Hildaites exilis VENTURI, 1973

Sassaroli, Stefano & Venturi, Federico, 2012, Early Toarcian (post-OAE) Hildoceratinae (Ammonitina) fauna from the Marchean Apennines (Italy), Revue de Paléobiologie 31 (1), pp. 85-114 : 98-99

Hildaites exilis VENTURI, 1973

Pl. II, figs. 7, 8a-c, 9, 10a-b, 11a-c, 12, 13; Fig. 6, J-L

? 1966. Protogrammoceras isseli FUCINI. -KOTTEK, pl. 12, fig. 6.

1973. Hildaites exilis VENTURI , p. 595, fig. 11 (holotype) and p. 600; figs. 14h-14i (holotype).

1981. Hildaites gr. exilis VENTURI. - VENTURI, p. 596, fig 4.

1985. Hildaites (?) cf. exilis VENTURI. - VENTURI, p. 72, fig. 104

2001. Hildaites exilis VENTURI. - VENTURI & FERRI, pl. 25, fig. e.

Material: Twenty specimens of medium and small size, some with the body chamber, all coming from Marconessa quarry.

Amended diagnosis: The several specimens collected allow us to improve the diagnosis. Moderate involute shell with elliptical and compressed whorl section and moderately overlap of the whorls. The low umbilical wall is almost vertical, the umbilical edge is rounded. It has an usually narrow and tabulate, sometimes gently sulcated venter, with raised and fine keel. The moderately fine, sparse and gently falcoid ribs arise from the umbilical wall with a gentle flexion and they end projected on the ventro-lateral edge. The ornament also shows weak bundles, and sometimes the ribs are joined by gentle bullae. The simple ammonitic suture shows the E lobe usually only just shorter than L; the U 2 and U 3 lobes are small.

Description: Moderately involute and more or less compressed shell, characterized by elliptical whorl section with a remarkable overlap of the whorls compared to other Hildaites such as H. striatus and H. subserpentinus . The narrow keeled venter varies from tabulate or little rounded (mainly in the later examples) to gently bisulcate. The low and almost vertical umbilical wall shows a rounded umbilical edge. The moderately fine, sparse and sigmo-falcoid ribs vary from single, mainly in the inner whorls and in the former samples, to bifurcating or bundled in the outer whorls and in the later samples. The ribs can be more or less projected on the ventro-lateral edge; they start ever little retroverted from the umbilical wall. Sometimes, mainly in the later samples, the ribs are joined by gentle bullae. The suture shows a wide and low L lobe, the E lobe only just shorter than L or as long as L, the A lobe and the umbilical lobes little developed.

Remarks and comparative diagnosis: We observe from the former to latter examples the remarkable evolutionary trend to develop a swelling whorl section, to lose the sulci on the venter, which is more flattened among the older specimens, and to increase the ribs bundles. This species clearly differs from gr. H. subserpentinus and H. striatus , mainly for the sparser and sigmo-falcoid ribs. The badly preserved Greek specimen figured by KOTTEK (1996, pl. 12, fig. 6) as Protogrammoceras isseli FUCINI, 1929 , is probably an example of H. exilis .

Occurrence: From lower to upper Striatus zone.