Gregoryceras aff. 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 : 532

Gregoryceras aff. devauxi Bert & Enay, 2004 ( Fig. 9F)

MATERIAL EXAMINED. — MI4W 6a/1.

STRATIGRAPHIC DISTRIBUTION. — The specimen has been collected in bed 6a of section Monte Inici East. According to Bert (2004) G. tranversarium is limited to

the base of the middle Oxfordian Transversarium Zone, Luciaeformis Subzone whilst G. devauxi has a vertical range coinciding with that of the Schilli Subzone. The specimen described can be assigned to the middle or upper part of the Transversarium Zone.

DESCRIPTION

Evolute shell with a subtrapezoidal whorl section. Flanks are gently rounded in the first half of the last whorl and then almost flat in the second half; they converge towards a relatively narrow, gently rounded venter. The umbilical edge is rounded; the umbilical wall is oblique and well developed in the last whorl. The sculpture is not visible up to a diameter of almost 34 mm. Rursiradiate ribs spring, single or in pairs, from rounded umbilical tubercles. In the last quarter of the last whorl ribs spring from strong, elevated umbilical bullae and tend to become rectiradiate. Due to the poor preservation of the umbilical wall it seems that ribs originate from the umbilical margin, except the four last preserved bullae that start from the upper part of the umbilical wall. No rib branching above the inner third of the flank is observed in the last two thirds of the last whorl (the first third is insufficiently preserved). Ribs thicken on the ventrolateral margin where they tend to form extremely weak tubercles. Ribs cross the venter strengthened and without interruption; they are here gently convex adapically, although in the last quarter of the last whorl they tend to become almost straight. As weak traces of sutures are visible up to a diameter of almost 45 mm only, it is impossible to know whether this specimen bears the body chamber or not. Measurements: see Table 18.

DISCUSSION

The ribbing of G. (G.) transversarium is strongly rursiradiate and often shows biplicate ribs in the outer third of the flank. The ribbing of the specimen described recalls G. (G.) fouquei , which differs because of its less evolute coiling and its vertical umbilical wall. Gregoryceras devauxi , the species that follows G. tranversarium in the anagenetic succession of chronospecies proposed by Bert (2004), differs because of its clearly developed tubercles on the ventrolateral margin. The described specimen likely corresponds to an immature individual of G. devauxi as suggested by the comparison, at the same diameter, with some of the specimens figured by Bert (2004: pl. 19, figs 1, 2).