The Taxonomic Characters of Coral Polyps and the Position of the Recently Described Ordovician Coral Lindaphylon Rozhnov, 2024 (Cnidaria: Anthozoa: Lindaphylonaceae: Lindaphylonidae)

axonomic characters used for classification of coral polyps are reviewed and summarized. When distinguishing large taxonomic groups within Anthozoa, the following characters were proposed at different times and are still used: presence of a calcareous massive aragonite external skeleton secreted by ectoderm cells; presence of calcareous calcite sclerites located in the mesoglea; presence or absence of pinnate tentacles; polyp symmetry; cessation of polyp development at the Edwardsia stage, leading to preservation of larval eight-ray bilateral symmetry, or transition to the Halcampula stage, leading to radial symmetry. The combination of these characters allows two classes to be recognized in Anthozoa corals: Alcyonaria Milne-Edwards et Haime, 1857 and Zoantharia Milne-Edwards et Haime, 1857, which largely correspond to the Octocorallia and Hexacorallia recognized by Haeckel. In a number of structural features, the colony of the fossil coral Lindaphylon is very similar to the most integrated and complexly structured recent representatives of the eight-rayed sea pens, which were long united in the order Pennatulacea. The main feature that does prevents this Ordovician form from being classified as a sea pen is the twelve-rayed (or six-rayed, which is the same) symmetry of the zooids. The inconsistent twelve-ray symmetry observed in the calcareous cups of Tubiporida does not correspond to the eight-rayed symmetry of their polyp. Therefore, the symmetry type cannot serve as an incontrovertible argument for classifying a coral into one of the two classes. The most reliable character for class separation is the arrested morphogenesis of the polyp during development at the Edwardsia stage, leading to the preservation of larval bilateral symmetry by the adult alcyonarian polyp. However, this cannot be applied to Lindaphylon, since the arrangement of the mesenteries and tentacles is unknown; the presence of pinnate tentacles in Lindaphylon could not be confirmed either; only the most variable aspect of the coral polyp organization, the structure of the skeleton, remains the most informative feature of the Lindaphylon colony structure, indicating its constructive or, possibly, phylogenetic affinity with sea pens. The presence in the colony of traces of a longitudinal organic rod, characteristic of many representatives of the class Alcyonaria among modern anthozoans, brings Lindaphylon closer to these animals. The question of whether this genus belongs to Alcyonaria or Zoantharia remains open