Hildaites striatus GUEX, 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

Hildaites striatus GUEX, 1973

Pl. II, figs. 1a-b, 2, 3a-c, 4, 5, 6; Fig. 6, E-F

?1913. Hildoceras (Fuciniceras) pectinatum MENEGHINI. - MEISTER, pl. 13, fig. 1

?1963. Hildaites serpentinum BUCKMAN. - ZANZUCCHI, pl. 18, fig. 6.

?1966. Hildaites acutus TATE. – KOTTEK, pl. 5, fig. 3 and pl. 6, fig.1.

1973. Hildaites striatus GUEX , pl. 7, fig. 3 (holotype); pl. 3, fig. 10 and pl. 10, fig. 2.

1973. Hildaites sp. ind. GUEX, pl. 6, fig. 3.

1980. Hildaites striatus GUEX. - WIEDENMAYER, pl. 32, figs. 1-4.

1992. Hildaites striatus GUEX. - JIMÉNEZ JIMÉNEZ & RIVAS CARRERA, pl. 4, figs. 11-15.

2002a. Hildaites wrighti SPATH. - MACCHIONI, fig. 4.6.

2010. Hildaites striatus GUEX. - BILOTTA et al., fig. 17 A-B.

Material: Eighteen specimens of small and medium size, five of which with the body chamber, from Marconessa quarry.

Amended diagnosis: Evolute compressed shell, elliptical whorl section with little overlap of the whorls. The coiling rapidly increase in height whorl. The venter is moderately narrow, tabulate with raised and coarse keel. The umbilical edge is rounded and the low umbilical wall is gently sloped. The moderately sinuous ribs are finest and dense, usually bifurcating or sometimes bundled mainly in the outer whorls and in the body chamber. The ribs start little retroverted from the umbilical wall, ending projected on the ventro-lateral edge. They become more sinuous and projected in the body chamber. The ammonitic suture is simple, little frilled and it shows a slanting ES saddle, which is wider twice as wide as LS 1 . Description: Evolute small sized shell. The elliptical very compressed shell shows a little overlap of the whorls and a rapid coiling increase. The low umbilical wall shows a gently sloped or rounded umbilical edge. The moderately sinuous, finest and dense ribs are sometime single or usually bifurcating; in the outer whorls and in the body chamber they become densest and sometime bundled forming the typical striatures. The venter is narrow and flattened with coarse keel, similar to H. subserpentinus , which instead has a more narrow and more acute venter. The ribs are sinuous in the fragmocone, but not much backward near the umbilical and moderately projected on the ventro-lateral edge, becoming more sinuous in the body chamber. The suture shows little jagged lobes, a lengthened L lobe, a E lobe shorter than L. The umbilical lobes are moderately developed, and the A lobe is slanting. The LS 1 saddle is narrow, and the ES saddle is about twice as wide as the former. The Apennines specimens are very close to GUEX’ s holotype for the ornament and the whorl section and coiling; they are less evolute than those figured by JIMÉNEZ JIMÉNEZ & RIVAS CARRERA (1992).

Remarks and comparative diagnosis: HOWARTH (1992) believes that H. striatus is a synonym of Hildaites wrighti . In our opinion these are two well separate species. The ribs of H. striatus are finer and denser, bifurcating or bundled with the typical striatures. On the contrary the ribs of H. wrighti are usually single, more falcoid and more projected on the ventro-lateral edge. Besides, H. wrighti shows a greater shell involution, a more overlap of the whorls, a lesser rapid coiling increase, while H. striatus has a more compressed and evolute shell. The venter of H. wrighti is bisulcate, narrower and more acute-keeled than of H. striatus , which is flat without sulci. The umbilical wall of H. striatus is lower and more sloped than of H. wrighti , which is higher and more rounded. Despite of the remarkable likeness between H. striatus and H. subserpentinus , which also have the same stratigraphic occurrence, the former differs from the later by having finer and denser ribs, which are from bifurcating to bundled, more sinuous and projected on the ventro-lateral edge chiefly in the outer whorls and in the body chamber. The two Grecian specimens assigned by KOTTEK (1966, pl. 5, fig. 3 and pl. 6, fig. 1) to Hildaites acutus (TATE, 1895) probably must be assigned to H. striatus .

Occurrence: Striatus zone. We choose Hildaites striatus as zonal index of early Toarcian (post-OAE) of the Apennines outcrops.