Hildaites planiventris, 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

Hildaites planiventris n. sp.

Pl. IV, figs. 3a-c, 4, 5; Fig. 7, A-C

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

Derivatio nominis: By reason of its typical wide and perfectly flattened venter.

Holotype: A well preserved shell MSA 61 of large size with the body chamber, coming from Marconessa quarry, section 3, bed 1 (Pl. IV, figs. 3a-b-c; Fig. 7, A-C).

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

Diagnosis: Evolute very compressed shell with little coiling overlap, high sub-rectangular whorl section by flattened sides. The wide and flattened not sulcated venter show a strong and raised keel. The falcoid, sparse, moderately strong usually single ribs start from a low and gently sloped umbilical wall. The umbilical edge is rounded. The ammonitic suture is simple, little frilled, characterized by a wide and little developed in height L lobe.

Holotype measurements. D (diameter): 87 mm - U (umbilical width): 44 mm - ratio U/D: 0.51 - Wb (whorl breadth): 15 mm - Wh (whorl height): 24 mm - ratio Wh/ Wb: 1.6 - ratio Wb/Wh: 0.62.

Description: Evolute very compressed shell with little coiling overlap and a high sub-rectangular whorl section by flattened sides. The venter is wide and perfectly flat, not sulcated with a raised, sharp and strong keel. The low umbilical wall is sloped and the umbilical edge is rounded. The shell shows a spiral groove in the body chamber, placed at 1/3 of height of the whorl side from the umbilical edge, which, contrary to some Hildoceras , is not smooth. The falcoid ribs are moderately strong and spaced with low rib-density (we count 45 ribs in the last whorl of the phragmocone) and they are usually single, rarely bifurcating, in the phragmocone, and not much projected on the ventro-lateral edge showing a relatively short proximal segment. In the body chamber ribs increase in number becoming finer, denser and usually bifurcating-trifurcating. The suture line is simple and little frilled; it shows a L lobe wide and short, and a E lobe shorter than L. The ES saddle is much wider than the LS 1 saddle.

Paratypes: The paratypes are very similar to the holotype, showing an evolute shell, a flat venter and a rounded umbilical edge. The ribs are more or less falcoid than the holotype. The suture line shows more or less short L and E lobes. Both small sized paratypes MSA64 and MSA78 show a flat venter, never bisulcate. The species is characterized in all growth stages by a tabulate venter (see Table 1 for measurements of all figured samples) .

Remarks and comparative diagnosis: The dorsal sight of holotype seems to be close to a specimen figured by HOWARTH (1992, pl. 30, figs. 9a-b) as Hildaites murleyi . Nevertheless, HOWARTH’ s specimen compared to H. planiventris ’s holotype has a narrower venter, a subquadrate whorls section and a stronger keel and its ribs are finer and denser. Furthermore, its unbilical wall is higher than planiventris . On the other hand, BUCKMAN’ s (1921, pl. 216, figs. 2-3) holotype of H. murleyi differs from our holotype and from HOWARTH’ s specimen, because it has an almost swollen sub-quadrate whorl section, a wider flat venter and coarse, sinuous not falcoid ribs from the inner whorls too, which end near the keel. On the contrary the wide venter of H. planiventris is smooth because the ribs end on the ventro-lateral edge. For these reasons we think that planiventris is not a synonym of H. murleyi . We believe also that HOWARTH has enlarged the morphological range of the British species H. murleyi too much in having figured specimens sometime very different from each other.

Occurrence: Lowermost Striatus zone.