Gorbyoceras Shimizu & Obata, 1935

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 51

Genus Gorbyoceras Shimizu & Obata, 1935

Type species

Orthoceras gorbyi Miller in Miller & Faber, 1894 , probably from the Saluda Formation, latest Katian, Franklin County, Indiana, USA; by original designation.

Diagnosis

Slender, circular or slightly compressed orthocones with symmetrically curved septa and straight transverse sutures; sutures parallel to annulations in each groove of the annulations; annulations with fine transverse ornament or growth lines; annulations more distinctive in adult growth stages; distinctive irregularly spaced longitudinal ridges that may form nodes at the ridges of the annulations; siphuncle subcentral, expanded in early growth stages, with short cyrtochoanitic septal necks, nearly tubular in later growth stages, with suborthochoanitic septal necks; mural cameral deposits developed much farther anteriorly than endosiphuncular deposits. (Adopted from Sweet 1964c; Kröger & Isakar 2006.)

Remarks

A large number of species of Gorbyoceras exist, which are in need of a revision. Potentially most of the Late Ordovician North American species of Spyroceras Hyatt, 1884 belong to Gorbyoceras , but their internal characters are not yet described. Additionally, often the ontogenetic variability in ornamentation is poorly known. Evans (1993) synonymized a number of Late Ordovician species with the aspect of Gorbyoceras from Ireland within G. gracile ( Portlock, 1843) and suggested that the number and strength of the longitudinal lirae are ontogenetically highly variable. Among the material assigned within G. gracile by Evans (1993) and listed in Evans (his table 5), not a single specimen shows any similarity with the material from the Boda Limestone. The specimens described by Evans (1993) may represent different species, but none of them can by synonymized with the Boda species of Gorbyoceras or Dawsonoceras .