Porkunioceras tuba, Kröger, 2007

Kröger, Björn, 2007, Concentrations of juvenile and small adult cephalopods in the Hirnantian cherts (Late Ordovician) of Porkuni, Estonia, Acta Palaeontologica Polonica 52 (3), pp. 591-608 : 602-603

Genus Porkunioceras nov.

Derivation of the name: From Porkuni, referring to the type locality. Type species: Porkunioceras tuba sp. nov., monotypic.

Diagnosis.— Porkunioceras differs from other Graciloceratidae in having nearly straight breviconic conch with angle of expansion of approximately 10 ° –30 ° that is ornamented with transverse raised lirae. Seven transverse lirae occur at one millimetre. Genus in consideration differs from Pomerantsoceras in having stumpy, ornamented shell. Cross section is compressed with ratio conch width/height 0.7. Septal spacing is narrow with approximately 0.2–0.25 of shell diameter. Septal curvature is very shallow. Sutures have shallow lateral lobe. Septal necks are achoanitic. Siphuncle is marginal, positioned at convex side of conch curvature and very thin, tubular with cross section 0.10 of conch. Apex is acute. Second septum of apex occurs at cross section diameter of approximately two millimetres. No endosiphuncular deposits known. Adult length of the conch amounts to approximately 15 mm.

Occurrences.—Siuge Member, Porkuni Regional Stage, Late Ordovician from Porkuni, north Estonia.