832


Journal of Paleontology 91(4):829–846


surrounding crinoid origins remain, this debate is moot with respect to the phylogeny-based definitions and classification presented herein and ultimately has no bearing on the focus and conclusions of this paper.


Toward a phylogenetic classification of the Crinoidea


From the perspective of their geologic history, crinoids are a bottom-heavy clade (Gould et al., 1987). In contrast to the tre- mendously diverse assemblage of stem lineages, comparatively few species are encompassed within the crown group (Fig. 1). Because of the enormous diversity of the stem group relative to the crown group, fossil crinoids have received much systematic attention compared to their extant representatives (but see Clark, 1915; David et al., 2006; Hess and Messing, 2011; Hemery et al., 2013; Rouse et al., 2013). Aside from a number of smaller studies examining relationships among species of middle to late Paleozoic genera (e.g., Gahn and Kammer, 2002; Kammer and Gahn, 2003; Ausich and Kammer, 2008), most investigations of crinoid phylogeny have focused on discerning relationships among Ordovician taxa (Brower, 1995; Ausich 1998b; Guensburg, 2012; Ausich et al., 2015; Cole, 2017). The Ordo- vician Period represents a key interval in crinoid evolution because species belonging to various groups of traditionally named taxa first appear in rocks of the Lower Ordovician (Tremadocian) (Guensburg and Sprinkle, 2003, 2009; Guensburg, 2010) and the majority of well-studied groups had originated prior to its close. The divergence between camerate and non-camerate


lineages forms a fundamental, early split in the history of crinoid evolution (Jaekel, 1918; Donovan, 1988; Guensburg, 2012; Ausich et al., 2015; Cole, 2017; Wright, 2017) (Fig. 1). For example, in the recent phylogeny of Ausich et al. (2015), taxa belonging to the Camerata (sensu Moore and Teichert, 1978) form the sister clade to all other crinoids, including the proto- crinoids (Guensburg and Sprinkle, 2003). Disparids were recovered as sister to a clade comprised of most ‘cladid’ taxa, and hybocrinids were recovered as sister to a group of ‘cyathocrine’ cladids (sensu Moore and Teichert, 1978). A similar pattern was recovered by Guensburg (2012, fig. 2). Building on these studies, Cole (2017) further assessed the


basal split between camerates and non-camerates and tested the taxonomic status of the Monobathrida and Diplobathrida (Fig. 1). Wright’s (2017) analysis of relationships among non- camerate crinoids offers a more nuanced perspective of this portion of the crinoid tree than previously recovered. Notably, many so-called Ordovician clades of Guensburg (2012) and Ausich et al. (2015) do not retain their status of monophyly when post-Ordovician taxa are considered (Wright, 2017). Recent molecular phylogenetic studies indicate broad


relationships among major clades of extant crinoids are also reaching a consensus, with the Isocrinida representing the sister clade to all other extant crinoids (Rouse et al., 2013, 2015). It is


interesting to note that divergence time estimation based on relaxed molecular clock models suggests the split between iso- crinids and other extant groups took place some 231–252 million years ago (Rouse et al., 2013). Thus, molecular phylogenetic analyses and paleontological evidence are in general agreement regarding an ancient origin of the crinoid crown group. A summary tree based on results presented by Rouse et al.


(2013), Ausich et al. (2015), Wright (2017), and Cole (2017) is depicted in the form of a simplified cladogram in Figure 2. This cladogram is annotated with the clade names we propose below. Terminal taxa in the cladogram were carefully chosen to maximize stability in phylogenetic nomenclature (Table 1). Sereno (2005) listed numerous criteria for choosing taxon specifiers in clade definitions. These recommendations include choosing specifiers that are nested rather than basal (if possible), represented by well-known or readily available material, and using multiple specifiers where necessary to accommodate phylogenetic uncertainty and/or alternative hypotheses. We have carefully chosen our clade definitions to not hinge on labile phylogenetic hypotheses or specific interpretations of unusual and/or problematic taxa. Classesofclade definitions used in phylogenetic taxonomy


and their graphical representations used herein closely follow Sereno (1999, 2005). Node-based clade definitions circumscribe the most recent common ancestor of at least two taxa and all of its descendants. Thus, node-based definitions form the least inclusive clade containing a minimum of two specifiers. By contrast, stem-based definitions circumscribe the most inclu- sive clade containing at least one internal specifier. In both cases, additional precision is obtained by identifying external specifiers falling outside the clade (i.e., the outgroup). For example, a stem- based definition for hypothetical Clade A with two internal and one external taxon specifiers can be stated as ‘all species sharing a more recent common ancestorwith species X and Y than Z,’ where X and Y are internal taxon specifiers and Z is an external specifier. In otherwords,Clade Ais stem-defined as themost inclusive clade containing X and Y but not Z. Note the presence of one species as an external specifier effectively eliminates the entire clade to which it belongs.By definition, a clade cannot contain an ancestor of its sister group. In phylogenetic taxonomy, clade membership is not


determined by the presence or absence of a ‘key’ morphologic feature unless that apomorphy (or set of apomorphies) is listed in the definition as a qualifying clause (Sereno, 2005). We avoid apomorphic qualifiers in our definitions for several reasons. First, incomplete preservation may lead to cases where it is unknown whether a fossil species has the key feature diagnostic of the clade in question. Thus, the inclusion or exclusion of a fossil species depends on character state optimizations rather than direct data. Second, a trait may be ‘absent’ in a taxon either because it was truly absent or because it was secondarily lost. Similarly, a trait may be ‘present’ because of convergent


Figure 1. Taxa representing major crinoid clades: (1) Pentacrinites fossilis Blumenbach, 1804, articulate, from Goldfuss (1831); (2) Taxocrinus colletti White, 1881, flexible, from Springer (1920); (3) Actinocrinites jugosus (Hall, 1859), monobathrid camerate, from Wachsmuth and Springer (1897); (4) Synbathocrinus swallovi Hall, 1858, disparid, from Wachsmuth and Springer (1897); (5) Dendrocrinus caduceus Hall, 1866, eucladid, from Meek (1873); (6) Hybocystites eldonensis Parks, 1908, hybocrinid, from Springer (1911); (7) Porocrinus shawi Schuchert, 1900, porocrinid, from Kesling and Paul (1968); (8) Archaeocrinus microbasalis (Billings, 1857), diplobathrid camerate, from Wachsmuth and Springer (1897). Scale bars = 0.5cm and applicable as indicated.


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