Streblites frotho (Oppel, 1863)

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 : 518

Streblites frotho (Oppel, 1863) ( Fig. 4E )

Ammonites tenuilobatus Oppel, 1862: 160 , pl. 50, fig. 1.

Ammonites frotho Oppel, 1863: 199 .

non Oppelia frotho – Gemmellaro 1872: 39, pl. 6, fig. 6.

Ammonites (Oppelia) frotho – Favre 1877: 28, pl. 2, figs 7, 8.

Oppelia frotho – Fontannes 1879: 24, pl. 3, fig. 7.

Oppelia frotho var. mediogranosa Fontannes, 1879: 24 , pl. 3, fig. 8.

Streblites frotho – Wegele 1929: 106, pl. 25, fig. 10. — Christ 1960: 88, pl. 5, fig. 5.

Streblites sp. gr. frotho – Olóriz 1978: 50.

MATERIAL EXAMINED. — MI4N 8b/1.

OCCURRENCE. — The specimen has been collected in bed 8b of section Monte Inici East, which has been assigned to the Bimammatum Zone. The species is reported from the Bimammatum up to the Planula Zone ( Olóriz 1978, 2002).

DESCRIPTION

Involute, compressed shell. The whorl section shows slightly convex sides up to mid-flank where they converge towards a fastigate ventral area whose margins correspond to sharp ventrolateral shoulders. The venter is partly preserved: it bears a high crenulated keel that is visible up to the first half of the body chamber. Due to poor preservation, the ornamentation is incompletely visible. Some small tubercles corresponding to the ventrolateral tips of secondary ribs are visible in the last portion of the phragmocone. In the body chamber, the lower side of the flank only shows weak traces of ribs that give rise to radial bullae at mid-flank. In the upper side of the flank, adorally concave ribs spring from bullae: between two of these concave ribs, finer secondary ribs of different length are visible. Some of the secondary ribs seem to spring from the lateral bullae. All ribs stop at the ventrolateral shoulders. Complex suture with E as long as half of L and S1 higher than the external saddle. Measurements: see Table 5.

DISCUSSION

The ornamentation of the phragmocone of the specimen from Monte Inici is almost invisible, thus making the comparison with the specimens figured in the literature (which are mostly phragmocones or seem to correspond to phragmocones) difficult. Our specimen has the closest morphological similarities with the one figured by Wegele (1929), which shows a weak sculpture in the last third of the last whorl and a comparable ontogenetical evolution of the ornamentation. As already observed by Christ (1960) and Olóriz (1978), it is not always easy to distinguish S. frotho from congeneric species like S. tenuilobatus (Oppel, 1863) and S. folgariacus (Oppel, 1863) . The weakness of the ornamentation of our specimen is not entirely due to preservation failure and reflects a morphological transition towards S. tenuilobatus , from which it differs by the lower number of primary ribs.

Figures

FIG. 4. — A, Calliphylloceras benacense (Catullo,1847),specimen MI4N 8b/2 from bed 8b of section Monte Inici East;B, Sowerbyceras tortisulcatum (d’Orbigny,1841),specimen MI4N 6c/1 from bed 6c of section Monte Inici East;C, Holcophylloceras polyolcum (Benecke, 1866), specimen MI4W 8b/c/1 from bed 8c of section Monte Inici East; D, Tornquistes (T.) cf. romani (Douvillé, 1912), specimen MI3 9/7 from bed 9 of section Castello Inici; E, Streblites frotho (Oppel, 1863), specimen MI4N 8b/1 from bed 8b of section Monte Inici East. All figures natural size. Arrows indicate the beginning of the body chamber.