Wright—Phylogeny of early to middle Paleozoic crinoids
features ‘cyathocrine’ morphologies. These results support (perhaps unfortunately) long-held suspicions of taxonomic anarchy among cladids recognized by previous authors (McIntosh, 1986, 2001; Kammer and Ausich, 1992, 1996; Simms and Sevastopulo, 1993; Webster and Maples, 2006). The clade of dendrocrine-grade crinoids containing the
most recent common ancestor of Thenarocrinus and Corematocrinus, and its constituent subclades, are supported with posterior probabilities between 0.68 and 0.84. This clade contains a subclade comprising members of the Rutkowskicri- nidae, Glossocrinidae, Corematocrinidae, and Amabilicrinidae. This clade is supported with posterior probability 0.84 and is equivalent to the superfamily Glossocrinacea originally recognized byWebster et al. (2003).
Discussion
It is generally appreciated that quantitative phylogenetic methods do not typically take full advantage of the complete spectrum of information supplied by the fossil record (Wagner and Marcot, 2010). However, recently developed probabilistic macroevolutionary models and powerful computational tools have provided major advancements for estimating phylogenies containing fossil taxa and accommodating paleontologic idiosyncrasies, such as sampling taxa (incompletely) over time (Stadler, 2010; Ronquist et al., 2012; Gavryushkina et al., 2014; Lee and Palci, 2015). This paper builds on these advances by implementing a Bayesian framework to estimate time-scaled phylogenetic hypotheses of early to middle Paleozoic fossil crinoids. The resulting phylogeny indicates extensive taxo- nomic revisions are necessary, especially among the ‘cladid’ crinoids, and points to several areas where further analysis at lower taxonomic levels are needed. In addition to evolutionary implications for crinoids, the results raise several key issues regarding probabilistic approaches to phylogenetic inference in the fossil record and suggest possible directions for future research.
Implications for crinoid evolution and systematics.—The phylogenetic analysis presented herein offers several insights into early to middle Paleozoic crinoid evolution and provides a basis for requisite taxonomic revisions. Although I provide suggestions for revisions below, attempting to more fully
resolve outstanding problems in crinoid systematics and classi- fication is beyond the scope of this paper. Instead, that topic is addressed in a companion paper (see Wright et al., 2017). A basal divergence between disparids and most other
non-camerate crinoids has been recovered in a number of recent phylogenetic studies (cf. Guensburg, 2012; Ausich et al., 2015), including the analysis herein. Although several nominal
‘cladids’ are stemward of this split (see Ausich et al., 2015, fig. 5), the overwhelming majority of nominal cladids (sensu Moore and Laudon, 1943) are not. Thus, disparids are nested within a clade comprising the common ancestor of all nominal ‘cladids’ (sensu Moore and Laudon, 1943) and all of its descendants. Following Simms and Sevastopulo (1993), the Flexibilia and Articulata are placed within the Cladida, but no previous phylogenetic hypothesis has considered the Disparida a subclade within the Cladida. In an effort to retain as much of
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the original intent and traditional use of taxonomic names as possible, a redefinition of the Cladida is necessary to prevent the Disparida from being considered a subclade of cladids, particularly since the Cladida is already in need of extensive revision for other reasons. A simple solution to remedy the problem could be obtained using phylogeny-based clade definitions that recognize the Disparida and Cladida (sensu Simms and Sevastopulo, 1993) as sister clades. This would require orphaning only a small number of so-called cladids (sensu Moore and Laudon, 1943) as stem taxa to the Disparida + Cladida clade and retain the majority of cladids (sensu Moore and Laudon, 1943) within the more inclusive Cladida (sensu Simms and Sevastopulo, 1993). TheDisparida is considered herein to include the most recent common ancestor of Alphacrinus and
Eustenocrinus.Thus, Merocrinus and Metabolocrinus (typically considered cladids; Ausich, 1998; but see Guensburg, 2012) are tentatively placed within the disparid clade (Fig. 4), but further work and character-based analyses are needed to confirm the precise phylogenetic position of these problematic taxa. A more comprehensive discussion with rigorous phylogenetic definitions andarevisedclassification for cladid and disparid clades is provided in Wright et al. (2017). Previous analyses of Ordovician taxa have recovered
monophyletic groups, such as a clade comprised of cyathocrine cladids and hybocrinids (Guensburg, 2012; Ausich et al., 2015). This offered some hope that potentially the Cyathocrinida might be monophyletic (Ausich, 1998; Guensburg, 2012; Ausich et al., 2015). However, the present analysis rejects the monophyly of the Cyathocrinida because some nominal cyathocrines are more closely related to nominal dendrocrines than to other cyathocrines. If these results are taken seriously, then an extensive revision of higher taxa within the Cladida is needed. To further test this issue, I conducted an additional analysis placing a hard topological constraint on all cyathocrine cladids (see Bergsten et al., 2013). Comparing this analysis to the best fit model described above, Bayes factors strongly reject a model where cyathocrines are forced to be monophyletic (BF =9.64). Thus, caution should be exercised when extra- polating results from an analysis considering one timeslice to subsequent time intervals. Nevertheless, there is strong support for a clade of cyathocrine-grade cladids and hybocrinids (Ausich, 1998; Guensburg, 2012; Ausich et al., 2015). This clade is characterized by a number of morphologic features convergent with blastozoan echinoderms, such as thecal respiratory structures, calyx and/or arm plate reduction, and recumbent ambulacra. Given the strong statistical support and morphologic distinctness of this clade, I propose the name Porocrinoidea to represent this idiosyncratic group of crinoids. The Hybocrinida is considered herein a subclade within the Porocrinoidea (Fig. 4). The position of Cupulocrinus at the base of the flexible
clade has strong statistical support and corroborates earlier studies linking Cupulocrinus with flexible crinoids (Springer, 1911, 1920; Brower, 1995; Ausich, 1998) (Fig. 4). For example, Springer (1920) considered Cupulocrinus to have traits inter- mediate between cladids and flexibles and hypothesized a species of Cupulocrinus was ancestral to the Flexibilia. Because phylogenetic relationships were estimated using methods that include the possibility of potentially sampling ancestral
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