Unbranched Holdfasts of Stemmed Echinoderms from the Ordovician of the Leningrad Region
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nbranched Holdfasts of Stemmed Echinoderms from the Ordovician of the Leningrad Region G. A. Anekeevaa, * and S. V. Rozhnova, ** a
Borrisiak Paleontological Institute, Russian Academy of Sciences, Moscow, 117997 Russia *e-mail: [email protected] **e-mail: [email protected] Received September 1, 2019; revised September 17, 2019; accepted September 17, 2019
Abstract—Based on examination of the collection of stemmed echinoderm holdfasts from the Ordovician of the Leningrad Region (Russia), all holdfasts are divided in two large groups: “unbranched” and “branched”. In total, 11 morphotypes of unbranched holdfasts are identified and described in detail. Keywords: Echinodermata, attachment, holdfast, benthos, hardground, Paleozoic, Ordovician DOI: 10.1134/S0031030120070023
INTRODUCTION An important feature of stemmed echinoderms is the attachment of their larvae to the substrate with the preoral lobe using secreted sticky mucus (similar in composition to the extracellular polymeric substance (EPS) of bacterial mats) and subsequent formation in this place of a calcite base, above which a stem is developed raising the theca with the food-gathering apparatus into the water column. Later, in some echinoderms (typical of the Cambrian) this discoidal base only increases in size without any change in structure and shape. In some adult forms, it is completely reduced when no longer needed for attachment, e.g., solutes rested on the ground. However, most stemmed echinoderms show wide morphological diversity in the shape and structure of their holdfasts. This diversity increased sharply at the beginning of Middle Ordovician (Rozhnov, 2018) as evidenced by the studied collection of Ordovician echinoderms from the Leningrad Region. In most cases, unfortunately, it is not possible to reliably associate holdfasts with skeletons of other parts of the animal’s body—stem and theca. Therefore, it is necessary to classify them formally. Nevertheless, the morphological types identified make it possible to obtain a better understanding of the evolutionary nature of the morphogenesis of stemmed echinoderms, as well as the features of their benthic communities. Finds of echinoderms from the Ordovician deposits are of particular interest in connection with the study of changes in the formation of marine substrates during this period, which led to the widespread occurrence of hardgrounds—the most suitable substrates for
the settling of many attached echinoderms, and the sharp increase in the diversity of benthic organisms in general and stemmed echinoderms in particular (Anekeeva and Rozhnov, 2019b). This work provides a general summary of the data available on the morphology and terminology of holdfasts and considers morphotypes that can be combined into a group of “unbranched” holdfasts. MATERIAL AND METHODS The large collection of specimens from the Lower (Latorp) and Middle (Volkhov and Kunda regional stages, a few finds from the Uhaku regional stage) Ordovician, exposed in the east of the Leningrad Region, collected for many years by S.V.
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