First Specimens of the Cornutan Stylophoran Phyllocystis (Echinodermata) in the Ordovician (Volkhov Regional Stage, Dapingian and Darriwilian) of Baltica and Special Aspects of Stylophoran Axial Symmetry
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Abstract
Two new species of cornutan stylophora of the genus Phyllocystis are described from the Volkhov Regional Stage (Middle Ordovician, Dapingian) of Baltica. This indicates a biogeographic connection between the Afro-European part of Gondwana and Baltica since the very beginning of the Middle Ordovician. Both species were confined to cold-water, shallow basins with low water mobility. Analysis of the axial symmetry of stylophorans and the location of the hydropore indicates the absence of torsion in their ontogeny and the location of the ambulacral system to the right of their anteroposterior axis. To explain this morphology, two alternative hypotheses are proposed: (1) inversion in the development of right and left coeloms; (2) inverted state of stylophorans compared to other invertebrates, in which respect they resemble chordates in terms of the position of the dorsal and ventral sides.
期刊介绍：
Paleontological Journal (Paleontologicheskii zhurnal) is the principal Russian periodical in paleontology. The journal publishes original work on the anatomy, morphology, and taxonomy of fossil organisms, as well as their distribution, ecology, and origin. It also publishes studies on the evolution of organisms, ecosystems, and the biosphere and provides invaluable information on global biostratigraphy with an emphasis on Eastern Europe and Asia.
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