Sowerbyceras tortisulcatum (d’Orbigny, 1841)

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 : 514-515

Sowerbyceras tortisulcatum (d’Orbigny, 1841) ( Fig. 4B )

Ammonites tortisulcatus d’Orbigny, 1841: 162, pl. 51, figs 4-6.

Non Phylloceras tortisulcatum – Gemmellaro 1871: 149, pl. 10, fig. 1; 1876: 49, pl. 10, fig. 1.

Sowerbyceras tortisulcatum – Joly 2000: 113, text-figs 236-242, pl. 28, figs 4-6, synonymy list.

MATERIAL EXAMINED. — MI4N 6c/1, MI4N 6e/4, MI4N 6e/5, MI4N 6e/6, MI4N 6e/7, MI4N 6f/1, MI4N 8a/1, MI4N 8a/3, MI4N 8top/1, MI4N 8a’/1, MI4W 6e/2, MI4W 8a’/1, MI4W 8c/2. Specimens determined (as S. cf. tortisulcatum due to insufficient preservation): MI3 9/6, MI4W 6e/3, MI4W 6e/4, MI4W 8c/1, MI4W 8d/2, MI4N 9/1, MI4W 9/1, MI4W 9top/1.

STRATIGRAPHIC DISTRIBUTION. — The different specimens collected come from beds ascribed to the Middle Oxfordian Plicatilis and Transversarium zones and the Late Oxfordian Bimammatum Zone.

DESCRIPTION

Phylloceratids with a relatively evolute coiling. The whorl section is rectangular with a large, slightly rounded venter and almost flat flanks, rounded ventrolateral margins and rounded umbilical edge. The inner molds collected in the studied sections show four to five deep furrows on the last whorl: these are prorsiradiate, from the umbilical edge up to mid-flank or slightly below, and from mid-flank they abruptly take a rursiradiate direction. The furrows further change their direction around the ventrolateral margin; on the venter they show a clear adoral convexity. The adoral side of the fur- row is steep and its edge is very sharp, thus forming ventral ridges. Measurements: see Table 2.

DISCUSSION

This is the most common species at Monte Inici. The preservation of the numerous collected specimens is far from satisfactory but clearly allows the identification of the d’Orbigny’s species. We refer the reader to Sarti (1993) for the clear definition of the limits of S. tortisulcatum and the similar, younger, species S. silenum (Fontannes, 1876) and S. loryi (Munier Chalmas in Pillet & De Fromentel, 1875) (but see Joly 2000 for further information about the creation of the latter species) and also for the detailed information on their biostratigraphic distribution. Sarti (2002) has also shown that the specimens ascribed by Gemmellaro (1871, 1876) to S. tortisulcatum actually belong to S. loryi .

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.