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Journal of Paleontology 91(4):829–846
evolution. Moreover, stem group taxa commonly have highly heterogeneous distributions of apomorphic traits, which may lead to instability when new taxa are sampled and/or alternative topologies are equally likely. Finally, the timing of a divergence event may not correspond with the acquisition of a diagnostic apomorphy. For example, the blastozoan Macrocystella is widely recognized as a basal glyptocystitoid rhombiferan even though it lacks the respiratory structures traditionally ‘diagnostic’ of the Glyptocystida (Paul, 1968; Sprinkle, 1973; Zamora et al., 2016). All of these considerations are highly importantwhen considering patterns of character evolution but may lead to nomenclatural instability if incorporated into clade definitions. Although we avoid the use of apomorphies to define clades,
we do discuss morphological traits potentially useful for taxo- nomic diagnoses. In some cases, our proposed clade definitions retain much of their traditional meaning and taxonomic content, with constituent taxa sharing numerous synapomorphies that form unambiguous taxonomic boundaries (e.g., the Flexibilia). However, in other cases, either substantial revision was necessary and/or a list of unambiguous diagnostic characterswas difficult or impossible to obtain (e.g., the Articulata). These challenges highlight the utility of phylogenetic taxonomy. For example, many authors have remarked that the Articulata has lacked a concise, unambiguous definition since it was first erected by Miller nearly 200 years ago (Simms, 1988; Webster and Jell, 1999; Hess and Messing, 2011; Rouse et al., 2013). A phylo- genetic definition of the Articulata provides a clearer criterion for clade membership and results in a framework for future phylo- genetic research assessing relationships among hypothesized stem clades, crown group synapomorphies, and subsequent morpho- logic transitions among crown group subclades. The clade definitions and revised classification proposed
herein represent the present state of knowledge, but systematics is a dynamic science and taxonomic theories are commonly reinterpreted in light of new discoveries. We fully expect our definitions to be refined and/or modified as more information becomes available. Some places of the crinoid tree still require extensive taxonomic revisions, such as upper Paleozoic ‘cladids’ (sensu Moore and Laudon, 1943) and stem articulates (Wright, 2015b). Despite these potential vicissitudes in the taxonomic content and/or definitions within our proposed clas- sification, we agree with G.G. Simpson’s sentiment: “It is pusillanimous to avoid making our best efforts today because they may appear inadequate tomorrow” (1944, p. xxx [sic]).
Systematic paleontology Crinoidea Miller, 1821
Definition.—The Crinoidea is stem-defined as the most inclusive clade containing Rhodocrinites verus Miller, 1821, Actinocrinites triacontadactylus Miller, 1821, and Pentacrinites fossilis Blumenbach, 1804 but not Rhopalocystis detombesi Ubaghs, 1963, Echinosphaerites aurantium (Gyllenhaal, 1772), Eumorphocystis multiporate Branson and Peck, 1940, Protocrinites ouiformis Eichwald, 1840, Cheirocystis antiqua Paul, 1972, Glyptocystella loeblichi (Bassler, 1943), Cambraster cannati Miquel, 1894, and Cambroblastus enubilatus Smith and Jell, 1990.
Remarks.—This definition captures J. S. Miller’s (1821) origi- nal concept based on fossil specimens and retains the name ‘Crinoidea’ as the clade comprising the crown group plus all extinct species sharing a more recent common ancestor with a living crinoid than any echinoderm taxon listed in the preceding as external specifiers (Fig. 2). Further, this definition closely resembles the traditional use and taxonomic content of the Cri- noidea as used by both biologists and paleontologists (Bather, 1899; Clark, 1915; Jaekel, 1918; Moore and Teichert, 1978; Hess et al., 1999; Rouse et al., 2013) and accommodates the current state of uncertainty regarding their nearest extinct sister group. In the interest of preserving the taxonomic content and common meaning of a widely used name, our Clade Crinoidea is preferred over Sumrall’s (1997) similarly defined Crinoido- formes (see Cantino and de Queiroz, 2010, p. 42). The Crinoi- dea is comprised of two major clades, the Camerata and the Pentacrinoidea, reflecting the early divergence between cam- erate and non-camerate crinoids (Jaekel, 1918; Donovan, 1988; Guensburg, 2012; Ausich et al., 2015). Because we provide the Crinoidea with a stem-based definition, the discovery of stem- ward fossils is accommodated within this definition. Internal taxon specifiers were chosen because they were
included in Miller’s (1821) original description and represent well-known, well-preserved, and highly nested members of their respective subclades. In contrast to the internal taxon specifiers, the choice of external specifiers is more complex. The use of external specifiers in this definition spanning various ‘blastozoan’ and edrioasteroid-grade groups reflects the current difficulty involved in postulating the nearest definitive sister group as well as the uncertain state of relationships among extinct stemmed echinoderms (Smith, 1984; Sumrall, 1997, 2014; Ausich, 1998a, 1998b; Guensburg and Sprinkle, 2009; Kammer et al., 2013; Ausich et al., 2015; Guensburg et al., 2016; O’Malley et al., 2016). The analysis of Ordovician crinoids by Ausich et al. (2015)
took a conservative approach to outgroup selection by sampling broadly across taxa nested within the Clade Pelmatozoa (Kammer et al., 2013; Sumrall, 2014). Similarly, we have chosen species from multiple pelmatozoan groups as external specifiers to help provide nomenclatural stability in the presence of phylogenetic uncertainty. Other taxa hypothesized to represent the crinoid sister group include the stylophorans (David et al., 2000) and edrioasteroids (Guensburg and Sprinkle, 2009; Guensburg et al., 2016). Stylophorans have long been considered non-radiate stem group echinoderms (e.g., Paul and Smith, 1984; Smith, 1984, 2008) and have been cogently demonstrated to lack crown group synapomorphies (Smith, 2005). Thus, we do not consider the stylophoran hypothesis further. Guensburg and Sprinkle (2009) and Guensburg et al. (2016) regard edrioasteroid echinoderms, such as the stromatocystidid Cambraster or the edrioblastoid Cambroblastus, to possess apomorphies indicating they share a more recent common ancestor with crinoids than with other echinoderms. Although this hypothesis contrasts with previous studies regarding edrioasteroids as stem group eleutherozoans (Paul and Smith, 1984; Smith 1984, 1985, 1990; Smith and Zamora, 2013), recent investigations suggest that edrioasteroids may comprise a para- or polyphyletic group (Kammer et al., 2013; Zamora, 2013; Zamora and Rahman, 2014). Some
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