Subdiscosphinctes, Malinowska, 1972

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 : 520

Subdiscosphinctes sp. ( Fig. 5D )

MATERIAL EXAMINED. — MI4N 9/4.

STRATIGRAPHICAL DISTRIBUTION. — The specimen has been collected in bed 9 of section Monte Inici East, which has been assigned to the Planula Zone. The specimens figured by Choffat (1893) were collected in layer 12 of Cabanas de Torres ( Portugal), together with species of the genus Subnebrodites , which indicate the Planula Zone.

DESCRIPTION

Compressed, evolute shell. The whorl section is subtrapezoidal, with flat flanks converging towards a narrow, gently rounded (almost flat) venter. Due to insufficient preservation, both the umbilical edge and the umbilical wall are not clearly visible. The ornamentation consists of numerous fine, prorsiradiate ribs. These spring from the umbilical edge and bifurcate at the upper third of the flank. Some ribs remain simple. Two ribs may be united on the umbilical edge to form rib bundles, which may be composed either by two bifurcate ribs or by a simple rib together with a bifurcate rib. According to Atrops (1982), these combinations are respectively called subpolyplocoid and incomplete subpolyplocoid ribs. At least in the first third of the body chamber a narrow smooth band interrupts ribs on the venter. Measurements: see Table 6.

DISCUSSION

This specimen shows some resemblances (coiling, whorl section) with some of the specimens figured by Choffat (1893). It has been provisionally identified in Cecca et al. (2001) as Perisphinctes sp. nov. aff. dybowskii Siemiradzki in Choffat (1893: pl. 10, fig. 1), from which our specimen differs because of its denser ribbing and the development of subpolyplocoid ribs. The latter are developed in S. castroi ( Choffat, 1893) (see Choffat 1893: pl. 10, figs 5, 6), which is clearly more involute.

Figures

FIG. 5. — A, B, Taramelliceras cf. hauffianum? (Oppel, 1863), specimen MI4W 8b/c/3 from bed 8c of section Monte Inici East: A, lateral (slightly oblique) view; B, ventral view; C, Perisphinctes (Dichotomosphinctes) gr. antecedens? Salfeld, 1914, specimen MI3 9/10 from bed 9 of section Castello Inici; D, Subdiscosphinctes sp., specimen MI4N 9/4 from bed 9 of section Monte Inici East. All figures natural size. Arrows indicate the beginning of the body chamber.