A Re‐Interpretation of the Ambulacral System of Eumorphocystis (Blastozoa, Echinodermata) and its Bearing on the Evolution of Early Crinoids

Sarah L. Sheffield, Colin D. Sumrall

Research output: Contribution to journalArticlepeer-review

Abstract

<p> Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. <em> Eumorphocystis </em> , a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well&hyphen;preserved specimens of <em> Eumorphocystis </em> from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that <em> Eumorphocystis </em> and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of <em> Eumorphocystis </em> suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.</p>
Original languageAmerican English
JournalPalaeontology
Volume62
DOIs
StatePublished - Jan 1 2019

Keywords

  • Blastozoa
  • crinoid
  • Eumorphocystis
  • echinoderm
  • homology
  • evolution

Disciplines

  • Earth Sciences

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