Early phylogeny of crinoids within the pelmatozoan clade

Abstract

Phylogenetic relationships among early crinoids are evaluated by maximizing parsimonious-informative characters that are unordered and unweighted. Primarily Tremadocian-Darriwilian (Early-Middle Ordovician) taxa are analysed. Stratigraphic congruence metrics support the best phylogenetic hypothesis derived using parsimony methods. This study confirms the traditionally recognized lineages of Palaeozoic crinoids and provides new information on the branching order of evolving lineages. Camerates are basal crinoids with progressively more tipward groups (from an Ordovician perspective) being protocrinoids, cladids (paraphyletic), hybocrinids and disparids. The Protocrinoida should be maintained, but the Aethocrinida should be placed within the Cladida. The results of this study identify phylogenetic structure amongst the major early crinoid lineages and delineate the relative positions of crinoid higher taxa along a tree. Each valid higher taxon discussed herein requires a comprehensive treatment to delimit within-lineage phylogenetic relationships.Key words: Echinodermata, Ordovician, phylogenetic methods, stratigraphic congruence.C R I N O I D S are regarded as the most primitive of the five living echinoderm classes (asteroids, crinoids, echinoids, holothurians and ophiuroids), but the phylogenetic position of crinoids amongst Palaeozoic echinoderms has been widely debated. The oldest known crinoids are from the early Tremadocian (Ordovician) (Guensburg and Sprinkle 2001, 2003, 2009 Guensburg 2010); and as part of the Great Ordovician Biodiversification Event (GOBE), they diversified to become the dominant stalked echinoderm clade by the early Late Ordovician (Sandbian). Crinoids became progressively more dominant amongst stalked echinoderms in most marine settings through the remainder of the Palaeozoic and were instrumental in establishment of the structure of Palaeozoic epifaunal suspension-feeding communities (Ausich and Bottjer 1982; Bottjer and Ausich 1987).However, a consensus has been difficult to reach concerning the phylogenetic position of crinoids amongst Ordovician clades and the early diversification of the Crinoidea. At the heart of this issue is a fundamental question of echinoderm phylogeny as a whole. Are stalked echinoderms a clade or a grade of echinoderm evolution? In this contribution, we adopt results from echinoderm universal elemental homology studies that have demonstrated that crinoids are nested within clades of other stalked echinoderms (Sumrall 2008(Sumrall , 2010 Sumrall and Waters 2012; Kammer et al. 2013; Sumrall 2014). With stalked echinoderms, including crinoids, recognized as a clade (the Pelmatozoa Leuckart, 1848), phylogenetic relationships of early crinoid evolution are examined.Various hypotheses for the origin and early evolution of crinoids have relied on the supposition that specific characters had special significance in identifying ancestordescendant relationships (e.g. ambulacral floor plates or lintels), that a priori interpretations of morphological tr...

Bayesian estimation of fossil phylogenies and the evolution of early to middle Paleozoic crinoids (Echinodermata)

“…Indeed, the Cladida (sensu Moore and Laudon, 1943) have long been considered a paraphyletic group because some nominal cladids are hypothesized to be more closely related to flexible and/or articulate crinoids than other cladids (Springer, 1920;Simms and Sevastopulo, 1993;Brower, 1995;Ausich, 1998;Wright, 2015). Unfortunately, recent phylogenetic analyses not only confirm that Ordovician cladids are a paraphyletic assemblage (Guensburg, 2012;Ausich et al, 2015), but also that the validity of the Cladida and their constituent higher taxa (i.e., Dendrocrinida and Cyathocrinida) cannot be fully remedied by simply adopting Simms and Sevastopulo's (1993) recommendation to place the Flexibilia and Articulata within the Cladida. In addition, because the monophyletic status of a taxon requires a temporal reference frame (conventionally taken as the present day), it is unknown whether some recovered 'clades' in Ordovician analyses retain their monophyletic status when 800 younger taxa are considered.…”

“…Thus, the Camerata is the sister clade to all non-camerate crinoids. Similarly, both Guensburg (2012) and Ausich et al (2015) recovered a monophyletic Hybocrinida as the sister clade to a subset of cladid taxa. Ordovician analyses also recovered a monophyletic Disparida as sister to the clade of cyathocrine cladids and hybocrinids (Guensburg, 2012;Ausich et al, 2015).…”

“…Similarly, both Guensburg (2012) and Ausich et al (2015) recovered a monophyletic Hybocrinida as the sister clade to a subset of cladid taxa. Ordovician analyses also recovered a monophyletic Disparida as sister to the clade of cyathocrine cladids and hybocrinids (Guensburg, 2012;Ausich et al, 2015). However, relationships among taxa placed currently within the Cladida, and their relationships to other higher taxa, have been a long-standing problem in crinoid systematics (McIntosh, 1986(McIntosh, , 2001Sevastopulo and Lane, 1988;Kammer andAusich, 1992, 1996;Simms and Sevastopulo, 1993).…”

“…Ever since Bather (1899) published his seminal work A Phylogenetic Classification of the Pelmatozoa, crinoid systematists have sought a robust evolutionary template for understanding the phylogenetic distribution of fossil and living species (Ausich and Kammer, 2001). Other than a few isolated studies conducted at low taxonomic levels (e.g., Kammer, 2001;Gahn and Kammer, 2002), most phylogenetic research using computational methods has focused on inferring relationships within two key time intervals: the Ordovician and the Recent (Ausich, 1998;Guensburg, 2012;Hemery et al, 2013;Rouse et al, 2013;Ausich et al, 2015;Summers et al, 2014;Cole, 2017). These intervals are significant because they bookend the evolutionary history of crinoids into their early diversification during the Ordovician Period and their present-day diversity in marine ecosystems.…”

“…Here, I examine phylogenetic relationships among early to middle Paleozoic crinoids (Echinodermata). Crinoids are particularly amenable for the purposes herein because: (1) they have a well-sampled fossil record (Foote and Raup, 1996); (2) their skeletal morphology is highly complex and character-rich (Ubaghs, 1978;Foote, 1994;Ausich et al, 2015); and (3) testing phylogenetic hypotheses among crinoid higher taxa requires sampling non-contemporaneous taxa over long timescales (>10 7 years), making them an ideal system for implementing a tip-dating approach (Ronquist et al, 2012). Because the approach taken herein is novel to the invertebrate fossil record, I provide a brief discussion on Bayesian tip-dating and the fossilized birth-death process tree prior to familiarize the reader with these emerging methods.…”