Gregoryceras riazi (de Grossouvre, 1917 )

Cecca, Fabrizio & Savary, Bérengère, 2007, Palaeontological study of Middle Oxfordian- Early Kimmeridgian (Late Jurassic) ammonites from the Rosso Ammonitico of Monte Inici (north-western Sicily, Italy), Geodiversitas 29 (4), pp. 507-548 : 529

Gregoryceras riazi (de Grossouvre, 1917) ( Fig. 9E)

Peltoceras transversarium – Gemmellaro 1877: 92, pl. 4bis, fig. 17 (non Peltoceras transversarium Quenstedt, 1847 ) [fig. 16 is a different species].

Peltoceras riazi de Grossouvre, 1917: 63 , pl. 9, figs 10- 12, pl. 10, figs 15, 16.

Gregoryceras riazi – Bert 2004: 57, 62, pl. 7, figs 1-5, pl. 8, figs 1-4, pl. 9, figs 1-4, text-fig. 10, synonymy list.

MATERIAL EXAMINED. — MI3 10/1.

STRATIGRAPHIC DISTRIBUTION. — Interval 10 of Castello Inici section. According to Bert (2004) G. riazi is limited to the Middle Oxfordian Plicatilis Zone, lower part of the Antecedens Subzone.

DESCRIPTION

Deformed specimen showing an evolute shell with subtrapezoidal whorl section. Large, slightly rounded umbilical wall, which is connected to the flank through a rounded umbilical margin. Flanks are almost flat in the last whorl; venter is almost flat. Sculpture is characterized by strong ribs, springing, simple or in pairs, from primary ribs that, at their turn, originate from umbilical tubercles or bullae. From the inner third of the flank, ribs are strongly rursiradiate. Rib branching is observed at the outer third of the flank. Ribbing is already developed from a diameter of 15 mm and probably (but preservation is insufficient) starts towards 12 mm. All ribs slightly thicken at the ventrolateral margin and then cross the venter without interruption. No traces of peristome are visible. Suture lines are not visible. Measurements: see Table 15.

DISCUSSION

The preservation of this specimen is far from satisfactory but observable characters allow the assignment to G. riazi . The species G. riaziformis Bert, 2004 differs from G. riazi on the basis of its larger adult size, the earlier development of the sculpture (10.5 instead of 12 mm), the less rursiradiate and flexuous ribbing, and finally its slightly younger age (upper part of the Antecedens Subzone). The distinction between G. riazi and G. riaziformis is however very difficult.