Formation of the Morphological Diversity of the Holdfast in Stemmed Echinoderms from the Ordovician of the Leningrad Region

This work examines the origin, development, diversity, and distribution of attachment structures in stemmed echinoderms, a composite group within Echinodermata that includes representatives of two ancient subphyla of this phylum: Crinozoa, some members of which persist in the present day, and the extinct Blastozoa. The paper presents a detailed formal classification of attachment structures proposed by the author (within which two groups are distinguished, one of which is also divided into two subgroups, and 22 morphotypes) and data on their stratigraphic range and facies confinement; the attribution of morphotypes of the attachment structure to representatives of specific taxa of stemmed echinoderms is discussed. The localities of the studied specimens are briefly described, and a historical background on the study of the Ordovician stratigraphy and paleontology of the region is provided.