Discosorids and Oncocerids (Cephalopoda) of the Middle Ordovician Kunda and Aseri Regional Stages of Baltoscandia and the early evolution of these groups

摘要

Discosorid and oncocerid cephalopods of the Kunda and Aseri Regional Stage (Darriwilian, Middle Ordovician) from Oland Island, Sweden and North Estonia are described for the first time. We demonstrate that a high generic and specific diversity of Oncocerida was already established in the earliest Darriwilian. The new oncocerid species Neumatoceras borense nov. sp., N. breviborense nov. sp., Paramiamoceras breviventrum nov. sp., Richardsonoceras gastroscopium nov. sp., R. gerhardi nov. sp., R. goldinanni nov. sp., R. haelluddenense nov. sp., Richardsonoceroides kingi nov. sp., R. oelandense nov. sp., and R. rhytium nov. sp. are erected; Richardsonoceroides schiefferdeckeri (Dewitz) is revised. The new graciloceratid Kundoceras evansi nov. gen., nov. sp. is erected. A review of records of early oncocerids from China shows that Richardsonoceras Foerste is probably the most primitive oncocerid and supports the hypothesis that oncocerids evolved in the early Floian or latest Tremadocian from a Bassleroceras Ulrich and Foerste-like ancestor. The new discosorid genus Paldoceras nov. gen. and species Paldoceras paldiskense nov. sp. and P. neptunsakerense nov. sp. are erected. These discosorids record the morphological link between the late Darriwilian Ruedemannoceras Flower and the early Floian Apocrinoceras Teichert and Glenister, which is considered to be the earliest discosorid.