Gregoryceras devauxi Bert & Enay, 2004

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 : 531-532

Gregoryceras devauxi Bert & Enay, 2004 ( Figs 9A, B; 10 )

? Gregoryceras fouquei – Atrops & Benest 1984: pl. 1, fig. 3 (non Peltoceras fouquei Killian, 1889 ).

Gregoryceras cf. fouquei – Schairer et al. 2003: 209, fig. 8 (3-5) (non Peltoceras fouquei Killian, 1889 ).

“ Gregoryceras devauxi ” – Bert 2004: 89, 98, pl. 17, fig. 1, pl. 18, figs 1-3, pl. 19, fig. 2, pl. 20, figs 1-3, pl. 21, figs 1-3, pl. 22, figs 1-5, text-fig. 17, 18, synonymy list.

Gregoryceras devauxi Bert & Enay, 2004: 456 , pl. 1, fig. 1, pl. 2, figs 1-3, pl. 3, figs 1, 2, pl. 4, figs 1-3, pl. 5, figs 1-3, pl. 6, figs 1-5.

MATERIAL EXAMINED. — MI4N 8a/2, MI4N 8a/3, MI4W 6c/2 ( G. cf. devauxi ).

STRATIGRAPHIC DISTRIBUTION. — The largest specimen has been collected from bed 6c of section Monte Inici East, whereas the smaller ones have been collected in bed 8a of the same section. According to Bert (2004), the range of G. devauxi coincides with the range of the Schilli Subzone that is the second Subzone of the Middle Oxfordian Tranversarium Zone.

DESCRIPTION

Evolute shell with a subtrapezoidal whorl section characterized by flat flanks and a relatively large, almost flat, venter. Strong sculpture made of rursiradiate ribs which spring, single or in pairs, from strong, prominent, umbilical tubercles and/or bullae. The latter clearly originate from the base of the umbilical wall. Ribs slightly thicken in the upper part of the flank giving rise to small, rounded ventrolateral tubercles. Simple ribs, which do not reach the umbilical margin, are intercalated between the ribs originating from the umbilical tubercles. No bifucations are observed. All ribs cross the venter without interruption forming a gentle adapical convexity, although in the last quarter of the last whorl they tend to become straight. It is impossible to know whether the studied specimens bear the body chamber or not, as no sutures are visible.

The three specimens found at Monte Inici are not morphologically identical. Specimen MI4N 8a/2 shows umbilical bullae (or thickened primary ribs), whereas MI4N 8a/3 has a steep umbilical wall, a more spaced ribbing and thicker bullae that mimic prominent tubercles.

A fragment of a larger individual (specimen MI4W 6c/2 determined as G. cf. devauxi ) is still septated at a diametre of about 100-110 mm. Ribs have lost their adapical convexity but are still gently rursiradiate and form a ventrolateral tubercle ( Fig. 10 ).

Measurements: see Table 17.

DISCUSSION

The described specimens correspond to three morphologies that can be ascribed to G. devauxi . Specimen MI4N 8a/2 ( Fig. 9A) recalls, at similar diameters, some specimens already figured in the literature: – the specimen figured by Bert (2004: pl. 22, fig. 4), which differs in the presence of bifurcate ribs from the umbilical margin;

– the specimen figured by Gygi (1990: fig. 7) with the name G. fouquei ;

– the specimen figured by Gemmellaro (1874: pl. 1, fig. 1) with the name G. fouquei (identification confirmed by D’Arpa & Meléndez 2002), although at the same diameter (i.e. up to the first third of the last whorl) our specimen is more evolute.

At a comparable diameter, specimen MI4N 8a/3 ( Fig. 9B), which is characterized by definitely rursiradiate ribs, strongly resembles the one figured by Bert (2004) in pl. 21, fig. 2. It also shows some morphological similarities with other specimens figured by Bert (2004: pl. 20, fig. 1; pl. 22, fig. 1) and also to the inner whorls of the holotype (Bert 2004: pl. 17, fig. 1c) but it differs because of its wider umbilicus. Specimen MI4W 6c/2 ( Fig. 10 ) resembles the specimen figured by Bert (2004) in pl. 17, fig. 1. It also may recall forms ascribed to G. fouquei or G. aff. fouquei (see D’Arpa & Meléndez 2004). However, no splitting of the ventrolateral tubercle occurs and its ribs are gently rursiradiate.

The differences between G. devauxi and the forms ascribed to both G. aff. fouquei and G. fouquei have been described by Bert (2004), to whose paper we refer the reader.

Figures

FIG. 10. — Entirely septated specimen identified as Gregoryceras cf. devauxi Bert & Enay, 2004, specimen MI4W 6c/2 from bed 6c of section Monte Inici East: A, B, lateral views; C, ventral view. All figures natural size.