First Specimens of the Cornutan Stylophora Phyllocystis (Echinodermata) in the Ordovician (Volkhov Regional Stage, Dapingian and Darrivilian) of Baltica and Special Aspects of Stylophora Axial Symmetry

Described two new species of cornutan stylophora of the genus Phyllocystis from the Volkhov Regional Stage (Middle Ordovician, Dapingian) of Baltica. This points to the biogeographic connection between Afro-European part of Gondwana and Baltica since the very beginning of the Middle Ordovician. Both species were confined to cold-water, shallow conditions with low water mobility. Analysis of the axial symmetry of the stylophora 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 structure, two alternative hypotheses have been proposed: 1) inversion inversion in the development of right and left coelomes; 2) inverted state of stylophora compared to other invertebrates, what makes them similar to the chordates in terms of the position of the dorsal and ventral sides.