A large and taxonomically diverse collection of Ordovician Asterozoa from France and Morocco enables new insights into the early evolution of the subphylum. Available specimens of both Asteroidea and Stenuroidea are comparatively few, the collection dominated by Ophiuroidea and to a lesser extent, Somasteroidea. Nearly all the asteroid specimens are assigned to Petraster Billings, 1858; P. caidramiensis n. sp. is differentiated based on the complexity of the extraxial skeleton. The genus Petraster is reviewed. Another asteroid fragment, Euaxosida sp. A, is too incomplete to assign below the ordinal level. Among species of Petraster, the extraxial skeleton suggests homoplastic emergence of ossicular series definition whereas evolution of ambulacral axial-adaxial series expressions was conservative. The stemward asteroid axial/adaxial configuration is considered largely a plesiomorphic derivative of the somasteroid condition. Ossicular details of all series, however, differ among Petraster species. Euaxosida sp. A also offers oral-surface expressions suggestive of a transitional status. Differences among ossicular series expressions exemplify concerns surrounding interpretation of early asterozoan phylogeny and taxonomy.
A large and taxonomically diverse collection of Ordovician Asterozoa from France and Morocco enables new insights into the early evolution of the subphylum. Available specimens of both Asteroidea and Stenuroidea are comparatively few, the collection dominated by Ophiuroidea and to a lesser extent, Somasteroidea. Nearly all the asteroid specimens are assigned to Petraster Billings, 1858; P. caidramiensis n. sp. is differentiated based on the complexity of the extraxial skeleton. The genus Petraster is reviewed. Another asteroid fragment, Euaxosida sp. A, is too incomplete to assign below the ordinal level. Among species of Petraster, the extraxial skeleton suggests homoplastic emergence of ossicular series definition whereas evolution of ambulacral axial-adaxial series expressions was conservative. The stemward asteroid axial/adaxial configuration is considered largely a plesiomorphic derivative of the somasteroid condition. Ossicular details of all series, however, differ among Petraster species. Euaxosida sp. A also offers oral-surface expressions suggestive of a transitional status. Differences among ossicular series expressions exemplify concerns surrounding interpretation of early asterozoan phylogeny and taxonomy.
RÉSUMÉ
Le genre ordovicien Petraster Billings, 1858 (Asteroidea: Echinodermation) et la différenciation du squelette chez les premiers astéroïdes. Les faunes particulièrement abondantes et diversifiées d'astérozoaires ordoviciens de France et du Maroc apportent des connaissances nouvelles sur les premiers stades de la diversification au sein de ce sous-phylum. Ces assemblages sont dominés par les Ophiuroidea et, dans une moindre mesure, les Somasteroidea, tandis qu'Asteroidea et Stenuroidea y sont très peu représentés. Parmi les astéroïdes, la quasi-totalité des échantillons sont attribués au genre Petraster Billings, 1858 ; ce genre est révisé et P. caidramiensis n. sp. est décrit. Un autre fragment d'astéroïde, Euaxosida sp. A, est trop incomplet pour être identifié en deçà du niveau ordinal. L'expression du squelette extraxial au sein des différentes espèces du genre Petraster permet de mettre en évidence plusieurs paliers évolutifs durant