Wright et al.—Phylogenetic classification of the Crinoidea
Eknomocrinus and Adelphicrinus renders the Monobathrida a clade (Cole, 2017). The internal and external specifiers defining this stem-based clade ensure the taxonomic content closely matches Moore and Laudon (1943). Monobathrids are a taxonomically diverse group of
camerates ranging from the Ordovician to Permian and are traditionally diagnosed as monocyclic camerates. Although other clades had similar trends in circlet reduction (e.g., the Disparida), the transformation from a dicyclic to monocyclic calyx likely represents a veritable synapomorphy of mono- bathrid camerates, as the dicyclic crinoid Gaurocrinus was recovered as the sister taxon to the Monobathrida by Cole (2017). Additional features diagnostic of a typical monobathrid species include having radial plates larger than other calyx plates, an upright basal circlet, an uninterrupted radial circlet (except in the posterior interray), and a posterior interray with anitaxis plating and an anitaxial ridge.
Diplobathrida Moore and Laudon, 1943
Definition.—The Diplobathrida is stem-defined as the most inclusive clade containing Archaeocrinus lacunosus (Billings, 1857) and Rhodocrinites verus Miller, 1821 but not Actinocri- nites triacontadactylus Miller, 1821 and Glyptocrinus decadactylus Hall, 1847.
Remarks.—Similar to the discussion above, Moore and Laudon (1943) placed all of Bather’s (1899) dicyclic camerate crinoids within the Diplobathrida. As with the monobathrids, Cole’s (2017) phylogenetic analysis of Ordovician camerates revealed Moore and Laudon’s (1943) Diplobathrida required revision. To achieve monophyly of diplobathrids while retaining much of Moore and Laudon’s (1943) taxonomic content, all dicyclic taxa equally related to bothmonobathrid and diplobathrid camerates sensu Cole (2017) are removed from the Diplobathrida (e.g., Eknomocrinus, Reteocrinids, etc.). Following Cole’s (2017) suggested revision, our stem-based definition stabilizes the long- held hypothesis that monobathrids and diplobathrids represent sister clades (Moore and Laudon, 1943; Cole, 2017). Diplobathrids range from the Ordovician through lower
Carboniferous (Serpukhovian). Cole’s (2017) discussion on the taxonomic distribution of diplobathrid morphologies suggests they are generally characterized by a combination of character states, including a dicyclic calyx, a concave calyx base either concealing or partially concealing the infrabasal plates, and the presence of additional plates interrupting the radial circlet in all interrays (e.g., Rhodocrinites). Some diplobathrids sensu Cole (2017), such as the Dimerocrinitidae, are similar to monobathrids in having their radial circlet interrupted only in the posterior interray but can easily be distinguished by their dicyclic calyx. A closer examination of post-Ordovician species indicates a substantial revision of subclades within the Diplobathrida is needed and additional research is currently underway (Cole, 2015)
Pentacrinoidea Jaekel, 1918
Definition.—The Pentacrinoidea is stem-defined and as the most inclusive clade containing Apektocrinus ubaghsi Guensburg and Sprinkle, 2009 and Pentacrinites fossilis Blumenbach, 1804
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but not Rhodocrinites verus Miller, 1821 and Actinocrinites triacontadactylus Miller, 1821.
Remarks.—The name ‘Pentacrinoidea’ originates from Jaekel’s (1894, 1918) prescient observation that camerate and non- camerate crinoids form distinct clades. Although authors after Jaekel (1918) did not adopt this name in subsequent classifica- tions (see Lane, 1978; Ausich and Kammer, 2001), Jaekel’s usage coincides with this strongly supported clade (Guensburg, 2012; Ausich et al., 2015; Cole, 2017; Wright, 2017). Thus, we propose to reinstate the name Pentacrinoidea with the preceding definition. Wehave chosen two phylogenetically distant non-camerate
species as internal specifiers. Pentacrinites fossilis is a well- known fossil species from rocks of Jurassic age and is closely related to extant isocrinid crinoids (David et al., 2006), placing it within the Crown Crinoidea (see Articulata below). The species Apektocrinus ubaghsi is a Lower Ordovician fossil and ranks among the stratigraphically oldest known crinoids (Guensburg and Sprinkle, 2009). However, all phylogenetic research indicates it is closer to non-camerates than to camerates and diverges stemward of other basal ‘cladid’ (sensu Moore and Teichert, 1978) taxa such as Aethocrinus (Guensburg and Sprinkle, 2009; Guensburg, 2012; Ausich et al., 2015; Wright, 2017). Our stem-based definition recognizes Jaekel’s (1918) priority of this concept and effectively places all known non-camerate species within the Pentacrinoidea. Pentacrinoids are a spectacularly diverse and morphologi-
cally heterogeneous clade ranging from the Early Ordovician to present-day marine communities. The primary apomorphies differentiating pentacrinoids from camerates relate to their distinctive posterior plating patterns, the degree of calyx plate suturing, and oral region rigidity (‘tegmen’ terminology here is from Ausich and Kammer, 2016). Posterior plates among pentacrinoids display a proximal relationship with the C-ray radial plate (Guensburg, 2010; Wright, 2015a). Subclades within the Pentacrinoidea express this affinity differently (cf. Cladida and Disparida), and extant crinoids do not retain posterior plates as adults. However, the ontogenetic trajectory of posterior plate development in extant crinoids is tightly linked with morphologic patterns among their Paleozoic precursors (Wright, 2015a). Pentacrinoid calyx plates are less closely sutured (i.e., ankylosed) than camerates and typically have a non-rigid to flexible oral region. In many pentacrinoids, the mouth is directly exposed on the oral surface rather than beneath a tegmen (Ausich and Kammer, 2016). There are several other morphologic features less diag-
nostic than those described above but still useful for distinguish- ing most pentacrinoid species from camerates. For example, some basal pentacrinoids such as Apektocrinus, Aethocrinus, and Alphacrinus incorporate additional plates within the calyx (similar to camerates). However, the overwhelming majority of
pentacrinoid clades do not. A major exception occurs among flexible crinoids, but flexibles are a derived group of pentacrinoids and can be differentiated from camerates by other apomorphies (see Flexibilia below). Similarly, eucamerate crinoids have pinnules, but most early to middle Paleozoic pentacrinoids do not. Pinnulation evolved at least once (and probably several times) during the middle to late Paleozoic
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