Oonoceratidae Flower, 1942

Manda, Štěpán & Turek, Vojtěch, 2009, Minute Silurian oncocerid nautiloids with unusual colour patterns, Acta Palaeontologica Polonica 54 (3), pp. 503-512 : 508-509

Family Oonoceratidae Flower, 1942

Emended diagnosis.—Oncocerids with an exogastrically cyrtoconic shell; shell curve varies only slightly within an species; siphuncle without deposits, marginal, in later ontogenetic stages slightly sub−marginal; phragmocone chambers are low; suture with broad lateral lobes; hyponomic sinus usually well developed; body chamber relatively short, slightly longer than wider; cross section laterally compressed. Embryonic shell is cup−like, early shell is less curved than adult shell.

Discussion.—Family Oonoceratidae was erected by Flower in 1942, but no diagnosis was given by him. Flower (1942) included three genera in the family, namely Richardsonoceras Foerste, 1933 (Ordovician), Oonoceras Hyatt, 1884 (Silurian–earlier Devonian), and Oocerina Foerste, 1926 (Silurian). However, Oocerina in fact belongs to the family Jovellaniidae Foord, 1888 (see Manda 2001). Sweet (1964) synonymised the Oonoceratidae with Oncoceratidae Hyatt, 1884 . The diagnosis of the latter family, as refined by Frey (1995), suggests that Oonoceras and allied genera cannot by placed here. Late Ordovician species assigned by Flower (1942) to Oonoceras probably belong to another genus because they differ from Oonoceras in having long body chambers, almost straight sutures and very thin siphonal tubes. The phyletic relationship between Richardsonoceras and Oonoceras proposed by Flower (1942) and followed by Dzik (1984) is still unclear. In our view, the family Oonoceratidae contains only Oonoceras (Silurian and earlier Devonian) and its allied genera. It should be noted that the majority of taxa are known from the Silurian of the Prague Basin, where the family reached its maximal diversity as well as disparity. New material shows that species grouped within Oonoceras in fact belong to other, still undescribed, genera that differ in shell curvature, cross section, sculpture and ratio of phragmocone/body chamber length. At last, two morphologically convergent groups of oncocerids with exogastrically curved shells exist. The Silurian jovellaniids including Oocerina differs from oonoceratids by the presence of actinosiphonate deposits. Similarly, Ordovician Richardsonoceras –“ Oonoceras ” (sensu Flower 1942) and Silurian Oonoceras may represent convergent morphotypes of oncocerids. However, without data concerning early shell ontogeny, the systematic position of Richardsonoceras and allied forms remain unclear.

Genera included.— Oonoceras Hyatt, 1884 (Silurian, earlier Devonian), Pomerantsoceras Kröger, 2007 (latest Ordovician, Silurian), Pleziorizoceras Chen, 1981 (middle Silurian), Shuranoceras Barskov, 1959 (Silurian).