Wright et al.—Phylogenetic classification of the Crinoidea
we propose herein to define the Articulata as the crinoid crown group containing the last common ancestor of the extant isocrinid Endoxocrinus parrae and the comatulid Antedon bifida, and all of its descendants. As discussed by Ruta et al. (2003), the concepts of stem
groups and crown groups are sometimes misinterpreted or misused in the paleontological literature. Used properly, crown groups are defined by extant taxon specifiers. Notably, crown groups may be comprised of many (or mostly) extinct fossil species. For example, if a fossil crinoid is more closely related to some extant species than others, it is a member of the crown group. According to Rouse et al. (2013), the most recent common ancestor of all extant crinoids lived sometime during the Middle to Upper Triassic. Thus, our node-based definition eliminates the non-phylogenetic concept of ‘post-Paleozoic Crinoidea’ while retaining the majority of post-Paleozoic crinoids traditionally included within the Articulata. The Clade Articulata is synonymous with the Crown Crinoidea (Sumrall, 2014), and we advocate workers use these terms interchange- ably depending on context (e.g., discussing relationships among crinoids or between crinoids and non-crinoids). Traits that may be present in the Articulate ancestor are listed in Simms (1988), Simms and Sevastopulo (1993), Webster and Jell (1999), and Rouse et al. (2013). The Articulata likely contains most post-Paleozoic taxa
traditionally considered articulates, including the ~600 or so extant species. Althoughwe define Articulata with precision and phylogenetic stability (Rouse et al., 2013, 2015), it remains difficult in practice to unambiguously identify fossil articulates, particularly among specimens near the base of the articulate tree. However, such difficulties are already present and have long obfuscated the origin of the crinoid crown group. The more important problem is resolving the phylogenetic position of the common ancestor of extant crinoids within the myriad of fossil lineages. Our definition provides a useful framework for future phylogenetic research to uncover relationships between poten- tial stem articulates, extinct crown group lineages, and extant species.
A revised rank-based classification of the Crinoidea
Crinoid clades identified herein confirm many long-held views on the major divisions among crinoids from both the founda- tional work of Moore and Laudon (1943) and Moore and Teichert (1978) to more recent analyses (i.e., Ausich, 1998a, 1998b; Guensburg and Sprinkle, 2003; Guensburg, 2012; Ausich et al., 2015). Results from all of these studies recognized the Camerata, Diplobathrida, Monobathrida, Hybocrinida, Disparida, Cladida, and Flexibilia. The challenge is to represent these widely recognized clades in a rank-based Linnaean clas- sification scheme that maximizes common usages of names for crinoid lineages (Moore and Teichert, 1978) and is consistent with a phylogenetic understanding of relationships (Wiley and Lieberman, 2011). In our revision, the Crinoidea remain a class and every attempt is made to retain orders as recognized in Moore and Teichert (1978). Unfortunately, the tree topology of Figure 2 prevented the attainment of the latter in all instances, but the addition of intermediate Linnaean ranks makes it easier to apply a phylogenetic perspective to rank-based crinoid
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classification. The use of intermediate ranks (e.g., Parvclass) follows traditional use in pre-existing taxonomic literature (see Carroll, 1988; Sibley, 1994; Benton, 2005). Two older taxonomic names, the Pentacrinoidea Jaekel, 1918 and Inad- unata Wachsmuth and Springer, 1885, are formally reinstated herein because they represent meaningful clades as described above.
Post-Ordovician cladids (sensu Moore and Laudon, 1943)
and the Protocrinoida (Guensburg and Sprinkle, 2003) remain problematic groups. Because the rank for a monophyletic Cladida must be above flexibles and articulates (Simms and Sevastopulo, 1993), we propose the name Cyathoformes to contain taxa traditionally placed within the Cladida that are sister to the Articulata. Relationships among these taxa are the subject of future phylogenetic research (Wright, 2015b) and are not treated further here. From their initial description (Guensburg and Sprinkle, 2003), the protocrinoids have been an important but confounding group of crinoids that display char- acteristics of both crinoids and other stalked echinoderms. Guensburg and Sprinkle (2003) regarded the protocrinoid as an “order (plesion)”. The validity of the protocrinoids was later questioned by Guensburg and Sprinkle (2009) and led Guensburg (2012) to formally place them within the Camerata. However, Ausich et al. (2015) recovered a sister group relationship between Titanocrinus and Glenocrinus, but with protocrinoids more closely related to non-camerates than cam- erates. In contrast, Cole’s (2017) analysis of Ordovician crinoids recovered the protocrinoids as more closely related to camerates than non-camerates. Thus, we have carefully chosen our clade definitions to not depend on a particular phylogenetic hypoth- esis or morphologic interpretation of these significant but problematic taxa. For the moment, we tentatively place both protocrinoid taxa as Crinoidea incertae sedis subclass Protocrinoida. In our present understanding of crinoid evolution, the first
major divergence occurs between camerates and all other cri- noids (Fig. 2). The subclass rank is retained for the Camerata; and the subclass Pentacrinoidea Jaekel, 1918 is proposed for its sister group (Table 2). Within the Camerata, the orders Diplobathrida and Monobathrida are retained as sister groups, and the infraclass Eucamerata Cole (2017) unites these two orders. In phylogenetic analyses of camerates, several taxa are not placed within the Monobathrida and Diplobathrida (sensu Cole, 2017). Thus, they are considered here to be stem euca- merates (see remarks for Eucamerata above). The subclass Camerata unites these stem taxa with eucamerates. In terms of species richness, the subclass Pentacrinoidea is
the largest crinoid clade. This includes the Disparida, Cladida, Hybocrinida, and Articulata of Moore and Teichert (1978), which coincides exactly with Jaekel’s (1918) concept of the Pentacrinoidea (see Lane, 1978). Hence, we have proposed the reinstatement of this name. The Pentacrinoidea is comprised of the infraclass Inadunata and their stem taxa (e.g., Apektocrinus). The concept for the Inadunata in Moore and Teichert (1978) united the Disparida and the Cladida. Here, the infraclass Inadunata unites the Disparida, Cladida, and all of their descendants. This usage circumvents the non-phylogenetic usage of the Inadunata (sensu Moore and Teichert, 1978) and is consistent with the phylogenetic conclusions of Simms and
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