Wright et al.—Phylogenetic classification of the Crinoidea
followed Simms and Sevastopulo’s (1993) interpretation of relationships among these taxa, only a few authors have since followed their revised rank-based classification (e.g., Brower, 2001, 2002; Donovan and Harper, 2003). Our stem-based definition of the Cladida is similar in
taxonomic content to Simms and Sevastopulo’s(1993) because it includes all species closer to Dendrocrinus than to the disparid Synbathocrinus. Thus, the Cladida spans the Ordovician to the Recent and contains themajor subclades Porocrinoidea, Flexibilia, and Articulata. Cladids are most easily distinguished from their sister group, theDisparida, in typically having a dicyclic calyx and posterior plates (as adults or during development) located below and/or in line with the radial plate circlet (Wright, 2015a). Lastly, many middle Paleozoic to Recent cladids have pinnules, whereas most disparids do not (Frest et al., 1979).
Porocrinoidea Wright, 2017
Definition.—The Porocrinoidea is node-defined as the least inclusive clade containing Carabocrinus radiatus Billings, 1857 and Hybocrinus conicus Billings, 1857.
Remarks.—In their description of crinoids belonging to Bather’s (1899) ‘Cyathocrinina’, Moore and Laudon (1943) speculated that ‘primitive’ cyathocrinids such as Carabocrinus might be closely related to the enigmatic taxon Hybocrinus. Sprinkle (1982b) argued the stem and calyx morphology of Hybocrinus suggested hybocrinids were ‘pseudomonocyclic’ and listed a number of characters linking hybocrinids with cla-
dids. Although hybocrinids have not traditionally been classi- fied within the Cladida, many phylogenetic analyses of Ordovician crinoids have recovered a clade of ‘cyathocrine’ grade cladids and hybocrinids (Guensburg, 2012; Ausich et al., 2015; Wright, 2017). Wright’s (2017) phylogenetic analysis of Ordovician through Devonian pentacrinoids recovered a clade comprised of Porocrinus, Carabocrinus, and the hybocrinids Hybocrinus and Hybocystites. Notably, this clade is stemward of the split between flexible and other cladid crinoids. Thus, Wright (2017) proposed the name ‘Porocrinoidea’ to encompass this early diverging and morphologically unique clade of Ordovician crinoids. Our node-based definition of the Porocrinoidea sets up a
node-stem triplet that stabilizes the sister clade relationship among the Porocrinida and Hybocrinida recovered by Ausich et al. (2015), which had denser taxon sampling of Ordovician crinoids than Wright (2017). The Clade Porocrinoidea is likely limited to the Ordovician Period, but additional analyses sampling younger species are needed to test the extent of their geologic duration. Porocrinoids are a subclade of cladids characterized by globose, conical, or ovate calyces that possess a number of apomorphies convergent with blastozoan echino- derms, such as having thecal respiratory structures, reduction in arm number and calyx plates, and/or recumbent ambulacra (see Moore and Teichert, 1978; Sprinkle, 1982a, 1982b).
Porocrinida Miller and Gurley, 1894
Definition.—The Porocrinida is stem-defined as the most inclusive clade containing Porocrinus conicus Billings, 1857
Definition.—The Flexibilia is stem-defined as the most inclu- sive clade containing Taxocrinus macrodactylus (Phillips, 1841) but not Dendrocrinus longidactylus Hall, 1852.
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and Carabocrinus radiatus Billings, 1857 but not Hybocrinus conicus Billings, 1857.
Remarks.—The Porocrinida comprise a small clade of Ordovi- cian porocrinoids with apomorphic endothecal and/or exothecal respiratory structures. Sprinkle (1982a) pointed to many similarities among Carabocrinus, Palaeocrinus, and the Porocrinidae and hypothesized they may be closely related. Ausich et al. (2015) recovered a topology supporting this hypothesis with the euspirocrinid Illemocrinus as their sister taxon. However, Wright (2017) recovered Euspirocrinus out- side the porocrinid clade within a different clade of ‘cyatho- crine’ grade cladids. Thus, Illemocrinus is tentatively placed within the Porocrinida, but other taxa within the Euspriocrinidae should not be placed within the Porocrinida at this time as additional revisions are necessary. Guensburg (2012) recovered a similar tree to Ausich et al. (2015) that suggested Perittocrinus may be also be a porocrinid. The stem-based definition of the Porocrinida makes them
sister to theHybocrinida and retains the taxonomicmembership of this clade recovered inAusich et al. (2015) andGuensburg (2012). Porocrinids can easily be distinguished from hybocrinids in having a dicyclic calyx and the presence of thecal respiratory structures (Kesling and Paul, 1968; Sprinkle, 1982a).
Hybocrinida Jaekel, 1918
Definition.—The Hybocrinida is stem-defined as the most inclusive clade containing Hybocrinus conicus Billings, 1857 and Hybocystites problematicus Wetherby, 1880 but not Porocrinus conicus Billings, 1857 and Carabocrinus radiatus Billings, 1857.
Remarks.—Hybocrinids comprise a small yet morphologically disparate clade of Ordovician crinoids. Although the mono- cyclic hybocrinids have been either considered disparids or classified outside the Inadunata (Moore and Laudon, 1943; Moore and Teichert, 1978; Ausich, 1998b), Sprinkle (1982b) suspected hybocrinids might be ‘pseudomonocyclic’ and potentially related to ‘cyathocrine’ cladids (see Sprinkle, 1982a, 1982b). Phylogenetic analyses by Guensburg (2012), Ausich et al. (2015), and Wright (2017) all support the monophyly of the Hybocrinida and their sister group relationship with taxa placed in the Porocrinida (see Sprinkle, 1982b). In addition to having a pseudomonocyclic calyx (infra-
basals absent), hybocrinids are characterized by a number of unusual apomorphies that distinguish them from Porocrinids (and all other crinoids). Many of these traits are similar to those typically present in blastozoan echinoderms, including reduc- tion in the number of arms, modification of food-gathering appendages to be recumbent (sometimes extending downward over calyx plates), and reduction in the number of calyx plates (Sprinkle and Moore, 1978).
Flexibilia Zittel, 1895
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