Hildaites subserpentinus BUCKMAN, 1921

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 : 96-97

Hildaites subserpentinus BUCKMAN, 1921

Pl. I, figs. 2a-b, 3, 4a-b, 5a-c, 6a-b, 7; Fig. 6, C-D

1919. Hildoceras serpentinum (REINECKE) . - BUCKMAN, pl. 138B.

1921. Hildaites subserpentinum BUCKMAN , pl. 217, figs. 1-2 (holotype).

1963. Hildaites serpentinus BUCKMAN. - ZANZUCCHI, pl. 18, figs. 7- 8.

?1973. Hildaites aff. striatus GUEX. - GUEX, pl. 9, fig. 2.

1973. Hildaites subserpentinus BUCKMAN. - GUEX, pl. 7, fig. 5 and pl. 8, fig.1.

1975. Hildaites subserpentinus BUCKMAN. - VENTURI, pl. 30, fig. 7.

1992. Hildaites subserpentinus BUCKMAN. - HOWARTH, pl. 33, figs. 1a-b.

2002a. Hildaites serpentinus (REINECKE) . - MACCHIONI, fig. 4.8

Material: Twenty-one specimens of small and medium size and three specimens of large size, all collected in the beds of the Hildaites striatus zone, from Marconessa quarry.

Remarks: We consider here the BUCKMAN’ s (1921, pl. 217) holotype only, refigured by HOWARTH (1992, pl. 33, figs. 1a-b), not the examples provided by HOWARTH (1992, pl. 32, figs. 5a-b; pl. 33, figs. 2a-b, 3a-b, 4a-b; pl. 34, fig.1) and BÉCAUD (2006, pl. 37, fig. 1a-c). In fact, these samples are very different from each other and mainly from the holotype for some morphological traits and also for stratigraphical occurrence. Therefore in the following description, we compare our specimens from Marconessa quarry with BUCKMAN’ s holotype only, which has not an exact stratigraphical occurrence (Upper Lias at Ilminster, Somerset), but was also stated that it occur from the Whitbian zone (Falciferum subzone?).

Amended diagnosis: Evolute, compressed shell, characterized by an elliptical whorl section and a narrow acute-keeled venter in the inner whorls of the microconch, more flattened in the outer ones of the macroconch. The low umbilical wall is characterized by a rounded umbilical edge. Sigmoidal, fine and dense ribs, which are usually single in the inner whorls, sometimes bifurcating in the outer ones, more or less projected on the ventro-lateral edge. Rib-density decrease in outer whorls of large sized samples. The ammonitic suture is simple, moderately frilled, showing a long L lobe, a shorter E lobe, and well developed umbilical lobes.

Description and comparative diagnosis: We set the Apennines specimens in the morphological range of the H. subserpentinus , because they have the same whorl section, the same whorls coiling and the same ribs appearance of BUCKMAN’ s holotype. The Apennines samples show the shell little more evolute than holotype, a compressed elliptical whorls section with little overlap. The narrow not sulcated venter shows a thin keel; in the microconch it has an almost acute-keeled appearance, whereas the larger sized samples show a moderately trend to flattening. The ornament has sigmoidal, moderately sinuous, little projected on the ventro-lateral edge, fine and dense ribs, which shows a trend to increase in number and to fade and to bifurcate in the body chamber of the small-medium sized specimens, chiefly the ones that early occur in the stratigraphical range. On the contrary, the outer whorls of the larger sized specimens show a decrease in rib-density and more sinuous and projected ribs, mainly the samples which occur later. The suture line is close to the holotype for the ratio of the lobes. The Apennines samples of H. subserpentinus are at a glance close to Hildaites striatus . Besides, they have the same stratigraphic occurrence. Nevertheless, H. subserpentinus differs from H. striatus for larger sized sample, the greater shell evolution, for the slower increase of the coiling and for the ornament showing in the inner whorls single ribs starting from the umbilical edge. On the contrary H. striatus shows bifurcating or gentle bundled ribs from the inner whorls, which start from the umbilical wall; they are also finer and denser than those of H. subserpentinus .

Discussion: The BUCKMAN’ s holotype (1921, pl. 217) of H. subserpentinus is very important from a historical point of view, because it is the type species of the genus Hildaites . It was collected by MOORE and it was recently refigured by HOWARTH (1992, pl. 33, figs. 1a-1b), who nevertheless has included in the range of the species several examples that, in our opinion, are

too different from the holotype for morphology and probably for stratigraphic occurrence. A synonymy was also established by HOWARTH (1992) between H. subserpentinus and the ammonites figured by REINECKE, THOMPSON and BUCKMAN: the holotype of Argonauta serpentinus REINECKE, 1818 , refigured by THOMPSON (1909, fig. 13) and by BUCKMAN (1919, fig. 138A) such as Hildoceras serpentinum ; Hildoceras serpentinum BUCKMAN, 1889 , refigured by BUCKMAN (1923, fig. 267B) as holotype of Hildaites serpentiniformis and the plesiotype of Hildoceras serpentinum REINECKE, 1818 , refigured by BUCKMAN (1919, fig. 138B). Nevertheless, this account is very questionable. KOTTEK has figured two samples as H. subserpentinus (1966, pl. 5, figs. 1-2) which are close to the Apennines examples. We note that these Greek examples were found in assemblage with two specimens which were assigned by KOTTEK (1966, pl. 5, fig. 3 and pl. 6, fig. 1) to Hildaites acutus TATE, 1895 , but they are probably Hildaites striatus .

Occurrence: The Apennines specimens of H. subserpentinus occur in the Striatus zone, from the beds immediately overlying the OAE recording. On the contrary, HOWARTH (1992) states that the British examples of H. subserpentinus occur in the mid- Falciferum subzone. He also states that the holotype occurred in the mid-Falciferum subzone, following MOORE, THOMPSON and BUCKMAN’ s poor informations. But we suspect that the holotype has a different occurrence in respect of several examples figured by HOWARTH, which are morphologically very different. Noticeably, HOWARTH (1992) found at Barrington (Ilminster, Somerset) H. subserpentinus in assemblage with Hildoceras lusitanicum . The Apennines examples of H. subserpentinus are clearly older; they were found in assemblage with H. striatus .