Hildaites evolutus, , 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 : 99-101

Hildaites evolutus n. sp.

Pl. III, figs. 1a-b, 2a-c, 3, 4a-b, 5, 6; Fig. 6, G-I

2010. Hildaites evolutus, SASSAROLI & VENTURI (in VENTURI et al.), p. 279, 355.

Material: Eleven specimens of various size from small to large, all coming from Marconessa quarry.

Derivatio nominis: For the reason of its very evolute shell without overlap of the whorls coiling.

Holotype: A large sized and well preserved specimen MSA426 with the body chamber, coming from Marconessa quarry, section 3, bed 1a (Pl. III, figs. 1a-b; Fig. 6, G-I).

Stratus typicus: Nodular limestone and grey marls of the lowermost Striatus zone (Bugarone Formation).

Diagnosis: Very evolute compressed shell by flattened sides; from sub-quadrate to sub-rectangular whorl section. No whorls overlap occurs. The shell shows also a slow-growth coiling. Tricarinate-bisulcate and moderately wide venter with a thin keel. The sulci are little deep. The low umbilical wall is gently sloped and the umbilical edge is rounded. The sparse, fine, single, falcoid and rursiradiate ribs are projected on the ventrolateral edge and they show a short proximal segment moderately angled backward. The ammonitic suture is very simple and little frilled.

Holotype measurements. D: 83 mm - U: 43 mm - U/D: 0.52 - Wb: 17,5 mm - Wh: 23 mm - Wh/Wb: 1.31 - Wb/ Wh: 0.76.

Description: Very evolute compressed shell showing from sub-quadrate to sub-rectangular whorl section without overlap of the whorls and flattened sides. The moderately wide venter is tricarinate-bisulcate with little deep sulci and a thin keel. The umbilical wall is low and sloped in the phragmocone, higher and angled in the body chamber. The umbilical edge is gently rounded. The ribs are spaced with a moderately rib-density (we count 54 ribs in the last phragmocone whorl), fine, single and rursiradiate with falcoid appearance. They show a short proximal segment gently angled backward and they are moderately projected on the ventro-lateral edge. The suture is simple, little frilled, with a developed L lobe and a wide E lobe shorter than L. The U 2 lobe is long about the half length of L. U 3 is dentiform and the A lobe is little developed.

Paratypes: The paratypes are very close to the holotype description. They vary mainly in having a more or less wide venter and ribs projection. The paratype MSA831 shows the sides gently convex. The small sized specimens show a sub-quadrate whorl section and ever a tricarinatebisulcate venter .

Remarks and comparative diagnosis: This species is characterized by having the most evolute shell among the earliest Hildaites of the Striatus zone. It differs from H. planiventris chiefly in having a tricarinate-bisulcate venter, finer ribs and a more evolute whorl section.

A specimen of H. evolutus , coming from Valdorbia outcrop, was firstly figured and not formally described in VENTURI et al. (2010: 279, 255). It was wrongly pointedout as holotype, but really it is a sample only, because the holotype MSA426 was already described and figured in a previous unpublished paper .

Occurrence: Lowermost Striatus zone.