The early growth stage of a Devonian ophiuroid and its bearing on echinoderm phylogeny

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Abstract
A central point in arguments for a crinoid ancestry of asteroids and ophiuroids is the similarity and presumed homology of the fully developed apical systems of asteroids and ophiuroids with the calyx of crinoids. A major obstacle to this view has been the lack of supporting evidence from Paleozoic ophiuroids. Hamling''s ophiuroid, Prostasteridae juvenile species, is the early growth stage of an Upper Devonian oegophiurid from North Devon, England [UK]. The largest plates on the aboral disc form a primary rosette comparable to that seen in the ontogeny of the living ophiuroid orders Ophiurida and Phrynophiurida. It is now no longer possible to consider the ophiuroid primary rosette a late evolutionary development without phylogenetic significance. The concept of a crinoid ancestry for the Asterozoa, previously rejected for lack of this paleontological datum, is reinstated as a useful working hypothesis.

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