ABSTRACT: Anew taxonomical evaluation of the rostrum symmetry and its ontogenetic changes in advanced species of Siphonodella on the basis of collections from the Polar and Subpolar Urals, Chernyshev and Pechora ridges, Russia, is presented. Morphology of dextral and sinistral P1 elements of advanced Siphonodella was studied with SEM, optical microscopy, and X-ray microtomography. The investigations demonstrate that the rostrum, appearing at early stages of ontogeny, promises to be a useful trait for taxonomy and discrimination of the juvenile forms. The changes in the rostrum symmetry and ornamentation of P1 elements during the evolution of Siphonodella are considered as the major morphological events. The significant diversification of the genus in the Late duplicata-crenulata zones coincides with the appearance of the new rostrum structures. The nearly synchronous increase in complexity of the rostrum suggests that all the advanced Siphonodella species possessed a common ancestor. Three new species are described: Siphonodella ludmilae, S. lanei, and S. gladia.
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