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Journal of Paleontology

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Stem Structure and Evolution in the Earliest Pelmatozoan Echinoderms

¹ Laboratoire de Paléontologie et Paléogéographie du Paléozoïque, Université des Sciences et Technologies de Lille, F-59655 Villeneuve d'Ascq Cedex, France, <Sebastien.Clausen{at}univ-lille1.fr>
² Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK, <a.smith{at}nhm.ac.uk>

Echinoderm skeletal debris from the Early–Middle Cambrian boundary Micmacca Breccia of Morocco includes the oldest known holomeric columnals. The original calcite of these ossicles is coated and replaced by iron oxides, occasionally overlain by a late coating of silica, and preserves with high fidelity fine details of their three-dimensional microstructure. Irrespective of their external morphology, columnals can be divided into three groups based on the distribution of stereom microfabrics, which we suggest indicates the presence of at least three different holomeric stemmed taxa. One of these columnal types has well-developed galleried stereom perpendicular to its articulation facets, a sure sign that long, penetrative collagen bundles bound columnals together, as in modern stemmed crinoids. This columnal morphology also shows a primitive type of interlocking articulation, which we term parasymplexy and which may have helped to counter torsional stresses. The two other columnals either lacked fibrous connective tissue or had shallow, non-penetrative fibers between columnals.