Pomerantsoceras Kröger, 2007

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

Genus Pomerantsoceras Kröger, 2007

Type species: Pomerantsoceras tibia Kröger, 2007 . Latest Ordovician of Estonia .

Discussion.— Kröger (2007) placed his new genus in the family Graciloceratidae Flower, 1950 . Unfortunately, Graciloceras Flower, 1943 (Middle–Late Ordovician) is a poorly known genus. The type species Graciloceras longidonum Flower, 1943 has a small exogastric slightly curved shell with relative high expansion rate, moderately compressed cross section, sub−ventral siphuncle, very weakly vaulted septa, and body chamber longer than the phragmocone. The shell of Pomerantsoceras expands with a markedly lower angle and during its late growth stage, the angle of expansion further decreases. Its cross section is much more laterally compressed, siphonal tube is in contact with the venter, septa are deeper, and body chamber is markedly shorter than the phragmocone. The sutures of Pomerantsoceras consist of broad lateral saddles separated by dorsal and ventral lobes, the ventral being deeper. These morphological features resemble early stages of cephalopods assigned to the genus Oonoceras Hyatt, 1884 (Silurian). Consequently Pomerantsoceras is transferred to family Oonoceratidae Flower, 1942 .

Pomerantsoceras strongly resembles Pleziorizoceras Chen, 1981 from the middle Silurian of China ( Chen et al. 1981) which is distinct in having shallower lateral lobes at well as ventral−dorsal saddles. Pleziorizoceras ovatum Chen, Liu, and Chen, 1981 is based on a single specimen, an internal mould of the phragmocone. Thus, information about sculpture and body chamber is missing. It is possible that Pomerantsoceras is a junior synonym of Pleziorizoceras, but additional information about the latter genus is needed.

Species included.— The type species and Pomerantsoceras pollux (Barrande, 1866) from the Silurian of Bohemia .