Oncoceratidae Hyatt, 1884

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 : 601

Family Oncoceratidae Hyatt, 1884

Gen. et sp. indet

Fig. 6E.

Material.— One specimen preserving the apex (TUG 1227/37) .

Description.—Specimen TUG 1227/37 ( Fig. 6E) is fragment of apex with total length of 0.14 mm, maximum dorso−ventral diameter of 0.16 mm, and maximum lateral diameter of 0.09 mm. Conch is strongly compressed, cyrtoconic, ornamented with transverse lirae, starting with initial diameter of 0.03 mm. Initial lira surrounds oval area that contains cicatrix with length of less than 0.03 mm. Up to 0.08 mm conch length simple lirae, spaced at distance of 5 ̊m and approximately 20 in number. Subsequent lirae more widely spaced, shallower, crenulated and more irregularly spaced. Only surface characters known.

Occurrence.—Cherts of the Siuge Member of the Porkuni Regional Stage, Late Ordovician of Estonia.

Remarks.—The genus and species assignation of the single fragment is impossible solely on surface characters. A similar ornamentation occurs in several other oncoceridans (see below). It may be noted that all known oncoceridans with a crenulated shell display a circular or slightly depressed cross section. Contrarily, the fragment described above is strongly compressed in cross section.