Journal of Paleontology, 91(4), 2017, p. 815–828 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.137


Phylogeny and morphologic evolution of the Ordovician Camerata (Class Crinoidea, Phylum Echinodermata)


Selina R. Cole


School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, USA ⟨cole.678@osu.edu⟩


Abstract.—The subclass Camerata (Crinoidea, Echinodermata) is a major group of Paleozoic crinoids that represents an early divergence in the evolutionary history and morphologic diversification of class Crinoidea, yet phylogenetic relationships among early camerates remain unresolved. This study conducted a series of quantitative phylogenetic analyses using parsimony methods to infer relationships of all well-preserved Ordovician camerate genera (52 taxa), establish the branching sequence of early camerates, and test the monophyly of traditionally recognized higher taxa, including orders Monobathrida and Diplobathrida. The first phylogenetic analysis identified a suitable outroup for rooting the Ordovician camerate tree and assessed affinities of the atypical dicyclic family Reteocrinidae. The second analysis inferred the phylogeny of all well-preserved Ordovician camerate genera. Inferred phylogenies confirm: (1) the Tremadocian genera Cnemecrinus and Eknomocrinus are sister to the Camerata; (2) as historically defined, orders Monobathrida and Diplobathrida do not represent monophyletic groups; (3) with minimal revision, Monobathrida and Diplobathrida can be re-diagnosed to represent monophyletic clades; (4) family Reteocrinidae is more closely related to camerates than to other crinoid groups currently recognized at the subclass level; and (5) several genera in subclass Camerata represent stem taxa that cannot be classified as either true monobathrids or true diplobathrids. The clade containing Monobathrida and Diplobathrida, as recognized herein, is termed Eucamerata to distinguish its constituent taxa from more basally positioned taxa, termed stem eucamerates. The results of this study provide a phylogenetic framework for revising camerate classification, elucidating patterns of morphologic evolution, and informing outgroup selection for future phylogenetic analyses of post-Ordovician camerates.


Introduction


Subclass Camerata Wachsmuth and Springer, 1885 is a major clade of Paleozoic crinoids comprising nearly 350 genera, including some of the earliest known crinoid taxa (Guensburg and Sprinkle, 2003). Camerates persisted from the Early Ordovician (Tremadocian) to the late Permian (Lopingian) and were ecologically significant constituents of lower and middle Paleozoic crinoid evolutionary faunas (CEF) (Ausich and Deline, 2012). Recent quantitative phylogenetic analysis of Early–Middle Ordovician crinoids indicates camerates form a monophyletic group and were the earliest clade to diverge within the Crinoidea (Ausich et al., 2015), making the Camerata the sister group to all other crinoids. Because of their ecological significance, taxonomic diversity, and early divergence from other crinoid clades, understanding the early evolutionary rela- tionships among camerate lineages is important for interpreting patterns of morphologic evolution at the base of the crinoid tree of life during initial diversification of the class. Phylogenetic relationships within the Camerata remain obscure, however, and the monophyly of higher camerate taxa remains untested. Camerates are united by the presence of rigidly ankylosed


thecal and tegminal plates, fixed brachials and interbrachials incorporated into the calyx, subtegminal mouth, and typically additional plates in the posterior interray (Ubaghs, 1978).


The subclass has traditionally been divided into order Diplobathrida, which is characterized by two circlets of plates (basals and infrabasals) below the radial circlet, and order Monobathrida, which possesses only one circlet of plates (basals) below the radial circlet (Moore and Laudon, 1943a). Although the known stratigraphic range of both orders begins in the Lower Ordovician (Tremadocian), diplobathrids went extinct at the end Mississippian (Serpukhovian) whereas monobathrids persisted until the end Permian (Lopingian). Diplobathrids are the less taxonomically diverse of the two orders, comprising roughly a third of the total generic diversity of camerates. During the Ordovician, however, diplobathrids exceeded monobathrids as constituents of the early Paleozoic CEF and in terms of taxonomic diversity, with nearly twice as many genera as the Monobathrida (Ausich and Deline, 2012). Following the end-Ordovician extinction, monobathrids replaced diplobathrids as the dominant camerate constituents of the middle Paleozoic CEF (Eckert, 1988; Ausich et al., 1994). Although some general patterns of camerate evolutionary


history have been established, the absence of a phylogeny for the Camerata has restricted investigation of both systematic and macroevolutionary questions within the clade. Many recent studies have highlighted that phylogeny provides the valuable context of shared evolutionary history (Carlson, 2001; Kelley et al., 2013). To this end, a phylogenetic analysis was conducted


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