ASTROVIOIDEA: A NEW SUPERORDER OF PALEOZOIC CEPHALOPODS

Abstract

A new superorder of Paleozoic ectochochlian cephalopods, Astrovioidea superordo nov., is designated. A distinctive feature of this superorder is that the soft cameral tissue enters the siphon cavity and secrets calcareous cameral deposits. This resulted in the destruction of the connecting rings that previously constituted, in conjunction with the septal necks, the compound wall of the siphon. In early (Ordovician) representatives of this superorder, the soft cameral tissue passed through the siphon to destroy, either partially or completely, the connecting rings and to progress occasionally along the internal surface of those rings that remained undestroyed as far as several chambers (the order Lituitida Starobogatov, 1983). In later (Silurian and Devonian) astrovioids, the cameral tissue entered the siphon cavity through the septal foramen. The episeptal portion of the cameral tissue destroyed the connecting ring and moved over the septal neck to the earlier chamber to be fused with the hyposeptal portion of the cameral tissue located in this chamber. Subsequently, it gradually secreted calcareous deposits, thus squeezing the soft siphon in the septal neck. As cameral deposits accumulated, they filled the cavities of chambers and often came out from them to move toward the siphon and squeeze the soft siphon not only in the septal necks, but (in the absence of connecting rings) in the interspaces between them as well (the order Pallioceratida Marek, 1998). A detailed examination of thin and polished sections of newly found and some previously published Silurian and Devonian pallioceratids by an optical microscope and, what is more important, by a scanning electron microscope has shown that, in those parts of the phragmocone within which cameral deposits reached into the siphon cavity, the connecting rings were completely destroyed, thus allowing direct connection between the soft cameral and soft siphonal tissues. In addition, some new taxonomically important details of the internal structure of shells have been revealed. Thus, it has been found that many taxa of this group of cephalopods formed in the siphon cavity a longitudinal layer of calcareous deposits that passed through all chambers over the epi- and hyposeptal deposits that filled the chambers. We propose the term metacameral deposits for these deposits. Furthermore, some pallioceratids contain calcareous deposits that in the absence of connecting rings were secreted by the siphon directly in the chamber cavity to meet the cameral deposits. These deposits will be referred to as ectosiphonal deposits. The microstructure of the cameral, metacameral, and ectosiphonal deposits have been examined with the aid of SEM. These new data on the internal structure of pallioceratid shells and a detailed analysis of the literature that has been made in the light of these data allow the extension of the composition of this order and the development of a new classification. Thus, the order Pallioceratida now includes 5 families, 15 genera, and 32 species. The following new taxa have been separated: the families Flowerinidae, Ostreioceratidae, Astroviidae, and Plicatoceratidae; the genera Astrovia (with the type species A. adorea sp. nov.), Ostreioceras (with the type species Sinoceras riphaeum Zhuravleva, 1978), and Syndikoceras (with the type species S. arcticum sp. nov.); and the species Astrovia marhoumensis sp. nov., Syndikoceras mutveii sp. nov., and Plicatoceras bublichenkoi sp. nov. At present pallioceratids are known from the Wenlock through the Eifelian of Europe, Asia, Africa, and North America. The fact that in the order Lituitida cameral deposits penetrated into the siphon cavity and resulted in a destruction of connecting rings during the life of the organism was convincingly shown by Sweet (1958), Flower (1978), Dzik (1984), and other authors. The superorder Astrovioidea existed from the Early Ordovician to approximately the Middle Devonian (Eifelian) and was virtually worldwide.