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‐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 language | American English |
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Journal | Palaeontology |
Volume | 62 |
DOIs | |
State | Published - Jan 1 2019 |
Keywords
- Blastozoa
- crinoid
- Eumorphocystis
- echinoderm
- homology
- evolution
Disciplines
- Earth Sciences