Hildaites bisulcatus, Sassaroli & Venturi, 2012

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 : 101

Hildaites bisulcatus n. sp.

Pl. III, figs. 7, 8, 9a-b, 10a-c; Pl. IV, figs. 1, 2;

Fig. 7, D-E

2010. Hildaites n. sp. - BILOTTA et al., fig. 17C

Material: Fourteen specimens of small and medium size, all coming from Marconessa quarry.

Derivatio nominis: From the bisulcate venter shown in all stages.

Holotype: A medium size and wholly septate specimen MSA351 , coming from Marconessa quarry, section 2, bed 3a (Pl. III, figs. 10a-b-c; Fig.7, D-E).

Stratus typicus: Nodular limestone and grey marls of the early Toarcian (Bugarone Formation).

Diagnosis: Moderately evolute compressed shell showing a little overlap of the whorls, from fastigate to sub-trapezoidal whorl section by flattened or little convex sides, wide and bisulcate venter with gentle keel. The shell shows a fast-growth coiling, mainly in the outer whorls. High and sloped umbilical wall and rounded umbilical edge. Falcoid, sparse and single ribs, strongly projected on the ventro-lateral edge. The moderately frilled suture is simple.

Holotype measurements. D: 74,5 mm - U: 33,5 mm - U/D: 0.45 - Wb: 18 cm - Wh: 23 mm - Wh/ Wb: 1.27 - Wb/Wh: 0.78.

Description: Moderately evolute shell by little convex sides and a sub-trapezoidal whorl section. The bisulcate venter is wide, showing narrow and little deep sulci and a gentle keel. The sloped umbilical wall is high and the umbilical edge is rounded. The falcoid ribs are sparse (moderately rib-density, we counted 46 ribs in the last whorl of the phragmocone), ever single, little coarse and strongly projected on the ventro-lateral edge. The ammonitic suture is slightly jagged, with the E lobe shorter than L lobe, which is wide and little developed in high.

Paratypes: The paratypes features coincide almost perfectly to the holotype’s description. They vary chiefly in having a more or less rib projection on the ventrolateral edge. All samples in all growth stages show a bisulcate venter, which is the typical character of the species.

Remarks and comparative diagnosis: This species clearly differs from BUCKMAN’ s holotype of Hildaites subserpentinus for whorl section, rib-sculpture and ribdensity. At a glance it has a resemblance with some specimens figured by HOWARTH (1992) as Hildaites subserpentinus (pl. 32, figs. 5a-5b; pl. 33, figs. 4a-4b and Pl. 34, figs. 1a-1b), mainly for the ornament. Nevertheless, H. bisulcatus differs for the wider and bisulcate venter. This species differs from H. evolutus chiefly in having a lesser evolute shell from a sub-trapezoidal whorl section, a narrower bisulcate venter and strongly projected and coarser ribs.

Occurrence: Upper Striatus zone.