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Crinoid phylogeny: a preliminary analysis (Echinodermata: Crinoidea)

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Abstract

We describe the first molecular and morphological analysis of extant crinoid high-level inter-relationships. Nuclear and mitochondrial gene sequences and a cladistically coded matrix of 30 morphological characters are presented, and analysed by phylogenetic methods. The molecular data were compiled from concatenated nuclear-encoded 18S rDNA, internal transcribed spacer 1, 5.8S rDNA, and internal transcribed spacer 2, together with part of mitochondrial 16S rDNA, and comprised 3,593 sites, of which 313 were parsimony-informative. The molecular and morphological analyses include data from the bourgueticrinid Bathycrinus; the antedonid comatulids Dorometra and Florometra; the cyrtocrinids Cyathidium, Gymnocrinus, and Holopus; the isocrinids Endoxocrinus, and two species of Metacrinus; as well as from Guillecrinus and Caledonicrinus, whose ordinal relationships are uncertain, together with morphological data from Proisocrinus. Because the molecular data include indel-rich regions, special attention was given to alignment procedure, and it was found that relatively low, gene-specific, gap penalties gave alignments from which congruent phylogenetic information was obtained from both well-aligned, indel-poor and potentially misaligned, indel-rich regions. The different sequence data partitions also gave essentially congruent results. The overall direction of evolution in the gene trees remains uncertain: an asteroid outgroup places the root on the branch adjacent to the slowly evolving isocrinids (consistent with palaeontological order of first appearances), but maximum likelihood analysis with a molecular clock places it elsewhere. Despite lineage-specific rate differences, the clock model was not excluded by a likelihood ratio test. Morphological analyses were unrooted. All analyses identified three clades, two of them generally well-supported. One well-supported clade (BCG) unites Bathycrinus and Guillecrinus with the representative (chimaeric) comatulid in a derived position, suggesting that comatulids originated from a sessile, stalked ancestor. In this connection it is noted that because the comatulid centrodorsal ossicle originates ontogenetically from the column, it is not strictly correct to describe comatulids as “unstalked” crinoids. A second, uniformly well-supported clade contains members of the Isocrinida, while the third clade contains Gymnocrinus, a well-established member of the Cyrtocrinida, together with the problematic taxon Caledonicrinus, currently classified as a bourgueticrinid. Another cyrtocrinid, Holopus, joins this clade with only weak molecular, but strong morphological support. In one morphological analysis Proisocrinus is weakly attached to the isocrinid clade. Only an unusual, divergent 18S rDNA sequence was obtained from the morphologically strange cyrtocrinid Cyathidium. Although not analysed in detail, features of this sequence suggested that it may be a PCR artefact, so that the apparently basal position of this taxon requires confirmation. If not an artefact, Cyathidium either diverged from the crinoid stem much earlier than has been recognised hitherto (i.e., it may be a Palaeozoic relic), or it has an atypically high rate of molecular evolution.

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Acknowledgements

We are grateful to J.M. Bohn, University of Munich, for the specimen of Bathycrinus and to the master and crew of R.V. “Alis” (IRD, Nouméa) for dredge and trawl samples. We are also indebted to J. Wilgenbusch (University of South Florida) for access to version 4.11 of PAUP* and to M. Robinson (University of Lyon) for a version of RRTree. A. Scouras (Simon Frazer University, Burnaby, BC, Canada) kindly provided DNA of Florometra together with 16S primer sequences and aliquots, and access to unpublished 16S sequences (e.g., of Gymnocrinus). Valuable advice, and criticism of an earlier version of this paper, were provided by D. Meyer (University of Cincinnati); M.J. Simms (National Museum of Wales), M.-C. Boisselier and F. Pleijel (Museum Nationale d’Histoire Naturelle, Paris), and S. Samadi (IRD, Nouméa). Three anonymous referees provided carefully detailed reviews of the submitted manuscript. B.L.C received travel grants from the Carnegie Trust for the Universities of Scotland and the John Robertson Bequest to the Senate of the University of Glasgow.

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Authors and Affiliations

  1. IBLS Division of Molecular Genetics, University of Glasgow, Pontecorvo Building, 56 Dumbarton Rd., Glasgow, G11 6NU, UK

    Bernard L. Cohen

  2. Département des Milieux et Peuplements Aquatiques, Museum national d’Histoire naturelle, UMR 5178 CNRS BOME “Biologie des Organismes Marins et Ecologie”, 55 rue Buffon, 75005, Paris, France

    Nadia Améziane & Marc Eleaume

  3. Institut de Recherche et Développement, BP A5, 98848, Nouméa, New Caledonia

    Bertrand Richer de Forges

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  1. Bernard L. Cohen
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Correspondence to Bernard L. Cohen.

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Communicated by J.P. Thorpe, Port Erin

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Appendix

Appendix

Brief descriptions and status of morphological characters and character-states, with homoplasy levels indicated by successive approximation weights

  1. 1.

    Skeletal structures that support the calyx: none (0); series of columnals (1); centrodorsal (2). State 0 describes the complete absence of a column or stalk. Informative, weight = 1.00.

  2. 2.

    Symplexies in series of columnals: absent (0); present (1). Symplexies are a particular form of articulation characteristically absent or present in different taxa. Informative, weight = 1.00.

  3. 3.

    Synostoses in series of columnals: absent (0); present (1). Synostoses are a particular form of articulation characteristically absent or present in different taxa. Informative, weight = 1.00.

  4. 4

    Syzygies in series of columnals: absent (0); present (1). Syzygies are a particular form of articulation characteristically absent or present in different taxa. Uninformative.

  5. 5.

    Synarthries in series of columnals: absent (0); present (1). Synarthries are a particular form of articulation characteristically absent or present in different taxa. Uninformative. (For details concerning characters 2–5 see Macurda and Meyer 1975).

  6. 6.

    Other joints in series of columnals: absent (0); present (1). Describes the existence of articulation types other than those described by 2–5, characteristically absent or present in different taxa. The codings for 2–6 imply the hypothesis that each articulation type evolves independently. Uninformative.

  7. 7.

    Constitution of elements in series of columnals: homeomorphic (0), heteromorphic (1), xenomorphic (2); homeomorphic and heteromorphic (3). Describes the nature of differentiation of columnar ossicles in the column; all similar, dissimilar, or divided into two parts, one all-similar, the other not. Uninformative.

  8. 8.

    Distalmost element in series of columnals: columnal (0); holdfast disc (1); rhizoids (2). Describes alternative structures, some of which serve for fixation. Coding as a multistate character may not be optimal because there is no evidence, and it is not assumed that each state can be derived from every other state, but coding as three separate a/p characters is also sub-optimal and may be less parsimonious. Informative, weight = 1.00.

  9. 9.

    Cirri: absent (0); present (1). An unambiguous absence/presence (a/p) character in which it is assumed that all cirri are homologues even when independently evolved in different lineages. This homology is based on the assumption that cirrus morphogenesis involves a single developmental program. Informative, weight = 0.33.

  10. 10.

    Location of cirri: whole stalk (0); proximal stalk only (1); centrodorsal (2). Coding comment as for character 8. Uninformative.

  11. 11.

    Arm length: uniform (0); variable (1). In most crinoids all arms are the same length. Informative, weight = 1.00.

  12. 12.

    Crown retraction: absent (0); present (1). In most crinoids the arms cannot fold inwards (retract) like a clenched fist. Informative, weight = 1.00.

  13. 13.

    Arms when retracted: partly closed (0); fully closed (1). Describes a clear-cut difference in the extent of retraction. Uninformative.

  14. 14.

    Maximum number of arm divisions: none (0); one (1); two (2). Each arm may or may not divide after leaving the calyx. When there is no division the crinoid has 5 arms; when one division, 10, etc. Coding comment similar to character 8. Informative, weight = 0.44.

  15. 15.

    Position of first axillary: on arm ossicle two (0); on arm ossicle seven (1). Describes the position of the first arm division, where present. Uninformative.

  16. 16.

    Infrabasal: evident (0); concealed (1). Describes alternative configurations of the infrabasal ossicles of the calyx. Uninformative.

  17. 17.

    Basal ossicle: absent (0); present (1). A clear-cut a/p character describing the circlet of calyx ossicles immediately below those that carry the arms. Informative, weight = 1.00.

  18. 18.

    Basal position: evident (0); concealed (1). Describes alternative configurations of the basal ossicles of the calyx, where present. Uninformative.

  19. 19.

    Basal fusion: not fused (0); fused (1). Describes alternative conditions of the basal ossicles of the calyx, where present. Uninformative.

  20. 20.

    Br1–2 articulation: synarthry (0); transverse synarthry (1); synostoses (2); fused (3). Describes the type of articulation present between the first two brachial ossicles. Coding comment similar as character 8. Informative, weight = 0.20.

  21. 21.

    Arm syzygies: absent (0); present (1). Describes the presence of syzygies between any pair of arm ossicles. Informative, weight = 1.00.

  22. 22.

    First syzygy: I Br 1+2 (0); II Br 3+4 (1). Describes the location of the first syzygy (where present) along the arm, in relation to the pattern of arm division. Uninformative.

  23. 23.

    Arm synostoses: (absent); present (1). Describes the presence of synostoses between arm ossicles. Informative, weight = 0.11.

  24. 24.

    Pinnule state: undifferentiated (0); differentiated (1). Describes whether arm pinnules are of morphologically distinguishable types or not. Informative, weight = 1.00.

  25. 25.

    Genital pinnules: absent (0); present (1). Describes whether gonads are confined to morphologically distinguishable pinnules or may occur on any pinnule. Informative, weight = 1.00.

  26. 26.

    Pinnule coverplates: absent (0); present (1). Coverplates over the ambulacral groove are characteristically absent or present on pinnules of different taxa. Informative, weight = 0.25.

  27. 27.

    Tegmen plating on the oral surface of the calyx: scattered (0); tessellated (1). Informative, weight = 0.16.

  28. 28.

    Oral plates: absent (0); present (1). Describes the presence on the surface of the tegmen of a complete or incomplete circlet of circum-oral plates. Informative, weight = 1.00.

  29. 29.

    Oral plate pores absent (0); present (1). Describes the presence or absence of pores in oral plates. Uninformative.

  30. 30.

    Number of oral plate pores: <15 (0); >15 (1). Reflects a bimodal distribution of oral plate pore number. Uninformative.

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Cohen, B.L., Améziane, N., Eleaume, M. et al. Crinoid phylogeny: a preliminary analysis (Echinodermata: Crinoidea). Marine Biology 144, 605–617 (2004). https://doi.org/10.1007/s00227-003-1212-7

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