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

Abstract

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.

Phylogenetic taxonomy and classification of the Crinoidea (Echinodermata)

“…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;Cole, 2017). The Ordovician 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) Sprinkle, 2003, 2009;Guensburg, 2010) and the majority of well-studied groups had originated prior to its close.…”

“…Multiple studies indicate strong support for camerate monophyly (Ausich, 1998b;Cole, 2017). However, Cole's (2017) analysis of Ordovician camerates did not find support for a strict division between monocyclic and dicyclic forms.…”

“…Following revision, the Monobathrida and Diplobathrida are sister clades that together comprise the more inclusive Eucamerata (Cole, 2017). Thus, the stem-based definition of the Camerata contains the Clade Eucamerata and their stem taxa, including representatives of the oldest known crinoid fossils (e.g., Eknomocrinus, Cnemecrinus), and genera placed within the problematic Reteocrinitidae (see Cole, 2017), and may or may not contain the protocrinoids (see Guensburg and Sprinkle, 2003;Guensburg, 2012;Cole, 2017 Remarks.-Cole (2017) revised the Monobathrida and Diplobathrida to represent monophyletic groups while attempting to preserve the greatest number of taxa traditionally included within each (Moore and Teichert, 1978). The name 'Eucamerata' was proposed to identify the clade of camerates comprised of the sister groups Monobathrida and Diplobathrida, which necessarily excludes stem taxa such as Cnemecrinus and Reteocrinus (Cole, 2017).…”

“…Cole's (2017) phylogenetic analysis of Ordovician camerate crinoids indicates a strict adherence to Moore and Laudon's (1943) concept of the Monobathrida is not monophyletic. However, removal of the stemward camerates Eknomocrinus and Adelphicrinus renders the Monobathrida a clade (Cole, 2017). The internal and external specifiers defining this stem-based clade ensure the taxonomic content closely matches Moore and Laudon (1943).…”

“…Revised rank-based classification of the Crinoidea.ClassCrinoidea Miller, 1821 †Subclass Camerata Wachsmuth and Springer, 1885 'Stem eucamerates' (e.g., Eknomocrinus) Infraclass EucamerataCole, 2017 Order Diplobathrida…”