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

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Abstract

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.

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About the authors

S. V. Rozhnov

Borissiak Paleontological Institute, Russian Academy of Sciences

Author for correspondence.
Email: rozhnov@paleo.ru
Russian Federation, Moscow, 117647

G. A. Anekeeva

Borissiak Paleontological Institute, Russian Academy of Sciences

Email: anekeeva@paleo.ru
Russian Federation, Moscow, 117647

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2. Fig. 1. Map of the Leningrad Region to the east of St. Petersburg. The dotted line shows the Baltic-Ladoga Klint. Find sites are marked with stars.

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3. Fig. 2. Stratigraphic scheme of the Lower and Middle Ordovician of the Leningrad Region (after: Dronov, 1998; Dronov et al., 2003, with modifications). Find levels are marked with arrows. Designations: a - pelitomorphic limestone, including highly clayey; b - clay; c - quartz sand and sandstone; d - glauconitic sand; d - bioclastic limestone; e - limestones and clays with ferruginous oolites; g - intensely bioturbated rocks with passages filled with material contrasting with the surrounding rock; h - surfaces of the hard bottom with Trypanites drillings; i – surfaces of the hard bottom with “pencil”-type drillings; k – irregular surfaces of the hard bottom with ferruginous impregnation; l – surface with amphora-shaped burrows (“glass”); m – Bergaueria tubercles on the lower surface of the beds.

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4. Fig. 3. Phyllocystis baltica sp. nov., holotype PIN, No. 4125/1034: a – from the ventral side, b – from the dorsal side; c, d – drawing of plates: c – from the ventral side, d – from the dorsal side; Leningrad Region, Putilov Quarry; Middle Ordovician, lower Dapingian Stage, lower Volkhovian Horizon (wild).

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5. Tabl. V

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6. Tabl. VI

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7. Fig. 4. Phyllocystis cellularis sp. nov.: a – holotype PIN, No. 4125/1028: a1 – general view from the ventral side, a2 – marginal plate M′2, view from the external side; b – specimen PIN, No. 4125/1029, isolated marginal plates, most likely belonging to a single specimen; characteristic second marginalia M2 and M′2 can be distinguished (among them, remains of other echinoderms are also observed – cylindrical segments and a specimen of an eocrinoid of the genus Bolboporites); c – specimen PIN, No. 4125/1024, separate marginal plate M′3: c1 - from the dorsal side, c2 - from the side of the proximal articular facet, c3 - from the side of the distal articular facet, c4 - from the ventral side (hereinafter, the proximal articular facet is taken to be the facet located at the end of the plate directed toward the aulacophore, and the distal one - at the end directed in the opposite direction); g - specimen PIN, No. 4125/988, separate marginal plate M5: g1 - from the dorsal side, g2 - from the side of the proximal facet, g3 - from the side of the distal facet, g4 - from the ventral side; Leningrad Region, Putilov quarry (a, b) and the left bank of the Lynna River near its mouth (c, d); Middle Ordovician, Darriwilian Stage, upper part of the Volkhov Horizon (friezes). Scale bar length is 1 mm.

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8. Fig. 5. Drawing of the holotype of Phyllocystis cellularis sp. nov. (a) and reconstruction of the position of its marginal plates (b).

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9. Fig. 6. Phyllocystis cellularis sp. nov., photographs of stylocones and microstructure: a – specimen PIN, No. 4125/1025d: a1 – view from the dorsal side, a2 – view from the proximal side, a3 – view from the distal side, a4 – view from the ventral side, a5 – microstructure of the proximal side, a6 – microstructure of the distal side; b – specimen PIN, No. 4125/1047: b1 – dorsal view, b2 – proximal view, b3 – distal view, b4 – ventral view; c, d – specimen PIN, No. 4125/995, microstructure of a cell on the outer surface of plate M′3 (c) and microstructure of the inner surface of the same plate M′3 (d); d, e – specimen PIN, No. 4125/988, microstructure of the inner surface of plate M5, at the origin of the zygal process; Leningrad Region, Putilov quarry (a) and the left bank of the Lynna River near its mouth (b–e); Middle Ordovician, Darriwilian Stage, upper part of the Volkhov Horizon (friezes).

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10. Fig. 7. Phyllocystis cellularis sp. nov., scattered stylocones and plates from the locality at the mouth of the Lynna River: a – stylocone, specimen PIN, No. 4125/1040; b – the smallest of the stylocones in the collection, specimen PIN, No. 4125/1048: b1 – dorsal view, b2 – proximal view, b3 – distal view, b4 – ventral view; c–e – second marginal plates: c – specimen PIN, No. 4125/1042, d – specimen PIN, No. 4125/1041, d – specimen PIN, No. 4125/1043, e – specimen PIN, No. 4125/1044; f, h – marginal plates of unknown position: f – specimen PIN, No. 4125/1045, h – specimen PIN, No. 4125/1047. Scale bar length – 1 mm.

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11. Fig. 8. Reconstruction of the stylophora of the genus Phyllocystis with the designation of the anteroposterior axis and the position of the right adoral opening (after: Rozhnov, 2018, with changes).

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