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Journal of Paleontology 91(4):829–846
Remarks.—Flexible crinoids are a morphologically homo- geneous clade that originated sometime during the Middle to Late Ordovician and range through the Permian. Springer (1911, 1920) was the first to recognize that flexible crinoids were closely related to inadunates. In his comprehensive 1920 monograph, The Crinoidea Flexibilia, Springer compared morphologic characteristics of the inadunate Cupulocrinus with the earliest known flexible Protaxocrinus, citing numerous similarities in calyx plating, interradial areas, and the arrangement of posterior plates. Springer (1920) concluded Cupulocrinus was potentially a transitional fossil that linked inadunates with flexibles, stating, “there is clearly an inter- mingling of the characters … and it is evident that in Cupulo- crinus we have to deal with a transition [sic] form whose exact status is difficult to decide” (Springer, 1920, p. 89). Subsequent taxonomic treatments have also recognized Cupulocrinus as occupying a proximal position to the base of the flexible tree (Moore and Laudon, 1943; Moore and Teichert, 1978). Phylogenetic analyses sampling flexible and other crinoid
taxa have invariably recovered tree topologies supporting Springer’s (1911, 1920) hypothesis, with Cupulocrinus recov- ered as the sister taxon to the Flexibilia (Brower, 1995, 2001; Ausich, 1998b; Ausich et al., 2015; Wright, 2017). Wright’s (2017) analysis used Bayesian methods to estimate the probability of Cupulocrinus being ancestral (sensu Foote, 1996) to the flexible clade. Results strongly support Cupulocri- nus as occupying an ancestral position (posterior probability = 0.99) (Wright, 2017). Given these results and our stem-based definition of the Flexibilia, species of Cupulocrinus are now placed within the flexibles. Flexible crinoids have loosely sutured calyx plating and a
remarkably uniform set of apomorphies relative to other crinoid clades. For example, flexibles differ from cladids in having interradial and intrabrachial plates and differ from other dicyclic crinoids in typically having their lowermost circlet comprised of three (rather than five) infrabasal plates. One infrabasal plate, the ‘azygous’, is smaller than the other two, and is located in the C ray (except for the derived Forbesiocrinus). Many flexibles retain posterior plate arrangements similar to other cladids, but posterior plates are sometimes absent in more derived flexibles. In contrast with cladids, arms of flexible crinoids are universally uniserial and lack pinnules, and the stem is nearly always transversely circular (Springer, 1920).
Eucladida Wright, 2017
Definition.—The Eucladida is stem-defined as the most inclu- sive clade containing Dendrocrinus longidactylus Hall, 1952 and Pentacrinites fossilis Blumenbach, 1804 but not Taxocrinus macrodactylus (Phillips, 1841).
Remarks.—The revision of the Cladida to be monophyletic requires placing the subclades Porocrinoidea, Flexibilia, and Articulata within a more inclusively defined Clade Cladida (Simms and Sevastopulo, 1993; Wright, 2017). However, the Cladida (sensu Moore and Laudon, 1943) is traditionally con- ceived as a Paleozoic-age paraphyletic group that excludes the Flexibilia. The Eucladida was proposed by Wright (2017) to comprise all species within the Clade Cladida sharing a more
recent common ancestor with Dendrocrinus and Pentacrinites than with Taxocrinus. Thus, the stem-defined clades Flexibilia and Eucladida are sister to one another and articulates are nested within the Eucladida. This Eucladida retains much of the meaning and taxonomic content of Moore and Laudon’s (1943) concept for Paleozoic cladids while eschewing paraphyly. In the Treatise on Invertebrate Paleontology, Moore et al.
(1978a) recognized three rank-based taxa within the Cladida: the Dendrocrinida, the Cyathocrinida, and the Poteriocrinida. However, it has long been questioned whether these taxa represent monophyletic groups (McIntosh, 1986, 2001; Sevastopulo and Lane, 1988; Kammer and Ausich, 1992, 1996; Simms and Sevastopulo, 1993; Wright, 2015a, 2015b; Wright, and Ausich, 2015). Indeed, the Poteriocrinida is depicted in the Treatise as a polyphyletic group (Moore et al., 1978a, fig. 412).Aphylogenetic analysis of Ordovician through Devonian pentacrinoids by Wright (2017) has confirmed the doubts over the monophyly of these taxa. Much of the problem arises from ambiguous and/or uninformative apomorphies chosen for these taxa that perpetuate taxonomic anarchy via ‘undiagnostic diagnoses’ (Wright, 2015b; see Lane, 1978, p. T295). Although much revision is needed, recent analyses indicate there is nevertheless considerable phylogenetic struc- ture among subclades of Paleozoic cladids, and additional work is under way to revise this diverse group (Wright, 2015b).
Articulata Miller, 1821
Definition.—The Articulata is node-defined as the least inclu- sive clade containing Endoxocrinus parrae (Gervais, 1835) and Antedon bifida (Pennant, 1777).
Remarks.—The Articulata was proposed by Miller (1821) and has since developed a longstanding reputation as a problematic group that lacks a concise and unambiguous definition (Rasmussen, 1978; Simms, 1988; Simms and Sevastopulo, 1993; Webster and Jell, 1999; Rouse et al., 2013). Although all extant crinoids are invariably recognized as articulates, much confusion surrounds the recognition of fossil articulates and the timing of their origin. The primary difficulties surround which apomorphy (or combination of apomorphies) is useful for diagnosing the Articulata. For example, it is widely appreciated that no apomorphy or unique set of apomorphies can presently diagnose fossil articulates without ambiguity (Simms, 1988; Simms and Sevastopulo, 1993; Webster and Jell, 1999; Rouse et al., 2013). Most crinoid workers have obviated this problem by simply treating the Articulata as synonymous with post- Paleozoic crinoids (see Simms and Sevastopulo, 1993). However, this usage is problematic because this definition is not based on any explicit phylogenetic hypothesis. Moreover, many Paleozoic groups of fossil cladids share different combinations of traits typically listed as ‘diagnostic’ for the Articulata (Webster and Jell, 1999; Webster and Lane, 2007). If the con- cept of what defines the Articulata depends on the choice of a particular combination of apomorphies alone, then questions regarding the ‘origin of the Articulata’ will always depend on which specific combination was chosen a priori to be diagnostic. Without a phylogenetic definition, it is impossible to objectively specify a precise set of synapomorphies for the Articulata. Thus,
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