Streptoiocrinus gen. nov. (Disparida, Crinoidea) from the Lower and Middle Ordovician of the Leningrad Region, and Fluct
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treptoiocrinus gen. nov. (Disparida, Crinoidea) from the Lower and Middle Ordovician of the Leningrad Region, and Fluctuating Asymmetry of Radial Symmetry S. V. Rozhnov* Borissiak Paleontological Institute, Russian Academy of Sciences, Moscow, 119647 Russia *e-mail: [email protected] Received September 25, 2019; revised October 7, 2019; accepted October 16, 2019
Abstract—A crown of Streptoiocrinus gen. nov., with its articulated stem, the columnals of which have long been known in the Billingen and Volkhov regional stages (Floian and Dapingian stages) of the Leningrad Region and Estonia are described. A detailed study of the crown and stem morphology showed that the new genus belongs to the disparid crinoid superfamily Ioсrinacea. This genus is the first mass crinoid invader into the Ordovician Baltic paleobasin. The fluctuating asymmetry in the manifestation of the pentaradial symmetry of the stem columnals is described.
Keywords: Echinoderms, crinoids, disparids, crown, stem, morphology, symmetry, fluctuating asymmetry, Ordovician, Floian, Dapingian, Baltic DOI: 10.1134/S0031030120070114
INTRODUCTION
MATERIAL
The Billingen and Lower Volkhov (Dikari Limestone) deposits of the Lower-Middle Ordovician (Floian and Dapingian stages) of the Baltic Region frequently contain characteristic lenticular crinoid columnals (Fig. 1). One of the beds of the Dikari Limestone is traditionally referred to by workers on the quarries of the Leningrad Region as “glazastick” (birds-eye limestone) because of the abundant white columnals scattered in the limestone. These columnals were one of the first fossils described from the Ordovician of the Baltic Region (Eichwald, 1860). Nevertheless, until recently, their unusual morphology and the absence of crowns and stems did not allow them to be taxonomically assigned. Recently two crowns with stem fragments composed of such columnals have been found. One of these was found by the present author and another was donated to the Paleontological Institute by P.V. Fedorov, a well-known scholar studying the Ordovician of the Leningrad Region. This material, as well as numerous columnals and several stem fragments, were discussed in this paper and were the basis for a new genus of disparid inadunates. The pentaradial symmetry of the columnals shows distortions similar to the manifestations of fluctuating asymmetries of the body of bilateral animals.
The collection is housed in the Borissiak Paleontological Institute, Russian Academy of Sciences, coll. no. PIN 4125.
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Fig. 1. Aggregation of columnals of Streptoiocrinus monile; specimen, PIN no. 4125/953. Quarry near the village of Putilovo. Sample from spoil heaps, Billingen Regional Stage or lower substage of the Volkhov Regional Stage. Scale bar length 5 mm.
STREPTOIOCRINUS GEN. NOV. (DISPARIDA, CRINOIDEA)
SYSTEMATIC PALEONTOLOGY Order Disparida Moore and Laudon, 1943 Family Iocrinidae Moore and Laudon, 1943 Genus Streptoiocrinus Rozhnov gen. nov. Haplocrinus (pars): Eichwald, 1860. Pentagonocyclicus (pars): Yeltyshewa, 1964.
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