Wright et al.—Phylogenetic classification of the Crinoidea


Ausich et al., 2015). With the exception of Simms and Sevastopulo (1993), these studies have been readjustments of the Moore and Teichert (1978) classification to accommodate rank changes, the addition of new groups, and delineation of clade membership defined by phylogenetic studies of extant species. The study of extant crinoids remains in the shadow of


A. H. Clark, who published more than 100 publications on their morphology, taxonomy, and classification during the early to middle twentieth century (e.g., Clark, 1915, 1921; Clark and Clark, 1967). The advent and application of molecular phylo- genetic methods to crinoid phylogeny has recently thrown light on relationships among extant species (Cohen et al., 2004; Hemery et al., 2013; Rouse et al., 2013; Summers et al., 2014). However, these analyses also point toward the need for exten- sive taxonomic revisions and an improved understanding of morphologic traits among living species (Messing and White, 2001; David et al., 2006; Roux et al., 2013; Summers et al., 2014; Hays et al., 2015). Remarkably, there has been little pre- vious work to combine molecular phylogenetic studies of extant crinoids with paleontologic data to assemble a more complete picture of post-Paleozoic crinoid evolutionary history. Efforts to integrate these rich sources of information present both chal- lenges and opportunities for future researchers to resolve pat- terns and processes shaping the crinoid tree of life (Lee and Palci, 2015; Pyron, 2015).


Crinoid origins and classification


Extant echinoderms include the Crinoidea, Echinoidea, Ophiuroidea, Asteroidea, and the Holothuroidea, with the latter four comprising the Eleutherozoa. Although it has been long established that crinoids form the sister group to the Eleuther- ozoa, the relationships among many fossil and extant echino- derm groups are controversial (Paul and Smith, 1984; Sumrall 1997; David et al., 2000; Smith, 2005; Pisani et al., 2012; Telford et al., 2014; Zamora and Rahman, 2014; Feuda and Smith, 2015; Reich et al., 2015). The phylogenetic position of crinoids within the Echinodermata was contested throughout the late twentieth century, with a focal question whether the Pelmatozoa (i.e., stalked echinoderms including blastozoans and crinoids) and/or the Blastozoa are monophyletic groups or a ‘grade’ of body plan organization. This is a fundamental ques- tion not only for understanding the origin of crinoids but also for resolving phylogenetic relationships among clades within the Echinodermata. One hypothesis of crinoid origins postulates that crinoids and blastozoan echinoderms independently evolved pelmatozoan-grade body plans (e.g., Sprinkle, 1973, 1976; Mooi and David, 1998, 2008; David et al., 2000; Guensburg and Sprinkle, 2003; Guensburg, 2012). This hypothesis proposes that blastozoans and crinoids each com- prise distinct monophyletic groups. By contrast, an alternative hypothesis postulates that blastozoans and crinoids are members of an inclusive pelmatozoan clade, with crinoids nested within a paraphyletic Blastozoa (Leuckart, 1848; Bather, 1899, 1900; Paul and Smith, 1984; Smith, 1984; Paul, 1988; Smith and Jell, 1990; Smith, 1994; Sumrall, 1997; Ausich, 1998a, 1998b; Clausen et al., 2009; Zamora and Smith, 2011; Kammer et al., 2013; O’Malley et al., 2016). In this hypothesis, the blastozoan


831


body plan represents a grade of organization within the more inclusive Pelmatozoa, a clade comprising all blastozoan-grade echinoderms and crinoids (including the crown group). Although the inclusive group of nominal ‘blastozoan’ taxa is not monophyletic, there are undoubtedly assemblages of blastozoan taxa that do correspond to monophyletic groups (Smith, 1984; Sumrall and Wray, 2007; Zamora and Smith, 2011; Sumrall and Waters, 2012; Zamora et al., 2016). Important to this debate are the differences among


researchers with respect to their underlying taxonomic princi- ples and systematic practices (see Smith, 1988). Those who support the monophyly of the Blastozoa and Crinoidea embrace systematic practices that emphasize differences (rather than similarities) among taxa, recognize plesiomorphic traits as taxonomically informative characters, exclude character data from consideration of relationships because of a priori beliefs regarding the distribution of homoplastic traits, and conflate sister group hypotheses with ancestor–descendant relationships (e.g., Guensburg and Sprinkle, 2003, 2007; Guensburg, 2012; Guensburg et al., 2016). These practices differ considerably from those that infer the Pelmatozoa as a clade. These workers tend to emphasize similarities (rather than differences) among taxa, minimize a priori assumptions regarding hypotheses of character evolution, and utilize the principles of phylogenetic systematics to rigorously test whether apparent similarities in form reflect synapomorphies or homoplasy (e.g., Sumrall and Waters, 2012; Sumrall, 2014; Ausich et al., 2015). Given the recent advances in homology assessment among pentaradiate echinoderms (e.g., Sumrall, 1997, 2008, 2010, 2014; Sumrall and Waters, 2012; Kammer et al., 2013) and computational phylogenetic analyses of echinoderm taxa based on a large ensemble of characters, it is becoming increasingly clear that a blastozoan-grade taxon likely forms the closest immediate out- group to the Crinoidea (Kammer et al., 2013; Sumrall, 2014). In the future, new developments in phylogenetic research along with a continued search for the oldest ‘crinoid’ fossils will continue to play a role in uncovering the sequence of morpho- logic transitions behind the assembly of the crinoid body plan. Despite desultory disagreements regarding crinoid origins


(Sprinkle, 1973; Ubaghs, 1978; Donovan, 1988; Ausich, 1998a, 1998b; Ausich and Babcock, 1998; Guensburg and Sprinkle, 2007, 2009; Guensburg, 2012; Kammer et al., 2013; Ausich et al., 2015; Guensburg et al., 2016), there is nevertheless con- siderable agreement among workers regarding the pattern of branching relationships within the crinoid ingroup. For exam- ple, the recent phylogenetic analyses of Guensburg (2012) and Ausich et al. (2015) reveal highly congruent patterns of branching relationships among crinoid higher taxa despite the use of alternative outgroups, different data sets, and alternative interpretations of homologous morphologic characters. We surmise this growing consensus stems from the improved taxonomic sampling of the oldest known crinoids (Guensburg and Sprinkle, 2003, 2009; Guensburg, 2010) and implementa- tion of more rigorous quantitative approaches to testing phylo- genetic hypotheses (Guensburg, 2012; Ausich et al., 2015; Cole, 2017; Wright, 2017). We conclude that congruence observed among tree topo-


logies obtained from researchers with different perspectives indicates strong support for these patterns. Although questions


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