First report of the cephalopod Order Ascocerida in the southern Hemisphere, Hirnantian of the Paraná Basin, Paraguay

Abstract

The ascocerid cephalopods were restrictively recognized in Ordovician andSilurian strata of North America (Canada, USA) and Europe (Bohemia, Estonia,Norway, Poland, Sweden, United Kingdom), being unknown from Gondwana. This workconstitutes the first mention of the Order Ascocerida Kuhn, 1949, from thesouthern hemisphere. Our material comes from the Itacurubí Groupof the Paraná Basin (southeastern Paraguay). At least two specimens were collectedat the Santa Elena quarry, from the Eusebio Ayala Formation (Hirnantian-Llandovery).Although poorly preserved, both specimens show an initial longiconic andslightly cyrtoconic phragmocone that abruptly increases its apical angle,turning wider at a certain point, although apparently with straight sutures.Adorad, the incomplete living chamber seems to be depressed in cross section.The lack of recognition of ascoceroid sutures indicates either a poor preservation,or the presence of primitive forms within the group, possibly close to theFamily Hebetoceratidae Flower, 1940. It is worth noting the preservation ofboth parts of the conch before the truncation, which is extremely rare. Thesefindings indicate a different paleogeographical dispersion of these forms thanpreviously assumed. More studies are needed to improve our understanding about themorphology of these specimens in order to clarify the dispersal of theseoccurrences to the southwestern Gondwana.