fixed proximal brachials, interradials, and sometimes intrabrachials, whereas most derived pentacrinoid clades lack these features. Multiple studies indicate strong support for camerate
monophyly (Ausich, 1998b; Ausich et al., 2015; Cole, 2017). However, Cole’s (2017) analysis of Ordovician camerates did not find support for a strict division between monocyclic and dicyclic forms. Cole’s (2017) phylogenetic revision proposed
narrower restrictions on clade membership to render these taxa monophyletic. Following revision, the Monobathrida and Diplo- bathrida are sister clades that together comprise themore inclusive Eucamerata (Cole, 2017). Thus, the stem-based definition of the Camerata contains the Clade Eucamerata and their stem taxa, including representatives of the oldest known crinoid fossils (e.g., Eknomocrinus, Cnemecrinus), and genera placed within the problematic Reteocrinitidae (seeCole, 2017), andmay ormay not contain the protocrinoids (see Guensburg and Sprinkle, 2003; Guensburg, 2012; Ausich et al., 2015; Cole, 2017).
Eucamerata Cole, 2017
Definition.—The Eucamerata is node-defined as the least inclusive clade containing Actinocrinites triacontadactylus Miller, 1821, Rhodocrinites verus Miller, 1821, and Rosfacrinus robustus Le Menn and Spjeldnaes, 1996.
Remarks.—Cole (2017) revised the Monobathrida and Diplo- bathrida to represent monophyletic groups while attempting to preserve the greatest number of taxa traditionally included within each (Moore and Teichert, 1978). The name ‘Eucamerata’ was proposed to identify the clade of camerates comprised of the sister groups Monobathrida and Diplobathrida, which necessarily excludes stem taxa such as Cnemecrinus and Reteocrinus (Cole, 2017). The Eucamerata comprise the majority of camerate taxa and span the Ordovician through Permian. Eucamerates are characterized generally by the traits listed above for the Camerata, but differ in typically having more strongly ankylosed calyx plate sutures, primaxils on the second primibrachial, holomeric stems, and pinnulate arms (cf. Actinocrinites and Rhodocrinites with Eknomocrinus and Reteocrinus). In an attempt to preserve the stability of sister group
relationships between monobathrid and diplobathrid clades, we provide a node-based definition for the Eucamerata and stem- based definitions for the Monobathrida and Diplobathrida. The internal taxon specifiers Actinocrinites and Rhodocrinites are highly nested constituents of their respective monobathrid and diplobathrid subclades (Moore and Laudon, 1943; Cole, 2017). Rosfacrinus is cautiously included as an additional external specifier because it occupies a somewhat uncertain position at the base of the eucamerate tree (see discussion in Cole, 2017).
Monobathrida Moore and Laudon, 1943
Definition.—The Monobathrida is stem-defined as the most inclusive clade containing Glyptocrinus decadactylus Hall, 1847 and Actinocrinites triacontadactylus Miller, 1821 but not Rhodocrinites verus Miller, 1821 and Archaeocrinus lacunosus (Billings, 1857).
Remarks.—When revising Bather’s (1899) polyphyletic divi- sion of crinoids into the Monocyclica and Dicyclica, Moore and Laudon (1943) placed all camerates with monocyclic calyces into the Monobathrida. Cole’s (2017) phylogenetic analysis of Ordovician camerate crinoids indicates a strict adherence to Moore and Laudon’s (1943) concept of the Monobathrida is not monophyletic. However, removal of the stemward camerates
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