Cantabrigiaster fezouataensis: A new Somasteroid Echinoderm from the Early Ordovician Fezouata Lagerstätte in Morocco.
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Asterozoans, whose most familiar members include Starfish and Brittle Stars, are the dominant group of extant Echinoderms based on their diversity, abundance and biogeographic distribution. Despite their ecological success and a fossil record spanning more than 480 million years, the origin and early evolution of Asterozoans, and those of crown-group Echinoderms more generally, remain uncertain given the difficulty of comparing the organisation of the calcified endoskeleton in diverse Lower Palaeozoic groups, such as the Edrioasteroids and Blastozoans. The extraxial–axial theory, which supports the homology of the biserial ambulacral ossicles of pentaradial and non-pentaradial Echinoderms based on embryonic and ontogenetic data, has been proposed as a developmentally informed model that facilitates comparisons among groups with disparate morphologies. Although the extraxial–axial theory can potentially clarify the early evolution of crown-group Echinodermata, the broad implications of this hypothesis have never been examined under a comprehensive quantitative phylogenetic framework. Consequently, the main phylogenetic predictions of the extraxial–axial theory, pertaining to the evolutionary relationships of Cambrian and Ordovician Echinoderms, such as the origin of the crown group from Edrioasteroid-like ancestors, although analysed with other homology schemes, have yet to be critically tested using the extraxial–axial theory.